[mudman.git] / language.md

% The Muddle Programming Language
% Greg Pfister
  S. W. Galley
  et al.
% 1979

MIT Technical Report 293

Laboratory for Computer Science\
Massachusetts Institute of Technology\
545 Technology Square\
Cambridge, Massachusetts 02139

Copyright
=========

This document is free of known copyright restrictions.

Abstract
========

The Muddle programming language began existence in late 1970 (under
the name Muddle) as a successor to Lisp (Moon, 1974), a candidate
vehicle for the Dynamic Modeling System, and a possible base for
implementation of Planner (Hewitt, 1969). The original design goals
included an interactive integrated environment for programming,
debugging, loading, and editing: ease in learning and use; facilities
for structured, modular, shared programs; extensibility of syntax,
data types and operators: data-type checking for debugging and
optional data-type declarations for compiled efficiency; associative
storage, coroutining, and graphics. Along the way to reaching those
goals, it developed flexible input/output (including the ARPA
Network), and flexible interrupt and signal handling. It now serves as
a base for software prototyping, research, development, education, and
implementation of the majority of programs at MIT-DMS: a library of
sharable modules, a coherent user interface, special research
projects, autonomous daemons, etc.

This document was originally intended to be a simple low-level
introduction to Muddle. It has, however, acquired a case of
elephantiasis and now amounts to a discursive description of the whole
interpreter, as realized in Muddle release numbers 55 (ITS version)
and 105 (Tenex and Tops-20 versions). (Significant changes from the
previous edition are marked in the margin.) A low-level introduction
may still be had by restricting one's attention to specially-marked
sections only. The scope of the document is confined as much as
possible to the interpreter itself. Other adjuncts (compiler,
assembler, pre-loaded user programs, library) are mentioned as little
as possible, despite their value in promoting the language seen by a
user from "basic survival" to "comfortable living". Indeed, Muddle
could not fulfill the above design goals without the compiler,
assembler, structure editor, control-stack printer, context printer,
pretty-printer, dynamic loader, and library system -- all of which are
not part of the interpreter but programs written in Muddle and
symbiotic with one another. Further information on these adjuncts can
be found in Lebling's (1979) document.

Acknowledgements
----------------

I was not a member of the original group which labored for two years
in the design and initial implementation of Muddle; that group was
composed principally of Gerald Sussman, Carl Hewit, Chris Reeve, Dave
Cressey, and later Bruce Daniels. I would therefore like to take this
opportunity to thank my Muddle mentors, chiefly Chris Reeve and Bruce
Daniels, for remaining civil through several months of verbal
badgering. I believe that I learned more than "just another
programming language" in learning Muddle, and I am grateful for this
opportunity to pass on some of that knowledge. What I cannot pass on
is the knowledge gained by using Muddle as a system; that I can only
ask you to share.

For editing the content of this document and correcting some
misconceptions, I would like to thank Chris Reeve, Bruce Daniels, and
especially Gerald Sussman, one of whose good ideas I finally did use.

Greg Pfister\
December 15, 1972

Since Greg left the fold, I have taken up the banner and updated his
document. The main sources for small revisions have been the on-line
file of changes to Muddle, for which credit goes to Neal Ryan as well
as Reeve and Daniels, and the set of on-line abstracts for interpreter
Subroutines, contributed by unnamed members of the Programming
Technology Division. Some new sections were written almost entirely by
others: Dave Lebling wrote chapter 14 and appendix 3, Jim Michener
section 14.3, Reeve chapter 19 and appendix 1, Daniels and Reeve
appendix 2. Brian Berkowitz section 22.7, Tak To section 17.2.2, and
Ryan section 17.1.3. Sue Pitkin did the tedious task of marking
phrases in the manuscript for indexing. Pitts Jarvis and Jack Haverty
advised on the use of PUB and the XGP. Many PTD people commented
helpfully on a draft version.

My task has been to impose some uniformity and structure on these
diverse resources (so that the result sounds less like a dozen hackers
typing at a dozen terminals for a dozen days) and to enjoy some of the
richness of Muddle from the inside. I especially thank Chris Reeve
("the oracle") for the patience to answer questions and resolve
doubts, as he no doubt as done innumerable times before.

S. W. Galley\
May 23, 1979

This work was supported by the Advanced Research Projects Agency of
the Department of Defense and was monitored by the Office of Naval
Research under contract N00014-75-C-0661.

This document was prepared using [the PUB
system](http://www.nomodes.com/pub_manual.html) (originally from the
Stanford Artificial Intelligence Laboratory) and printed on the Xerox
Graphics Printer of the M.I.T. Artificial Intelligence Laboratory.

Foreword
--------

Trying to explain Muddle to an uninitiate is somewhat like trying to
untie a Gordian knot. Whatever topic one chooses to discuss first,
full discussion of it appears to imply discussion of everything else.
What follows is a discursive presentation of Muddle in an order
apparently requiring the fewest forward references. It is not perfect
in that regard; however, if you are patient and willing to accept a
few, stated things as "magic" until they can be explained better, you
will probably not have too many problems understanding what is going
on.

There are no "practice problems"; you are assumed to be learning
Muddle for some purpose, and your work in achieving that purpose will
be more useful and motivated than artificial problems. In several
cases, the examples contain illustrations of important points which
are not covered in the text. Ignore examples as your peril.

This document does not assume knowledge of any specific programming
language on the \[sic\] your part. However, "computational literacy"
is assumed: you should have written at least one program before. Also
very little familiarity is assumed with the interactive time-sharing
operating systems under which Muddle runs -- ITS, Tenex, and Tops-20
-- namely just file and user naming conventions.

### Notation

Sections marked \[1\] are recommended for any uninitiate's first
reading, in lieu of a separate introduction for Muddle. \[On first
reading, text within brackets like these should be ignored.\]

Most specifically indicated examples herein are composed of pairs of
lines. The first line of a pair, the input, always ends in `$` (which
is how the ASCII character `ESC` is represented, and which always
represents it). The second line is the result of Muddle's groveling
over the first. If you were to type all the first lines at Muddle, it
would respond with all the second lines. (More exactly, the "first
line" is one or more objects in Muddle followed by `$`, and the
"second line" is everything up to the next "first line".)

Anything which is written in the Muddle language or which is typed on
a computer terminal appears herein in a fixed width font, as in
`ROOT`. A metasyntactic variable -- something to be replaced in actual
use by something else -- appears as *radix:fix*, in an italic font;
often the variable will have both a meaning and a data type (as here),
but sometimes one of those will be ommitted, for obvious reasons.

An ellipsis (...) indicates that something uninteresting has been
omitted. The character `^` means that the following character is to be
"controllified": it is usually typed by holding down a terminal's
`CTRL` key and striking the other key.

Chapter 1. Basic Introduction
=============================

The purpose of this chapter is to provide you with that minimal amount
of information needed to experiment with Muddle while reading this
document. It is strongly recommended that you do experiment,
especially upon reaching chapter 5 (Simple Functions).

1.1. Loading Muddle \[1\]
-------------------------

First, catch your rabbit. Somehow get the interpreter running -- the
program in the file `SYS:TS.Muddle` in the ITS version or
`SYS:Muddle.SAV` in the Tenex version or `SYS:Muddle.EXE` in the
Tops-20 version. The interpreter will first type out some news
relating to Muddle, if any, then type

    LISTENING-AT-LEVEL 1 PROCESS 1

and then wait for you to type something.

The program which you are now running is an interpreter for the
language Muddle. **All** it knows how to do is interpret Muddle
expressions. There is no special "command language"; you communicate
with the program -- make it do things for you -- by actually typing
legal Muddle expressions, which it then interprets. **Everything** you
can do at a terminal can be done in a program, and vice versa, in
exactly the same way.

The program will be referred to as just "Muddle" (or "the
interpreter") from here on. There is no ambiguity, since the program
is just an incarnation of the concept "Muddle".

1.2. Typing \[1\]
-----------------

Typing a character at Muddle normally just causes that character to be
echoed (printed on your terminal) and remembered in a buffer. The only
characters for which this is normally not true act as follows:

Typing `$` (`ESC`) causes Muddle to echo dollar-sign and causes the
contents of the buffer (the characters which you've typed) to be
interpreted as an expression(s) in Muddle. When this interpretation is
done, the result will be printed and Muddle will wait for more typing.
`ESC` will be represented by the glyph `$` in this document.

Typing the rubout character (`DEL` in the ITS and Top-20 versions,
`CTRL`+`A` in the Tenex version) causes the last character in the
buffer -- the one most recently typed -- to be thrown away (deleted).
If you now immediately type another rubout, once again the last
character is deleted -- namely the second most recently typed. Etc.
The character deleted is echoed, so you can see what you're doing. On
some "display" terminals, rubout will "echo" by causing the deleted
character to disappear. If no characters are in the buffer, rubout
echoes as a carriage-return line-feed.

Typing `^@` (`CTRL`+`@`) deletes everything you have typed since the
last `$`, and prints a carriage-return line-feed.

Typing `^D` (`CTRL`+`D`) causes the current input buffer to be typed
back out at you. This allows you to see what you really have, without
the confusing re-echoed characters produced by rubout.

Typing `^L` (`CTRL`+`L`) produces the same effect as typing `^D`,
except that, if your terminal is a "display" terminal (for example,
IMLAC, ARDS, Datapoint), it firsts clears the screen.

Typing `^G` (`CTRL`+`G`) causes Muddle to stop whatever it is doing
and act as if an error had occurred ([section
1.4](#14-errors-simple-considerations-1)). `^G` is generally most
useful for temporary interruptions to check the progress of a
computation. `^G` is "reversible" -- that is, it does not destroy any
of the "state" of the computation it interrupts. To "undo" a `^G`,
type the characters

    <ERRET T>$

(This is discussed more fully far below, in section 16.4.)

Typing `^S` (`CTRL`+`S`) causes Muddle to **throw away** what it is
currently doing and return a normal "listening" state. (In the Tenex
and Tops-20 versions, `^O` also should have the same effect.) `^S` is
generally most useful for aborting infinite loops and similar terrible
things. `^S` **destroys** whatever is going on, and so it is **not**
reversible.

Most expressions in Muddle include "brackets" (generically meant) that
must be correctly paired and nested. If you end your typing with the
pair of characters `!$` (`!`+`ESC`), all currently unpaired brackets
(but not double-quotes, which bracket strings of characters) will
automatically be paired and interpretation will start. Without the
`!`, Muddle will just sit there waiting for you to pair them. If you
have improperly nested parentheses, brackets, etc., within the
expression you typed, an error will occur, and Muddle will tell you
what is wrong.

Once the brackets are properly paired, Muddle will immediately echo
carriage-return and line-feed, and the next thing it prints will be
the result of the evaluation. Thus, if a plain `$` is not so echoed,
you have some expression unclosed. In that case, if you have not typed
any characters beyond the `$`, you can usually rub out the `$` and
other characters back to the beginning of the unclosed expression.
Otherwise, what you have typed is beyond the help of rubout and `^@`;
if you want to abort it, use `^S`.

Muddle accepts and distinguishes between upper and lower case. All
"built-in functions" must be referenced in upper case.

1.3. Loading a File \[1\]
-------------------------

If you have a program in Muddle that you have written as an ASCII file
on some device, you can "load" it by typing

    <FLOAD file>$

where *file* is the name of the file, in standard operating-system
syntax, enclosed in "s (double-quotes). Omitted parts of the file name
are taken by default from the file name `DSK: INPUT >` (in the ITS
version) or `DSK: INPUT.MUD` (in the Tenex and Tops-20 versions) in
the current disk directory.

Once you type `$`, Muddle will process the text in the file (including
`FLOAD`s) exactly as if you had typed it on a terminal and followed it
with `$`, except that "values" produced by the computations are not
printed. When Muddle is finished processing the file, it will print
`DONE`.

When Muddle starts running, it will `FLOAD` the file `MUDDLE INIT`
(ITS version) or `MUDDLE.INIT` (Tenex and Tops-20 versions), if it
exists.

1.4. Errors — Simple Considerations \[1\]
-----------------------------------------

When Muddle decides for some reason that something is wrong, the
standard sequence of evaluation is interrupted and an error function
is called. This produces the following terminal output:

    *ERROR*
    often-hyphenated-reason
    function-in-which-error-occurred
    LISTENING-AT-LEVEL integer PROCESS integer

You can now interact with Muddle as usual, typing expressions and
having them evaluated. There exist facilities (built-in functions)
allowing you to find out what went wrong, restart, or abandon whatever
was going on. In particular, you can recover from an error -- that is,
undo everything but side effects and return to the initial typing
phase -- by typing the following first line, to which Muddle will
respond with the second line:

    <ERRET>$
    LISTENING-AT-LEVEL 1 PROCESS 1

If you type the following first line while still in the error state
(before `<ERRET>`), Muddle will print, as shown, the arguments (or
"parameters or "inputs" or "independent variables") which gave
indigestion to the unhappy function:

    <ARGS <FRAME <FRAME>>>$
    [ arguments to unhappy function ]

This will be explained by and by.

Chapter 2. Read, Evaluate, and Print
====================================

2.1. General \[1\]
------------------

Once you type `$` and all brackets are correctly paired and nested,
the current contents of the input buffer go through processing by
three functions successively: first `READ`, which passes its output to
`EVAL` ("evaluate"), which passes its output to `PRINT`, whose output
is typed on the terminal.

\[Actually, the sequence is more like `READ`, `CRLF`, `EVAL`, `PRIN1`,
`CRLF` (explained in chapter 11); Muddle gives you a carriage-return
line-feed when the `READ` is complete, that is, when all brackets are
paired.\]

Functionally:

-   `READ`: printable representations → Muddle objects
-   `EVAL`: Muddle objects → Muddle objects
-   `PRINT`: Muddle objects → printable representations

That is, `READ` takes ASCII text, such as is typed in at a terminal,
and creates the Muddle objects represented by that text. `PRINT` takes
Muddle objects, creates ASCII text representations of them, and types
them out. `EVAL`, which is the really important one, performs
transformations on Muddle objects.

2.2. Philosophy (TYPEs) \[1\]
-----------------------------

In a general sense, when you are interacting with Muddle, you are
dealing with a world inhabited only by a particular set of objects:
Muddle objects.

Muddle objects are best considered as abstract entities with abstract
properties. The properties of a particular Muddle object depend on the
class of Muddle objects to which it belongs. This class is the `TYPE`
of the Muddle object. Every Muddle object has a `TYPE`, and every
`TYPE` has its own peculiarities. There are many different `TYPE`s in
Muddle; they will gradually be introduced below, but in the meantime
here is a representative sample: `SUBR` (the `TYPE` of `READ`, `EVAL`,
and `PRINT`), `FSUBR`, `LIST`, `VECTOR`, `FORM`, `FUNCTION`, etc.
Since every object has a `TYPE`, one often abbreviates "an object of
`TYPE` *type*" by saying "a *type*".

The laws of the Muddle world are defined by `EVAL`. In a very real
sense, `EVAL` is the only Muddle object which "acts", which "does
something". In "acting", `EVAL` is always "following the directions"
of some Muddle object. Every Muddle object should be looked upon as
supplying a set of directions to `EVAL`; what these directions are
depends heavily on the `TYPE` of the Muddle object.

Since `EVAL` is so ever-present, an abbreviation is in order:
"evaluates to *something*" or "`EVAL`s to *something*" should be taken
as an abbreviation for "when given to `EVAL`, causes `EVAL` to return
*something*".

As abstract entities, Muddle objects are, of course, not "visible".
There is, however, a standard way of representing abstract Muddle
objects in the real world. The standard way of representing any given
`TYPE` of Muddle object will be given below when the `TYPE` is
introduced. These standard representations are what `READ`
understands, and what `PRINT` produces.

2.3. Example (TYPE FIX) \[1\]
-----------------------------

    1$
    1

The following has occurred:

First, `READ` recognized the character `1` as the representation for
an object of `TYPE` `FIX`, in particular the one which corresponds to
the integer one. (`FIX` means integer, because the decimal point is
understood always to be in a fixed position: at the right-hand end.)
`READ` built the Muddle object corresponding to the decimal
representation typed, and returned it.

Then `EVAL` noted that its input was of `TYPE` `FIX`. An object of
`TYPE` `FIX` evaluates to itself, so `EVAL` returned its input
undisturbed.

Then `PRINT` saw that its input was of `TYPE` `FIX`, and printed on
the terminal the decimal character representation of the corresponding
integer.

2.4. Example (TYPE FLOAT) \[1\]
-------------------------------

    1.0$
    1.0

What went on was entirely analogous to the preceding example, except
that the Muddle object was of `TYPE` `FLOAT`. (`FLOAT` means a real
number (of limited precision), because the decimal point can float
around to any convenient position: an internal exponent part tells
where it "really" belongs.)

2.5. Example (TYPE ATOM, PNAME) \[1\]
-------------------------------------

    GEORGE$
    GEORGE

This time a lot more has happened.

`READ` noted that what was typed had no special meaning, and therefore
assumed that it was the representation of an identifier, that is, an
object of `TYPE` `ATOM`. ("Atom" means more or less *indivisible*.)
`READ` therefore attempted to look up the representation in a table it
keeps for such purposes \[a `LIST` of `OBLISTS`, available as the
local value of the `ATOM` `OBLIST`\]. If `READ` finds an `ATOM` in its
table corresponding to the representation, that `ATOM` is returned as
`READ`'s value. If `READ` fails in looking up, it creates a new
`ATOM`, puts it in the table with the representation read \[`INSERT`
into `<1 .OBLIST>` usually\], and returns the new `ATOM`. Nothing
which could in any way be referenced as a legal "value" is attached to
the new `ATOM`. The initially-typed representation of an `ATOM`
becomes its `PNAME`, meaning its name for `PRINT`. One often
abbreviates "object of `TYPE` `ATOM` with `PNAME` *name*" by saying
"`ATOM` *name*".

`EVAL`, given an `ATOM`, returned just that `ATOM`.

`PRINT`, given an `ATOM`, typed out its `PNAME`.

At the end of this chapter, the question "what is a legal `PNAME`"
will be considered. Further on, the methods used to attach values to
`ATOM`s will be described.

2.6. FIXes, FLOATs, and ATOMs versus READ: Specifics
----------------------------------------------------

### 2.6.1. READ and FIXed-point Numbers

`READ` considers any grouping of characters which are solely digits to
be a `FIX`, and the radix of the representation is decimal by default.
A `-` (hyphen) immediately preceding such a grouping represents a
negative `FIX`. The largest `FIX` representable on the PDP-10 is two
to the 35th power minus one, or 34,359,738,367 (decimal): the smallest
is one less than the negative of that number. If you attempt to type
in a `FIX` outside that range, `READ` converts it to a `FLOAT`; if a
program you write attempts to produce a `FIX` outside that range, an
overflow error will occur (unless it is disabled).

The radix used by `READ` and `PRINT` is changeable by the user;
however, there are two formats for representations of `FIX`es which
cause `READ` to use a specified radix independent of the current one.
These are as follows:

1.  If a group of digits is immediately followed by a period (`.`),
    `READ` interprets that group as the decimal representation of a
    `FIX`. For example, `10.` is always interpreted by `READ` as the
    decimal representation of ten.

2.  If a group of digits is immediately enclosed on both sides with
    asterisks (`*`), `READ` interprets that group as the octal
    representation of a `FIX`. For example, `*10*` is always
    interpreted by `READ` as the octal representation of eight.

### 2.6.2. READ and PRINT versus FLOATing-point Numbers

`PRINT` can produce, and `READ` can understand, two different formats
for objects of `TYPE` `FLOAT`. The first is "decimal-point" notation,
the second is "scientific" notation. Decimal radix is always used for
representations of `FLOAT`s.

"Decimal-point" notation for a `FLOAT` consists of an arbitrarily long
string of digits containing one `.` (period) which is followed by at
least one digit. `READ` will make a `FLOAT` out of any such object,
with a limit of precision of one part in 2 to the 27th power.

"Scientific" notation consists of:

1.  a number,

2.  immediately followed by `E` or `e` (upper or lower case letter E),

3.  immediately followed by an exponent,

where a "number" is an arbitrarily long string of digits, with or
without a decimal point (see following note): an an "exponent" is up
to two digits worth of `FIX`. This notation represents the "number" to
the "exponent" power of ten. Note: if the "number" as above would by
itself be a `FIX`, and if the "exponent" is positive, and if the
result is within the allowed range of `FIX`es, then the result will be
a `FIX`. For example, `READ` understands `10E1` as `100` (a `FIX`),
but `10E-1` as `1.0000000` (a `FLOAT`).

The largest-magnitude `FLOAT` which can be handled without overflow is
`1.7014118E+38` (decimal radix). The smallest-magnitude `FLOAT` which
can be handled without underflow is `.14693679E-38`.

### 2.6.3. READ and PNAMEs

The question "what is a legal `PNAME`?" is actually not a reasonable
one to ask: **any** non-empty string of **arbitrary** characters can
be the `PNAME` of an `ATOM`. However, some `PNAME`s are easier to type
to `READ` than others. But even the question "what are easily typed
`PNAME`s?" is not too reasonable, because: `READ` decides that a group
of characters is a `PNAME` by **default**; if it can't possibly be
anything else, it's a `PNAME`. So, the rules governing the
specification of `PNAME`s are messy, and best expressed in terms of
what is not a `PNAME`. For simplicity, you can just consider any
uninterrupted group of upper- and lower-case letters and (customarily)
hyphens to be a `PNAME`; that will always work. If you neither a
perfectionist nor a masochist, skip to the next chapter.

#### 2.6.3.1. Non-PNAMEs

A group of characters is **not** a `PNAME` if:

1.  It represents a `FLOAT` or a `FIX`, as described above -- that is,
    it is composed wholly of digits, or digits and a single `.`
    (period) or digits and a `.` and the letter `E` or `e` (with
    optional minus signs in the right places).

2.  It begins with a `.` (period).

3.  It contains -- if typed interactively -- any of the characters
    which have special interactive effects: `^@`, `^D`, `^L`, `^G`,
    `^O`, `$` (`ESC`), rubout.

4.  It contains a format character -- space, carriage-return,
    line-feed, form-feed, horizontal tab, vertical tab.

5.  It contains a `,` (comma) or a `#` (number sign) or a `'` (single
    quote) or a `;` (semicolon) or a `%` (percent sign).

6.  It contains any variety of bracket -- `(` or `)` or `[` or `]` or
    `<` or `>` or `{` or `}` or `"`.

In addition, the character `\` (backslash) has a special
interpretation, as mentioned below. Also the pair of characters `!-`
(exclamation-point hyphen) has an extremely special interpretation,
which you will reach at chapter 15.

The characters mentioned in cases 4 through 6 are "separators" -- that
is, they signal to `READ` that whatever it was that the preceding
characters represented, it's done now. They can also indicate the
start of a new object's representation (all the opening "brackets" do
just that).

#### 2.6.3.2. Examples

The following examples are not in the "standard format" of "*line
typed in*`$` *result printed*", because they are not, in some cases,
completed objects; hence, `READ` would continue waiting for the
brackets to be closed. In other cases, they will produce errors during
`EVAL`uation if other -- currently irrelevant -- conditions are not
met. Instead, the right-hand column will be used to state just what
`READ` thought the input in the left-hand column really was.

  ------------------------------------------------------------------------------------
  Input                       Explanation
  --------------------------- --------------------------------------------------------
  `ABC$`                      an `ATOM` of `PNAME` `ABC`

  `abc$`                      an `ATOM` of `PNAME` `abc`

  `ARBITRARILY-LONG-PNAME$`   an `ATOM` of `PNAME` `ARBITRARILY-LONG-PNAME`

  `1.2345$`                   a `FLOAT`, `PRINT`ed as `1.2345000`

  `1.2.345$`                  an `ATOM` of `PNAME` `1.2.345`

  `A.or.B$`                   a `ATOM` of `PNAME` `A.or.B`

  `.A.or.B$`                  not an `ATOM`, but (as explained later) a `FORM`
                              containing an `ATOM` of `PNAME` `A.or.B`.

  `MORE THAN ONE$`            three `ATOM`s, with `PNAME`s `MORE`, and `THAN`, and
                              `ONE`.

  `ab(cd$`                    an `ATOM` of `PNAME` `ab`, followed by the start of
                              something else (The something else will contain an
                              `ATOM` of `PNAME` beginning `cd.`)

  `12345A34$`                 an `ATOM` of `PNAME` `12345A35` (If the A had been an E,
                              the object would have been a `FLOAT`.)
  ------------------------------------------------------------------------------------

#### 2.6.3.3.  (Backslash) in ATOMs

If you have a strange, uncontrollable compulsion to have what were
referred to as "separators" above as part of the `PNAME`s of your
`ATOM`s, you can do so by preceding them with the character `\`
(backslash). `\` will also magically turn an otherwise normal `FIX` or
`FLOAT` into an `ATOM` if it appears amongst the digits. In fact,
backslash in front of **any** character changes it from something
special to "just another character" (including the character `\`). It
is an escape character.

When `PRINT` confronts an `ATOM` which had to be backslashed in order
to be an `ATOM`, it will dutifully type out the required `\`s. They
will not, however, necessarily be where you typed them; they will
instead be at those positions which will cause `READ` the least grief.
For example, `PRINT` will type out a `PNAME` which consists wholly of
digits by first typing a `\` and then typing the digits - no matter
where you originally typed the `\` (or `\`s).

#### 2.6.3.4. Examples of Awful ATOMs

The following examples illustrate the amount of insanity that can be
perpetrated by using `\`. The format of the examples is again
non-standard, this time not because anything is unfinished or in
error, but because commenting is needed: `PRINT` doesn't do it full
justice.

  -------------------------------------------------------------------------------
  Input                    Explanation
  ------------------------ ------------------------------------------------------
  `a\ one\ and\ a\ two$`   one `ATOM`, whose `PNAME` has four spaces in it

  `1234\56789$`            an `ATOM` of `PNAME` `123456789`, which `PRINT`s as
                           `\1233456789`

  `123\ $`                 an `ATOM` of `PNAME` `123space`, which `PRINT`s as
                           `\123\`, with a space on the end

  `\\$`                    an `ATOM` whose `PNAME` is a single backslash
  -------------------------------------------------------------------------------

Chapter 3. Built-in Functions
=============================

3.1. Representation \[1\]
-------------------------

Up to this point, all the objects we have been concerned with have had
no internal structure discernible in Muddle. While the characteristics
of objects with internal structure differ greatly, the way `READ` and
`PRINT` handle them is uniform, to wit:

-   `READ`, when applied to the representation of a structured object,
    builds and returns an object of the indicated `TYPE` with elements
    formed by applying `READ` to each of their representations in
    turn.

-   `PRINT`, when applied to a structured object, produces a
    representation of the object, with its elements represented as
    `PRINT` applied to each of them in turn.

A Muddle object which is used to represent the application of a
function to its arguments is an argument of `TYPE` `FORM`. Its printed
representation is

    < func arg-1 arg-2 ... arg-N >

where *func* is an object which designates the function to be applied,
and *arg-1* through *arg-N* are object which designate the arguments
or "actual parameters" or "inputs". A `FORM` is just a structured
object which is stored and can be manipulated like a `LIST` (its
"primitive type" is `LIST` -- chapter 6). The application of the
function to the arguments is done by `EVAL`. The usual meaning of
"function" (uncapitalized) in this document will be anything
applicable to arguments.

3.2. Evaluation \[1\]
---------------------

`EVAL` applied to a `FORM` acts as if following these directions:

First, example the *func* (first element) of the `FORM`. If it is an
`ATOM`, look at its "value" (global or local, in that order -- see
next chapter). If it is not an `ATOM`, `EVAL` it and look at the
result of the evaluation. If what you are looking at is not something
which can be applied to arguments, complain (via the `ERROR`
function). Otherwise, inspect what you are looking at and follow its
directions in evaluating or not evaluating the arguments (chapters 9
and 19) and then "apply the function" -- that is, `EVAL` the body of
the object gotten from *func*.

3.3. Built-in Functions (TYPE SUBR, TYPE FSUBR) \[1\]
-----------------------------------------------------

The built-in functions of Muddle come in two varieties: those which
have all their arguments `EVAL`ed before operating on them (`TYPE`
`SUBR`, for "subroutine", pronounced "subber") and those which have
none of their arguments `EVAL`ed (`TYPE` `FSUBR`, historically from
Lisp (Moon, 1974), pronounced "effsubber"). Collectively they will be
called `F/SUBR`s, although that term is not meaningful to the
interpreter. See appendix 2 for a listing of all `F/SUBR`s and short
descriptions. The term "Subroutine" will be used herein to mean both
`F/SUBR`s and compiled user programs (`RSUBR`s and `RSUBR-ENTRY`s --
chapter 19).

Unless otherwise stated, **every** Muddle built-in Subroutine is of
`TYPE` **`SUBR`**. Also, when it is stated that an argument of a
`SUBR` must be of a particular `TYPE`, note that this means that
`EVAL` of what is there must be of the particular `TYPE`.

Another convenient abbreviation which will be used is "the `SUBR`
*pname*" in place of "the `SUBR` which is initially the 'value' of the
`ATOM` of `PNAME` *pname*". "The `FSUBR` *pname*" will be used with a
similar meaning.

3.4. Examples (+ and FIX; Arithmetic) \[1\]
-------------------------------------------

    <+ 2 4 6>$
    12

The `SUBR` `+` adds numbers. Most of the usual arithmetic functions
are Muddle `SUBR`s: `+`, `-`, `*`, `/`, `MIN`, `MAX`, `MOD`, `SIN`,
`COS`, `ATAN`, `SQRT`, `LOG`, `EXP`, `ABS`. (See appendix 2 for short
descriptions of these.) All except `MOD`, which wants `FIX`es, are
indifferent as to whether their arguments are `FLOAT` or `FIX` or a
mixture. In the last case they exhibit "contagious `FLOAT`ing": one
argument of `TYPE` `FLOAT` forces the result to be of `TYPE` `FLOAT`.

    <FIX 1.0>$
    1

The `SUBR` `FIX` explicitly returns a `FIX`ed-point number
corresponding to a `FLOAT`ing-point number. `FLOAT` does the opposite.

    <+ 5 <* 2 3>>$
    11
    <SQRT <+ <* 3 3> <* 4 4>>>$
    5.0
    <- 5 3 2>$
    0
    <- 5>$
    -5
    <MIN 1 2.0>$
    1.0
    </ 11 7 2.0>$
    0.5

Note this last result: the division of two `FIX`es gives a `FIX` with
truncation, not rounding, of the remainder: the intermediate result
remains a `FIX` until a `FLOAT` argument is encountered.

3.5. Arithmetic Details
-----------------------

`+`, `-`, `*`, `/`, `MIN`, and `MAX` all take any number of arguments,
doing the operation with the first argument and the second, then with
that result and the third argument, etc. If called with no arguments,
each returns the identity for its operation (`0`, `0`, `1`, `1`, the
greatest `FLOAT`, and the least `FLOAT`, respectively); if called with
one argument, each acts as if the identity and the argument has been
supplied. They all will cause an overflow or underflow error if any
result, intermediate or final, is too large or too small for the
machine's capacity. (That error can be disabled if necessary --
section 16.9).

One arithmetic function that always requires some discussion is the
pseudo-random-number generator. Muddle's is named `RANDOM`, and it
always returns a `FIX`, uniformly distributed over the whole range of
`FIX`es. If `RANDOM` is never called with arguments, it always returns
the exact same sequence of numbers, for convenience in debugging.
"Debugged" programs should give `RANDOM` two arguments on the first
call, which become seeds for a new sequence. Popular choices of new
seeds are the numbers given by `TIME` (which see), possibly with bits
modified (chapter 18). Example ("pick a number from one to ten"):

    <+ 1 <MOD <RANDOM> 10>>$
    4

Chapter 4. Values of Atoms
==========================

4.1. General \[1\]
------------------

There are two kinds of "value" which can be attached to an `ATOM`. An
`ATOM` can have either, both, or neither. They interact in no way
(except that alternately referring to one and then the other is
inefficient). These two values are referred to as the **local value**
and the **global value** of an `ATOM`. The terms "local" and "global"
are relative to `PROCESS`es (chapter 20), not functions or programs.
The `SUBR`s which reference the local and global values of an `ATOM`,
and some of the characteristics of local versus global values, follow.

4.2. Global Values
------------------

### 4.2.1. SETG \[1\]

A global value can be assigned to an `ATOM` by the `SUBR` `SETG` ("set
global"), as in

    <SETG atom any>

where *atom* must `EVAL` to an `ATOM`, and *any* can `EVAL` to
anything. `EVAL` of the second argument becomes the global value of
`EVAL` of the first argument. The value returned by the `SETG` is its
second argument, namely the new global value of *atom*.

Examples:

    <SETG FOO <SETG BAR 500>>$
    500

The above made the global values of both the `ATOM` `FOO` and the
`ATOM` `BAR` equal to the `FIX`ed-point number 500.

    <SETG BAR FOO>$
    FOO

That made the global value of the `ATOM` `BAR` equal to the `ATOM`
`FOO`.

### 4.2.2. GVAL \[1\]

The `SUBR` `GVAL` ("global value") is used to reference the global
value of an `ATOM`.

    <GVAL atom>

returns as a value the global value of *atom*. If *atom* does not
evaluate to an `ATOM`, or if the `ATOM` to which it evaluates has no
global value, an error occurs.

`GVAL` applied to an `ATOM` anywhere, in any `PROCESS`, in any
function, will return the same value. Any `SETG` anywhere changes the
global value for everybody. Global values are context-independent.

`READ` understands the character `,` (comma) as an abbreviation for an
application of `GVAL` to whatever follows it. `PRINT` always
translates an application of `GVAL` into the comma format. The
following are absolutely equivalent:

    ,atom        <GVAL atom>

Assuming the examples in section 4.2.1 were carried out in the order
given, the following will evaluate as indicated:

    ,FOO$
    500
    <GVAL FOO>$
    500
    ,BAR$
    FOO
    ,,BAR$
    500

### 4.2.3. Note on SUBRs and FSUBRs

The initial `GVAL`s of the `ATOM`s used to refer to Muddle "built-in"
Subroutines are the `SUBR`s and `FSUBR`s which actually get applied
when those `ATOM`s are referenced. If you don't like the way those
supplied routines work, you are perfectly free to `SETG` the `ATOM`s
to your own versions.

### 4.2.4. GUNASSIGN

    <GUNASSIGN atom>

("global unassign") causes *atom* to have no assigned global value,
whether or not it had one previously. The storage used for the global
value can become free for other uses.

4.3. Local Values
-----------------

### 4.3.1. SET \[1\]

The `SUBR` `SET` is used to assign a local value to an `ATOM`.
Applications of `SET` are of the form

    <SET atom any>

`SET` returns `EVAL` of *any* just like `SETG`.

Examples:

    <SET BAR <SET FOO 100>>$
    100

Both `BAR` and `FOO` have been given local values equal to the
`FIX`ed-point number 100.

    <SET FOO BAR>$
    BAR

`FOO` has been given the local value `BAR`.

Note that neither of the above did anything to any global values `FOO`
and `BAR` might have had.

### 4.3.2. LVAL \[1\]

The `SUBR` used to extract the local value of an `ATOM` is named
`LVAL`. As with `GVAL`, `READ` understands an abbreviation for an
application of `LVAL`: the character `.` (period), and `PRINT`
produces it. The following two representations are equivalent, and
when `EVAL` operates on the corresponding Muddle object, it returns
the current local value of *atom*:

    <LVAL atom>        .atom

The local value of an `ATOM` is unique within a `PROCESS`. `SET`ting
an `ATOM` in one `PROCESS` has no effect on its `LVAL` in another
`PROCESS`, because each `PROCESS` has its own "control stack"
(chapters 20 and 22).

Assume **all** of the previous examples in this chapter have been
done. Then the following evaluate as indicated:

    .BAR$
    100
    <LVAL BAR>$
    100
    .FOO$
    BAR
    ,.FOO$
    FOO

### 4.3.3. UNASSIGN

    <UNASSIGN atom>

causes *atom* to have no assigned local value, whether or not it had
one previously.

4.4. VALUE
----------

`VALUE` is a `SUBR` which takes an `ATOM` as an argument, and then:

1.  if the `ATOM` has an `LVAL`, returns the `LVAL`;
2.  if the `ATOM` has no `LVAL` but has a `GVAL`, returns the `GVAL`;
3.  if the `ATOM` has neither a `GVAL` nor an `LVAL`, calls the
    `ERROR` function.

This order of seeking a value is the **opposite** of that used when an
`ATOM` is the first element of a `FORM`. The latter will be called the
G/LVAL, even though that name is not used in Muddle.

Example:

    <UNASSIGN A>$
    A
    <SETG A 1>$
    1
    <VALUE A>$
    1
    <SET A 2>$
    2
    <VALUE A>$
    2
    ,A$
    1

Chapter 5. Simple Functions
===========================

5.1. General \[1\]
------------------

The Muddle equivalent of a "program" (uncompiled) is an object of
`TYPE` `FUNCTION`. Actually, full-blown "programs" are usually
composed of sets of `FUNCTION`s, with most `FUNCTION`s in the set
acting as "subprograms".

A `FUNCTION` may be considered to be a `SUBR` or `FSUBR` which you
yourself define. It is "run" by using a `FORM` to apply it to
arguments (for example, &lt;*function arg-1 arg-2 ...*&gt;), and it
always "returns" a single object, which is used as the value of the
`FORM` that applied it. The single object may be ignored by whatever
"ran" the `FUNCTION` -- equivalent to "returning no value" -- or it
may be a structured object containing many objects -- equivalent to
"returning many values". Muddle is an "applicative" language, in
contrast to "imperative" languages like Fortran. In Muddle it is
impossible to return values through arguments in the normal case; they
can be returned only as the value of the `FORM` itself, or as side
effects to structured objects or global values.

In this chapter a simple subset of the `FUNCTION`s you can write is
presented, namely `FUNCTION`s which "act like" `SUBR`s with a fixed
number of arguments. While this class corresponds to about 90% of the
`FUNCTION`s ever written, you won't be able to do very much with them
until you read further and learn more about Muddle's control and
manipulatory machinery. However, all that machinery is just a bunch of
`SUBR`s and `FSUBR`s, and you already know how to "use" them; you just
need to be told what they do. Once you have `FUNCTION`s under your
belt, you can immediately make use of everything presented from this
point on in the document. In fact, we recommend that you do so.

5.2. Representation \[1\]
-------------------------

A `FUNCTION` is just another data object in Muddle, of `TYPE`
`FUNCTION`. It can be manipulated like any other data object. `PRINT`
represents a `FUNCTION` like this:

    #FUNCTION (elements)

that is, a number sign, the `ATOM` `FUNCTION`, a left parenthesis,
each of the elements of the `FUNCTION`, and a right parenthesis. Since
`PRINT` represents `FUNCTION`s like this, you can type them in to
`READ` this way. (But there are a few `TYPE`s for which that
implication is false.)

The elements of a `FUNCTION` can be "any number of anythings";
however, when you **use** a `FUNCTION` (apply it with a `FORM`),
`EVAL` will complain if the `FUNCTION` does not look like

    #FUNCTION (act:atom arguments:list decl body)

where *act* and *decl* are optional (section 9.8 and chapter 14);
*body* is **at least one** Muddle object -- any old Muddle object;
and, in this simple case, *arguments* is

    (any number of ATOMs)

that is, something `READ` and `PRINT`ed as: left parenthesis, any
number -- including zero -- of `ATOM`s, right parenthesis. (This is
actually a normal Muddle object of `TYPE` `LIST`, containing only
`ATOM`s.)

Thus, these `FUNCTION`s will cause errors -- but only **when used**:

  Input                       Explanation
  --------------------------- ----------------------------------
  `#FUNCTION ()`              -- no argument `LIST` or body
  `#FUNCTION ((1) 2 7.3)`     -- non-`ATOM` in argument `LIST`
  `#FUNCTION ((A B C D))`     -- no body
  `#FUNCTION (<+ 1 2> A C)`   -- no argument `LIST`

These `FUNCTION`s will never cause errors because of format:

    #FUNCTION (() 1 2 3 4 5)
    #FUNCTION ((A) A)
    #FUNCTION (()()()()()()()())
    #FUNCTION ((A B C D EE F G H HIYA) <+ .A .HIYA>)
    #FUNCTION ((Q) <SETG C <* .Q ,C>> <+ <MOD ,C 3> .Q>)

and the last two actually do something which might be useful. (The
first three are rather pathological, but legal.)

5.3. Application of FUNCTIONs: Binding \[1\]
--------------------------------------------

`FUNCTION`s, like `SUBR`s and `FSUBR`s, are applied using `FORM`s. So,

    <#FUNCTION ((X) <* .X .X>) 5>$
    25

applied the indicated `FUNCTION` to 5 and returned 25.

What `EVAL` does when applying a `FUNCTION` is the following:

1.  Create a "world" in which the `ATOM`s of the argument `LIST` have
    been **`SET`** to the values applied to the `FUNCTION`, and all
    other `ATOM`s have their original values. This is called
    "binding".

-   In the above, this is a "world" in which `X` is `SET` to `5`.

2.  In that new "world", evaluate all the objects in the body of the
    `FUNCTION`, one after the other, from first to last.

-   In the above, this means evaluate `<* .X .X>` in a "world" where
    `X` is `SET` to `5`.

3.  Throw away the "world" created, and restore the `LVAL`s of all
    `ATOM`s bound in this application of the `FUNCTION` to their
    originals (if any). This is called "unbinding".

-   In the above, this simply gives `X` back the local value, if any,
    that it had before binding.

4.  Return as a value the **last value obtained** when the
    `FUNCTION`'s body was evaluated in step (2).

-   In the above, this means return `25` as the value.

The "world" mentioned above is actually an object of `TYPE`
`ENVIRONMENT`. The fact that such "worlds" are separate from the
`FUNCTION`s which cause their generation means that **all** Muddle
`FUNCTION`s can be used recursively.

The only thing that is at all troublesome in this sequence is the
effect of creating these new "worlds", in particular, the fact that
the **previous** world is completely restored. This means that if,
inside a `FUNCTION`, you `SET` one of its argument `ATOM`s to
something, that new `LVAL` will **not** be remembered when `EVAL`
leaves the `FUNCTION`. However, if you `SET` an `ATOM` which is
**not** in the argument `LIST` (or `SETG` **any** `ATOM`) the new
local (or global) value **will** be remembered. Examples:

    <SET X 0>$
    0
    <#FUNCTION ((X) <SET X <* .X .X>>) 5>$
    25
    .X$
    0

On the other hand,

    <SET Y 0>$
    0
    <#FUNCTION ((X) <SET Y <* .X .X>>) 5>$
    25
    .Y$
    25

By using `PRINT` as a `SUBR`, we can "see" that an argument's `LVAL`
really is changed while `EVAL`uating the body of a `FUNCTION`:

    <SET X 5>$
    5
    <#FUNCTION ((X) <PRINT .X> <+ .X 10>) 3>$
    3 13
    .X$
    5

The first number after the application `FORM` was typed out by the
`PRINT`; the second is the value of the application.

Remembering that `LVAL`s of `ATOM`s **not** in argument `LIST`s are
not changed, we can reference them within `FUNCTION`s, as in

    <SET Z 100>$
    100
    <#FUNCTION ((Y) </ .Z .Y>) 5>$
    20

`ATOM`s used like `Z` or `Y` in the above examples are referred to as
"free variables". The use of free variables, while often quite
convenient, is rather dangerous unless you know **exactly** how a
`FUNCTION` will **always** be used: if a `FUNCTION` containing free
variables is used within a `FUNCTION` within a `FUNCTION` within ...,
one of those `FUNCTION`s might just happen to use your free variable
in its argument `LIST`, binding it to some unknown value and possibly
causing your use of it to be erroneous. Please note that "dangerous",
as used above, really means that it may be effectively **impossible**
(1) for other people to use your `FUNCTION`s, and (2) for **you** to
use your `FUNCTION`s a month (two weeks?) later.

5.4. Defining FUNCTIONs (FUNCTION and DEFINE) \[1\]
---------------------------------------------------

Obviously, typing `#FUNCTION (...)` all the time is neither reasonable
nor adequate for many purposes. Normally, you just want a `FUNCTION`
to be the `GVAL` of some `ATOM` -- the way `SUBR`s and `FSUBR`s are --
so you can use it repeatedly (and recursively). Note that you
generally do **not** want a `FUNCTION` to be the `LVAL` of an `ATOM`;
this has the same problems as free variables. (Of course, there are
always cases where you are being clever and **want** the `ATOM` to be
re-bound....)

One way to "name" a `FUNCTION` is

    <SETG SQUARE #FUNCTION ((X) <* .X .X>)>$
    #FUNCTION ((X) <* .X .X>

So that

    <SQUARE 5>$
    25
    <SQUARE 100>$
    10000

Another way, which is somewhat cleaner in its typing:

    <SETG SQUARE <FUNCTION (X) <* .X .X>>>$
    #FUNCTION ((X) <* .X .X>

`FUNCTION` is an `FSUBR` which simply makes a `FUNCTION` out of its
arguments and returns the created `FUNCTION`.

This, however, is generally the **best** way:

    <DEFINE SQUARE (X) <* .X .X>>$
    SQUARE
    ,SQUARE$
    #FUNCTION ((X) <* .X .X>

The last two lines immediately above are just to prove that `DEFINE`
did the "right thing".

`DEFINE` is an `FSUBR` which `SETG`s `EVAL` of its first argument to
the `FUNCTION` it makes from the rest of its arguments, and then
returns `EVAL` of its first argument. `DEFINE` obviously requires the
least typing of the above methods, and is "best" from that standpoint.
However, the real reason for using `DEFINE` is the following: If
`EVAL` of `DEFINE`'s first argument **already has** a `GVAL`, `DEFINE`
produces an error. This helps to keep you from accidentally redefining
things -- like Muddle `SUBR`s and `FSUBR`s. The `SETG` constructions
should be used only when you really do want to redefine something.
`DEFINE` will be used in the rest of this document.

\[Actually, if it is absolutely necessary to use `DEFINE` to
"redefine" things, there is a "switch" which can be used: if the
`LVAL` of the `ATOM` `REDEFINE` is `T` (or anything not of `TYPE`
`FALSE`), `DEFINE` will produce no errors. The normal state can be
restored by evaluating `<SET REDEFINE <>>`. See chapter 8.\]

5.5. Examples (Comments) \[1\]
------------------------------

Using `SQUARE` as defined above:

    <DEFINE HYPOT (SIDE-1 SIDE-2)
            ;"This is a comment. This FUNCTION finds the
              length of the hypotenuse of a right triangle
              of sides SIDE-1 and SIDE-2."
        <SQRT <+ <SQUARE .SIDE-1> <SQUARE .SIDE-2>>>>$
    HYPOT
    <HYPOT 3 4>$
    5.0

Note that carriage-returns, line-feeds, tabs, etc. are just
separators, like spaces. A comment is **any single** Muddle object
which follows a `;` (semicolon). A comment can appear between any two
Muddle objects. A comment is totally ignored by `EVAL` but remembered
and associated by `READ` with the place in the `FUNCTION` (or any
other structured object) where it appeared. (This will become clearer
after chapter 13.) The `"`s (double-quotes) serve to make everything
between them a single Muddle object, whose `TYPE` is `STRING` (chapter
7). (`SQRT` is the `SUBR` which returns the square root of its
argument. It always returns a `FLOAT`.)

A whimsical `FUNCTION`:

    <DEFINE ONE (THETA) ;"This FUNCTION always returns 1."
            <+ <SQUARE <SIN .THETA>>
               <SQUARE <COS .THETA>>>>$
    ONE
    <ONE 5>$
    0.99999994
    <ONE 0.23>$
    0.99999999

`ONE` always returns (approximately) one, since the sum of the squares
of sin(x) and cos(x) is unity for any x. (`SIN` and `COS` always
return `FLOAT`s, and each takes its argument in radians. `ATAN`
(arctangent) returns its value in radians. Any other trigonometric
function can be compounded from these three.)

Muddle doesn't have a general "to the power" `SUBR`, so let's define
one using `LOG` and `EXP` (log base e, and e to a power, respectively;
again, they return `FLOAT`s).

    <DEFINE ** (NUM PWR) <EXP <* .PWR <LOG .NUM>>>>$
    **
    <** 2 2>$
    4.0000001
    <** 5 3>$
    125.00000
    <** 25 0.5>$
    5.0000001

Two `FUNCTION`s which use a single global variable (Since the `GVAL`
is used, it cannot be rebound.):

    <DEFINE START () <SETG GV 0>>$
    START
    <DEFINE STEP () <SETG GV <+ ,GV 1>>>$
    STEP
    <START>$
    0
    <STEP>$
    1
    <STEP>$
    2
    <STEP>$
    3

`START` and `STEP` take no arguments, so their argument `LIST`s are
empty.

An interesting, but pathological, `FUNCTION`:

    <DEFINE INC (ATM) <SET .ATM <+ ..ATM 1>>>$
    INC
    <SET A 0>$
    0
    <INC A>$
    1
    <INC A>$
    2
    .A$
    2

`INC` takes an **`ATOM`** as an argument, and `SET`s that `ATOM` to
its current `LVAL` plus `1`. Note that inside `INC`, the `ATOM` `ATM`
is `SET` to the `ATOM` which is its argument; thus `..ATM` returns the
`LVAL` of the **argument**. However, there is a problem:

    <SET ATM 0>$
    0
    <INC ATM>$

    *ERROR*
    ARG-WRONG-TYPE
    +
    LISTENING-AT-LEVEL 2 PROCESS 1
    <ARGS <FRAME <FRAME>>>$
    [ATM 1]

The error occurred because `.ATM` was `ATM`, the argument to `INC`,
and thus `..ATM` was `ATM` also. We really want the outermost `.` in
`..ATM` to be done in the "world" (`ENVIRONMENT`) which existed **just
before** `INC` was entered -- and this definition of `INC` does both
applications of `LVAL` in its own "world". Techniques for doing `INC`
"correctly" will be covered below. Read on.

Chapter 6. Data Types
=====================

6.1. General \[1\]
------------------

A Muddle object consists of two parts: its `TYPE` and its "data part"
(appendix 1). The interpretation of the "data part" of an object
depends of course on its `TYPE`. The structural organization of an
object, that is, the way it is organized in storage, is referred to as
its "primitive type". While there are many different `TYPE`s of
objects in Muddle, there are fewer primitive types.

All structured objects in Muddle are ordered sequences of elements. As
such, there are `SUBR`s which operate on all of them uniformly, as
ordered sequences. On the other hand, the reason for having different
primitive types of structured objects is that there are useful
qualities of structured objects which are mutually incompatible. There
are, therefore, `SUBR`s which do not work on all structured objects:
these `SUBR`s exist to take full advantage of those mutually
incompatible qualities. The most-commonly-used primitive types of
structured objects are discussed in chapter 7, along with those
special `SUBR`s operating on them.

It is very easy to make a new Muddle object that differs from an old
one only in `TYPE`, as long as the primitive type is unchanged. It is
relatively difficult to make a new structured object that differs from
an old one in primitive type, even if it has the same elements.

Before talking any more about structured objects, some information
needs to be given about `TYPE`s in general.

6.2. Printed Representation \[1\]
---------------------------------

There are many `TYPE`s for which Muddle has no specific
representation. There aren't enough different kinds of brackets. The
representation used for `TYPE`s without any special representation is

    #type representation-as-if-it-were-its-primitive-type

`READ` will understand that format for **any** `TYPE`, and `PRINT`
will use it by default. This representational format will be referred
to below as "\# notation". It was used above to represent `FUNCTION`s.

6.3. SUBRs Related to TYPEs
---------------------------

### 6.3.1. TYPE \[1\]

    <TYPE any>

returns an **`ATOM`** whose `PNAME` corresponds to the `TYPE` of
*any*. There is no `TYPE` "TYPE". To type a `TYPE` (aren't homonyms
wonderful?), just type the appropriate `ATOM`, like `FIX` or `FLOAT`
or `ATOM` etc. However, in this document we will use the convention
that a metasyntactic variable can have *type* for a "data type": for
example, *foo:type* means that the `TYPE` of *foo* is `ATOM`, but the
`ATOM` must be something that the `SUBR` `TYPE` can return.

Examples:

    <TYPE 1>$
    FIX
    <TYPE 1.0>$
    FLOAT
    <TYPE +>$
    ATOM
    <TYPE ,+>$
    SUBR
    <TYPE GEORGE>$
    ATOM

### 6.3.2. PRIMTYPE \[1\]

    <PRIMTYPE any>

evaluates to the primitive type of *any*. The `PRIMTYPE` of *any* is
an `ATOM` which also represents a `TYPE`. The way an object can be
**manipulated** depends solely upon its `PRIMTYPE`; the way it is
**evaluated** depends upon its `TYPE`.

Examples:

    <PRIMTYPE 1>$
    WORD
    <PRIMTYPE 1.0>$
    WORD
    <PRIMTYPE ,+>$
    WORD
    <PRIMTYPE GEORGE>$
    ATOM

### 6.3.3. TYPEPRIM \[1\]

    <TYPEPRIM type>

returns the `PRIMTYPE` of an object whose `TYPE` is *type*. *type* is,
as usual, an `ATOM` used to designate a `TYPE`.

Examples:

    <TYPEPRIM FIX>$
    WORD
    <TYPEPRIM FLOAT>$
    WORD
    <TYPEPRIM SUBR>$
    WORD
    <TYPEPRIM ATOM>$
    ATOM
    <TYPEPRIM FORM>$
    LIST

### 6.3.4. CHTYPE \[1\]

    <CHTYPE any type>

("change type") returns a new object that has `TYPE` *type* and the
same "data part" as *any* (appendix 1).

    <CHTYPE (+ 2 2) FORM>$
    <+ 2 2>

An error is generated if the `PRIMTYPE` of *any* is not the same as
the `TYPEPRIM` of *type*. An error will also be generated if the
attempted `CHTYPE` is dangerous and/or senseless, for example,
`CHTYPE`ing a `FIX` to a `SUBR`. Unfortunately, there are few useful
examples we can do at this point.

\[`CHTYPE`ing a `FIX` to a `FLOAT` or vice versa produces, in general,
nonsense, since the bit formats for `FIX`es and `FLOAT`s are
different. The `SUBR`s `FIX` and `FLOAT` convert between those
formats. Useful obscurity: because of their internal representations
on the PDP-10, `<CHTYPE <MAX> FIX>` gives the least possible `FIX`,
and analogously for `MIN`.\]

Passing note: "\# notation" is just an instruction to `READ` saying
"`READ` the representation of the `PRIMTYPE` normally and (literally)
`CHTYPE` it to the specified `TYPE`". \[Or, if the `PRIMTYPE` is
`TEMPLATE`, "apply the `GVAL` of the `TYPE` name (which should be a
`TEMPLATE` constructor) to the given elements of the `PRIMTYPE`
`TEMPLATE` as arguments."\]

6.4. More SUBRs Related to TYPEs
--------------------------------

### 6.4.1. ALLTYPES

    <ALLTYPES>

returns a `VECTOR` (chapter 7) containing just those `ATOM`s which can
currently be returned by `TYPE` or `PRIMTYPE`. This is the very
"`TYPE` vector" (section 22.1) that the interpreter uses: look, but
don't touch. No examples: try it, or see appendix 3.

### 6.4.2. VALID-TYPE?

    <VALID-TYPE? atom>

returns `#FALSE ()` if *atom* is not the name of a `TYPE`, and the
same object that `<TYPE-C atom>` (section 19.5) returns if it is.

### 6.4.3. NEWTYPE

Muddle is a type-extensible language, in the sense that the programmer
can invent new `TYPE`s and use them in every way that the predefined
`TYPE`s can be used. A program-defined `TYPE` is called a `NEWTYPE`.
New `PRIMTYPE`s cannot be invented except by changing the interpreter;
thus the `TYPEPRIM` of a `NEWTYPE` must be chosen from those already
available. But the name of a `NEWTYPE` (an `ATOM` of course) can be
chosen freely -- so long as it does not conflict with an existing
`TYPE` name. More importantly, the program that defines a `NEWTYPE`
can be included in a set of programs for manipulating objects of the
`NEWTYPE` in ways that are more meaningful than the predefined `SUBR`s
of Muddle.

Typically an object of a `NEWTYPE` is a structure that is a model of
some entity in the real world -- or whatever world the program is
concerned with -- and the elements of the structure are models of
parts or aspects of the real-world entity. A `NEWTYPE` definition is a
convenient way of formalizing this correspondence, of writing it down
for all to see and use rather than keeping it in your head. If the
defining set of programs provides functions for manipulating the
`NEWTYPE` objects in all ways that are meaningful for the intended
uses of the `NEWTYPE`, then any other program that wants to use the
`NEWTYPE` can call the manipulation functions for all its needs, and
it need never know or care about the internal details of the `NEWTYPE`
objects. This technique is a standard way of providing modularity and
abstraction.

For example, suppose you wanted to deal with airline schedules. If you
were to construct a set of programs that define and manipulate a
`NEWTYPE` called `FLIGHT`, then you could make that set into a
standard package of programs and call on it to handle all information
pertaining to scheduled airline flights. Since all `FLIGHT`s would
have the same quantity of information (more or less) and you would
want quick access to individual elements, you would not want the
`TYPEPRIM` to be `LIST`. Since the elements would be of various
`TYPE`s, you would not want the `TYPEPRIM` to be `UVECTOR` -- nor its
variations `STRING` or `BYTES`. The natural choice would be a
`TYPEPRIM` of `VECTOR` (although you could gain space and lose time
with `TEMPLATE` instead).

Now, the individual elements of a `FLIGHT` would, no doubt, have
`TYPE`s and meanings that don't change. The elements of a `FLIGHT`
might be airline code, flight number, originating-airport code, list
of intermediate stops, destination-airport code, type of aircraft,
days of operation, etc. Each and every `FLIGHT` would have the airline
code for its first element (say), the flight number for its second,
and so on. It is natural to invent names (`ATOM`s) for these elements
and always refer to the elements by name. For example, you could
`<SETG AIRLINE 1>` or `<SETG AIRLINE <OFFSET 1 FLIGHT>>` -- and in
either case `<MANIFEST AIRLINE>` so the compiler can generate more
efficient code. Then, if the local value of `F` were a `FLIGHT`,
`<AIRLINE .F>` would return the airline code, and `<AIRLINE .F AA>`
would set the airline code to `AA`. Once that is done, you can forget
about which element comes first: all you need to know are the names of
the offsets.

The next step is to notice that, outside the package of `FLIGHT`
functions, no one needs to know whether `AIRLINE` is just an offset or
in fact a function of some kind. For example, the scheduled duration
of a flight might not be explicitly stored in a `FLIGHT`, just the
scheduled times of departure and arrival. But, if the package had the
proper `DURATION` function for calculating the duration, then the call
`<DURATION .F>` could return the duration, no matter how it is found.
In this way the internal details of the package are conveniently
hidden from view and abstracted away.

The form of `NEWTYPE` definition allows for the `TYPE`s of all
components of a `NEWTYPE` to be declared (chapter 14), for use both by
a programmer while debugging programs that use the `NEWTYPE` and by
the compiler for generating faster code. It is very convenient to have
the type declaration in the `NEWTYPE` definition itself, rather than
replicating it everywhere the `NEWTYPE` is used. (If you think this
declaration might be obtrusive while debugging the programs in the
`NEWTYPE` package, when inconsistent improvements are being made to
various programs, you can either dissociate any declaration from the
`NEWTYPE` or turn off Muddle type-checking completely. Actually this
declaration is typically more useful to a programmer during
development than it is to the compiler.)

    <NEWTYPE atom type>

returns *atom*, after causing it to become the representation of a
brand-new `TYPE` whose `PRIMTYPE` is `<TYPEPRIM type>`. What `NEWTYPE`
actually does is make *atom* a legal argument to `CHTYPE` and
`TYPEPRIM`. (Note that names of new `TYPE`s can be blocked lexically
to prevent collision with other names, just like any other `ATOM`s --
chapter 15.) Objects of a `NEWTYPE`-created `TYPE` can be generated by
creating an object of the appropriate `PRIMTYPE` and using `CHTYPE`.
They will be `PRINT`ed (initially), and can be directly typed in, by
the use of "\# notation" as described above. `EVAL` of any object
whose `TYPE` was created by `NEWTYPE` is initially the object itself,
and, initially, you cannot `APPLY` something of a generated `TYPE` to
arguments. But see below.

Examples:

    <NEWTYPE GARGLE FIX>$
    GARGLE
    <TYPEPRIM GARGLE>$
    WORD
    <SET A <CHTYPE 1 GARGLE>>$
    #GARGLE *000000000001*
    <SET B #GARGLE 100>$
    #GARGLE *000000000144*
    <TYPE .B>$
    GARGLE
    <PRIMTYPE .B>$
    WORD

### 6.4.4. PRINTTYPE, EVALTYPE and APPLYTYPE

    <PRINTTYPE type how>

    <EVALTYPE type how>

    <APPLYTYPE type how>

all return *type*, after specifying *how* Muddle is to deal with it.

These three `SUBR`s can be used to make newly-generated `TYPE`s behave
in arbitrary ways, or to change the characteristics of standard Muddle
`TYPE`s. `PRINTTYPE` tells Muddle how to print *type*, `EVALTYPE` how
to evaluate it, and `APPLYTYPE` how to apply it in a `FORM`.

*how* can be either a `TYPE` or something that can be applied to
arguments.

If *how* is a `TYPE`, Muddle will treat *type* just like the `TYPE`
given as *how*. *how* must have the same `TYPEPRIM` as *type*.

If *how* is applicable, it will be used in the following way:

For `PRINTTYPE`, *how* should take one argument: the object being
output. *how* should output something without formatting
(`PRIN1`-style); its result is ignored. (Note: *how* cannot use an
output `SUBR` on *how*'s own *type*: endless recursion will result.
`OUTCHAN` is bound during the application to the `CHANNEL` in use, or
to a pseudo-internal channel for `FLATSIZE` -- chapter 11.) If *how*
is the `SUBR` `PRINT`, *type* will receive no special treatment in
printing, that is, it will be printed as it was in an initial Muddle
or immediately after its defining `NEWTYPE`.

For `EVALTYPE`, *how* should take one argument: the object being
evaluated. The value returned by *how* will be used as `EVAL` of the
object. If *how* is the `SUBR` `EVAL`, *type* will receive no special
treatment in its evaluation.

For `APPLYTYPE`, *how* should take at least one argument. The first
argument will be the object being applied: the rest will be the
objects it was given as arguments. The result returned by *how* will
be used as the result of the application. If *how* is the `SUBR`
`APPLY`, *type* will receive no special treatment in application to
arguments.

If any of these `SUBR`s is given only one argument, that is if *how*
is omitted, it returns the currently active *how* (a `TYPE` or an
applicable object), or else `#FALSE ()` if *type* is receiving no
special treatment in that operation.

Unfortunately, these examples are fully understandable only after you
have read through chapter 11.

    <DEFINE ROMAN-PRINT (NUMB)
    <COND (<OR <L=? .NUMB 0> <G? .NUMB 3999>>
           <PRINC <CHTYPE .NUMB TIME>>)
          (T
           <RCPRINT </ .NUMB 1000> '![!\M]>
           <RCPRINT </ .NUMB  100> '![!\C !\D !\M]>
           <RCPRINT </ .NUMB   10> '![!\X !\L !\C]>
           <RCPRINT    .NUMB       '![!\I !\V !\X]>)>>$
    ROMAN-PRINT

    <DEFINE RCPRINT (MODN V)
    <SET MODN <MOD .MODN 10>>
    <COND (<==? 0 .MODN>)
          (<==? 1 .MODN> <PRINC <1 .V>>)
          (<==? 2 .MODN> <PRINC <1 .V>> <PRINC <1 .V>>)
          (<==? 3 .MODN> <PRINC <1 .V>> <PRINC <1 .V>> <PRINC <1 .V>>)
          (<==? 4 .MODN> <PRINC <1 .V>> <PRINC <2 .V>>)
          (<==? 5 .MODN> <PRINC <2 .V>>)
          (<==? 6 .MODN> <PRINC <2 .V>> <PRINC <1 .V>>)
          (<==? 7 .MODN> <PRINC <2 .V>> <PRINC <1 .V>> <PRINC <1 .V>>)
          (<==? 8 .MODN>
           <PRINC <2 .V>>
           <PRINC <1 .V>>
           <PRINC <1 .V>>
           <PRINC <1 .V>>)
          (<==? 9 .MODN> <PRINC <1 .V>> <PRINC <3 .V>>)>>$
    RCPRINT

    <PRINTTYPE TIME FIX> ;"fairly harmless but necessary here"$
    TIME
    <PRINTTYPE FIX ,ROMAN-PRINT>    ;"hee hee!"$
    FIX
    <+ 2 2>$
    IV
    1984$
    MCMLXXXIV
    <PRINTTYPE FIX ,PRINT>$
    FIX

    <NEWTYPE GRITCH LIST>   ;"a new TYPE of PRIMTYPE LIST"$
    GRITCH
    <EVALTYPE GRITCH>$
    #FALSE ()
    <EVALTYPE GRITCH LIST>  ;"evaluated like a LIST"$
    GRITCH
    <EVALTYPE GRITCH>$
    LIST
    #GRITCH (A <+ 1 2 3> !<SET A "ABC">)    ;"Type in one."$
    #GRTICH (A 6 !\A !\B !\C)

    <NEWTYPE HARRY VECTOR>  ;"a new TYPE of PRIMTYPE VECTOR"$
    HARRY
    <EVALTYPE HARRY #FUNCTION ((X) <1 .X>)>
        ;"When a HARRY is EVALed, return its first element."$
    HARRY
    #HARRY [1 2 3 4]$
    1

    <NEWTYPE WINNER LIST>   ;"a TYPE with funny application"$
    WINNER
    <APPLYTYPE WINNER>$
    #FALSE ()
    <APPLYTYPE WINNER <FUNCTION (W "TUPLE" T) (!.W !.T)>>$
    WINNER
    <APPLYTYPE WINNER>$
    #FUNCTION ((W "TUPLE" T (!.W !.T))
    <#WINNER (A B C) <+ 1 2> q>$
    (A B C 3 q)

The following sequence makes Muddle look just like Lisp. (This example
is understandable only if you know Lisp (Moon, 1974); it is included
only because it is so beautiful.)

    <EVALTYPE LIST FORM>$
    LIST
    <EVALTYPE ATOM ,LVAL>$
    ATOM

So now:

    (+ 1 2)$
    3
    (SET 'A 5)$
    5
    A$
    5

To complete the job, of course, we would have to do some `SETG`'s:
`car` is `1`, `cdr` is `,REST`, and `lambda` is `,FUNCTION`. If you
really do this example, you should "undo" it before continuing:

    <EVALTYPE 'ATOM ,EVAL>$
    ATOM
    <EVALTYPE LIST ,EVAL>$
    LIST

Chapter 7. Structured Objects
=============================

This chapter discusses structured objects in general and the five
basic structured `PRIMTYPE`s. \[We defer detailed discussion of the
structured `PRIMTYPE`s `TUPLE` (section 9.2) and `STORAGE` (section
22.2.2).\]

7.1. Manipulation
-----------------

The following `SUBR`s operate uniformly on all structured objects and
generate an error if not applied to a structured object. Hereafter,
*structured* represents a structured object.

### 7.1.1. LENGTH \[1\]

    <LENGTH structured>

evaluates to the number of elements in *structured*.

### 7.1.2. NTH \[1\]

    <NTH structured fix>

evaluates to the *fix*'th element of *structured*. An error occurs if
*fix* is less than 1 or greater than `<LENGTH structured>`. *fix* is
optional, 1 by default.

### 7.1.3. REST \[1\]

    <REST structured fix>

evaluates to *structured* without its first *fix* elements. *fix* is
optional, 1 by default.

Obscure but important side effect: `REST` actually returns
*structured* "`CHTYPE`d" (but not through application of `CHTYPE`) to
its `PRIMTYPE`. For example, `REST` of a `FORM` is a `LIST`. `REST`
with an explicit second argument of `0` has no effect except for this
`TYPE` change.

### 7.1.4. PUT \[1\]

    <PUT structured fix anything-legal>

first makes *anything-legal* the *fix*'th element of *structured*,
then evaluates to *structured*. *anything-legal* is anything which can
legally be an element of *structured*; often, this is synonymous with
"any Muddle object", but see below. An error occurs if *fix* is less
than 1 or greater than `<LENGTH structured>`. (`PUT` is actually more
general than this -- chapter 13.)

### 7.1.5. GET

    <GET structured fix>

evaluates the same as `<NTH structured fix>`. It is more general than
`NTH`, however (chapter 13), and is included here only for symmetry
with `PUT`.

### 7.1.6. APPLYing a FIX \[1\]

`EVAL` understands the application of an object of `TYPE` `FIX` as a
"shorthand" call to `NTH` or `PUT`, depending on whether it is given
one or two arguments, respectively \[unless the `APPLYTYPE` of `FIX`
is changed\]. That is, `EVAL` considers the following two to be
identical:

    <fix structured>
    <NTH structured fix>

and these:

    <fix structured object>
    <PUT structured fix object>

\[However, the compiler (Lebling, 1979) cannot generate efficient code
from the longer forms unless it is sure that *fix* is a `FIX` (section
9.10). The two constructs are not identical even to `EVAL`, if the
order of evaluation is significant: for example, these two:

    <NTH .X <LENGTH <SET X .Y>>>        <<LENGTH <SET X .Y>> .X>

are **not** identical.\]

### 7.1.7. SUBSTRUC

`SUBSTRUC` ("substructure") facilitates the construction of structures
that are composed of sub-parts of existing structures. A special case
of this would be a "substring" function.

    <SUBSTRUC from:structured rest:fix amount:fix to:structured>

copies the first *amount* elements of `<REST from rest>` into another
object and returns the latter. All arguments are optional except
*from*, which must be of `PRIMTYPE` `LIST`, `VECTOR`, `TUPLE` (treated
like a `VECTOR`), `STRING`, `BYTES`, or `UVECTOR`. *rest* is `0` by
default, and *amount* is all the elements by default. *to*, if given,
receives the copied elements, starting at its beginning; it must be an
object whose `TYPE` is the `PRIMTYPE` of *from* (a `VECTOR` if *from*
is a `TUPLE`). If *to* is not given, a new object is returned, of
`TYPE` `<PRIMTYPE from>` (a `VECTOR` if *from* is a `TUPLE`), which
**never** shares with *from*. The copying is done in one fell swoop,
not an element at a time. Note: due to an implementation restriction,
if *from* is of `PRIMTYPE` `LIST`, it must not share any elements with
*to*.

7.2. Representation of Basic Structures
---------------------------------------

### 7.2.1. LIST \[1\]

    ( element-1 element-2 ... element-N )

represents a `LIST` of *N* elements.

### 7.2.2. VECTOR \[1\]

    [ element-1 element-2 ... element-N ]

represents a `VECTOR` of *N* elements. \[A `TUPLE` is just like a
`VECTOR`, but it lives on the control stack.\]

### 7.2.3. UVECTOR \[1\]

    ![ element-1 element-2 ... element-N !]

represents a `UVECTOR` (uniform vector) of *N* elements. The second
`!` (exclamation-point) is optional for input. \[A `STORAGE` is an
archaic kind of `UVECTOR` that is not garbage-collected.\]

### 7.2.4. STRING \[1\]

    "characters"

represents a `STRING` of ASCII text. A `STRING` containing the
character `"` (double-quote) is represented by placing a `\`
(backslash) before the double-quote inside the `STRING`. A `\` in a
`STRING` is represented by two consecutive backslashes.

### 7.2.5. BYTES

    #n {element-1 element-2 ... element-N}

represents a string of *N* uniformly-sized bytes of size *n* bits.

### 7.2.6. TEMPLATE

    { element-1 element-2 ... element-N }

represents a `TEMPLATE` of *N* elements when output, not input -- when
input, a `#` and a `TYPE` must precede it.

7.3. Evaluation of Basic Structures
-----------------------------------

This section and the next two describe how `EVAL` treats the basic
structured `TYPE`s \[in the absence of any modifying `EVALTYPE` calls
(section 6.4.4)\].

`EVAL` of a `STRING` \[or `BYTES` or `TEMPLATE`\] is just the original
object.

`EVAL` acts exactly the same with `LIST`s, `VECTOR`s, and `UVECTOR`s:
it generates a **new** object with elements equal to `EVAL` of the
elements it is given. This is one of the simplest means of
constructing a structure. However, see section 7.7.

7.4. Examples \[1\]
-------------------

    (1 2 <+ 3 4>)$
    (1 2 7)
    <SET FOO [5 <- 3> <TYPE "ABC">]>$
    [5 -3 STRING]
    <2 .FOO>$
    -3
    <TYPE <3 .FOO>>$
    ATOM
    <SET BAR ![("meow") (.FOO)]>$
    ![("meow") ([5 -3 STRING])!]
    <LENGTH .BAR>$
    2
    <REST <1 <2 .BAR>>>$
    [-3 STRING]
    [<SUBSTRUC <1 <2 .BAR>> 0 2>]$
    [[5 -3]]
    <PUT .FOO 1 SNEAKY>          ;"Watch out for .BAR !"$
    [SNEAKY -3 STRING]
    .BAR$
    ![("meow") ([SNEAKY -3 STRING])!]
    <SET FOO <REST <1 <1 .BAR>> 2>>$
    "ow"
    .BAR$
    ![("meow") ([SNEAKY -3 STRING])!]

7.5. Generation of Basic Structures
-----------------------------------

Since `LIST`s, `VECTOR`s, `UVECTOR`s, and `STRING`s \[and `BYTES`es\]
are all generated in a fairly uniform manner, methods of generating
them will be covered together here. \[`TEMPLATE`s cannot be generated
by the interpreter itself: see Lebling (1979).\]

### 7.5.1. Direct Representation \[1\]

Since `EVAL` of a `LIST`, `VECTOR`, or `UVECTOR` is a new `LIST`,
`VECTOR`, or `UVECTOR` with elements which are `EVAL` of the original
elements, simply evaluating a representation of the object you want
will generate it. (Care must be taken when representing a `UVECTOR`
that all elements have the same `TYPE`.) This method of generation was
exclusively used in the examples of section 7.4. Note that new
`STRING`s \[and `BYTES`es\] will not be generated in this manner,
since the contents of a `STRING` are not interpreted or copied by
`EVAL`. The same is true of any other `TYPE` whose `TYPEPRIM` happens
to be `LIST`, `VECTOR`, or `UVECTOR` \[again, assuming it neither has
been `EVALTYPE`d nor has a built-in `EVALTYPE`, as do `FORM` and
`SEGMENT`\].

### 7.5.2. QUOTE \[1\]

`QUOTE` is an `FSUBR` of one argument which returns its argument
unevaluated. `READ` and `PRINT` understand the character `'`
(single-quote) as an abbreviation for a call to `QUOTE`, the way
period and comma work for `LVAL` and `GVAL`. Examples:

    <+ 1 2>$
    3
    '<+ 1 2>$
    <+ 1 2>

Any `LIST`, `VECTOR`, or `UVECTOR` in a program that is constant and
need not have its elements evaluated should be represented directly
and **inside a call to `QUOTE`.** This technique prevents the
structure from being copied each time that portion of the program is
executed. Examples hereafter will adhere to this dictum. (Note: one
should **never** modify a `QUOTE`d object. The compiler will one day
put it in read-only (pure) storage.)

### 7.5.3. LIST, VECTOR, UVECTOR, and STRING (the SUBRs) \[1\]

Each of the `SUBR`s `LIST`, `VECTOR`, `UVECTOR`, and `STRING` takes
any number of arguments and returns an object of the appropriate
`TYPE` whose elements are `EVAL` of its arguments. There are
limitations on what the arguments to `UVECTOR` and `STRING` may `EVAL`
to, due to the nature of the objects generated. See sections 7.6.5 and
7.6.6.

`LIST`, `VECTOR`, and `UVECTOR` are generally used only in special
cases, since Direct Representation usually produces exactly the same
effect (in the absence of errors), and the intention is more apparent.
\[Note: if `.L` is a `LIST`, `<LIST !.L>` makes a copy of `.L` whereas
`(!.L)` doesn't; see section 7.7.\] `STRING`, on the other hand,
produces effect very different from literal `STRING`s.

Examples:

    <LIST 1 <+ 2 3> ABC>$
    (1 5 ABC)
    (1 <+ 2 3> ABC)$
    (1 5 ABC)
    <STRING "A" <2 "QWERT"> <REST "ABC"> "hello">$
    "AWBChello"
    "A <+ 2 3> (5)"$
    "A <+ 2 3> (5)"

### 7.5.4. ILIST, IVECTOR, IUVECTOR, and ISTRING \[1\]

Each of the `SUBR`s `ILIST`, `IVECTOR`, `IUVECTOR`, and `ISTRING`
("implicit" or "iterated" whatever) creates and returns an object of
the obvious `TYPE`. The format of an application of any of them is

    < Ithing number-of-elements:fix expression:any >

where *Ithing* is one of `ILIST`, `IVECTOR`, `IUVECTOR`, or `ISTRING`.
An object of `LENGTH` *number-of-elements* is generated, whose
elements are `EVAL` of *expression*.

*expression* is optional. When it is not specified, `ILIST`,
`IVECTOR`, and `IUVECTOR` return objects filled with objects of `TYPE`
`LOSE` (`PRIMTYPE` `WORD`) as place holders, a `TYPE` which can be
passed around and have its `TYPE` checked, but otherwise is an illegal
argument. If *expression* is not specified in `ISTRING`, you get a
`STRING` made up of `^@` characters.

When *expression* is supplied as an argument, it is re-`EVAL`uated
each time a new element is generated. (Actually, `EVAL` of
*expression* is re-`EVAL`uated, since all of these are `SUBR`s.) See
the last example for how this argument may be used.

\[By the way, in a construct like `<IUVECTOR 9 '.X>`, even if the
`LVAL` of `X` evaluates to itself, so that the `'` could be omitted
without changing the result, the compiler is much happier with the `'`
in place.\]

`IUVECTOR` and `ISTRING` again have limitations on what *expression*
may `EVAL` to; again, see sections 7.6.5 and 7.6.6.

Examples:

    <ILIST 5 6>$
    (6 6 6 6 6)
    <IVECTOR 2>$
    [#LOSE *000000000000* #LOSE *000000000000*]

    <SET A 0>$
    0
    <IUVECTOR 9 '<SET A <+ .A 1>>>$
    ![1 2 3 4 5 6 7 8 9!]

### 7.5.5. FORM and IFORM

Sometimes the need arises to create a `FORM` without `EVAL`ing it or
making it the body of a `FUNCTION`. In such cases the `SUBR`s `FORM`
and `IFORM` ("implicit form") can be used (or `QUOTE` can be used).
They are entirely analogous to `LIST` and `ILIST`. Example:

    <DEFINE INC-FORM (A)
            <FORM SET .A <FORM + 1 <FORM LVAL .A>>>>$
    INC-FORM
    <INC-FORM FOO>$
    <SET FOO <+ 1 .FOO>>

7.6. Unique Properties of Primitive TYPEs
-----------------------------------------

### 7.6.1. LIST (the PRIMTYPE) \[1\]

An object of `PRIMTYPE` `LIST` may be considered as a "pointer chain"
(appendix 1). Any Muddle object may be an element of a `PRIMTYPE`
`LIST`. It is easy to add and remove elements of a `PRIMTYPE` `LIST`,
but the higher N is, the longer it takes to refer to the Nth element.
The `SUBR`s which work only on objects of `PRIMTYPE` `LIST` are these:

#### 7.6.1.1. PUTREST \[1\]

    <PUTREST head:primtype-list tail:primtype-list>

changes *head* so that `<REST head>` is *tail* (actually
`<CHTYPE tail LIST>`), then evaluates to *head*. Note that this
actually changes *head*; it also changes anything having *head* as an
element or a value. For example:

    <SET BOW [<SET ARF (B W)>]>$
    [(B W)]
    <PUTREST .ARF '(3 4)>$
    (B 3 4)
    .BOW$
    [(B 3 4)]

`PUTREST` is probably most often used to splice lists together. For
example, given that `.L` is of `PRIMTYPE` `LIST`, to leave the first
*m* elements of it intact and take out the next *n* elements of it,
`<PUTREST <REST .L <- m 1>> <REST .L <+ m n>>>`. Specifically,

    <SET NUMS (1 2 3 4 5 6 7 8 9)>$
    (1 2 3 4 5 6 7 8 9)
    <PUTREST <REST .NUMS 3> <REST .NUMS 7>>$
    (4 8 9)
    .NUMS$
    (1 2 3 4 8 9)

#### 7.6.1.2. CONS

    <CONS new list>

("construct") adds *new* to the front of *list*, without copying
*list*, and returns the resulting `LIST`. References to *list* are not
affected.

\[Evaluating `<CONS .E .LIST>` is equivalent to evaluating
`(.E !.LIST)` (section 7.7) but is less preferable to the compiler
(Lebling, 1979).\]

### 7.6.2. "Array" PRIMTYPEs \[1\]

`VECTORS`, `UVECTOR`s, and `STRING`s \[and `BYTES`es and `TEMPLATE`s\]
may be considered as "arrays" (appendix 1). It is easy to refer to the
Nth element irrespective of how large N is, and it is relatively
difficult to add and delete elements. The following `SUBR`s can be
used only with an object of `PRIMTYPE` `VECTOR`, `UVECTOR`, or
`STRING` \[or `BYTES` or `TEMPLATE`\]. (In this section *array*
represents an object of such a `PRIMTYPE`.)

#### 7.6.2.1. BACK \[1\]

    <BACK array fix>

This is the opposite of `REST`. It evaluates to *array*, with *fix*
elements put back onto its front end, and changed to its `PRIMTYPE`.
*fix* is optional, 1 by default. If *fix* is greater than the number
of elements which have been `REST`ed off, an error occurs. Example:

    <SET ZOP <REST '![1 2 3 4] 3>>$
    ![4!]
    <BACK .ZOP 2>$
    ![2 3 4!]
    <SET S <REST "Right is might." 15>>$
    ""
    <BACK .S 6>$
    "might."

#### 7.6.2.2. TOP \[1\]

    <TOP array>

"`BACK`s up all the way" -- that is, evaluates to *array*, with all
the elements which have been `REST`ed off put back onto it, and
changed to its `PRIMTYPE`. Example:

    <TOP .ZOP>$
    ![1 2 3 4!]

### 7.6.3. "Vector" PRIMTYPEs

#### 7.6.3.1. GROW

    <GROW vu end:fix beg:fix>

adds/removes elements to/from either or both ends of *vu*, and returns
the entire (`TOP`ped) resultant object. *vu* can be of `PRIMTYPE`
`VECTOR` or `UVECTOR`. *end* specifies a lower bound for the number of
elements to be added to the **end** of *vu*; *beg* specifies the same
for the **beginning**. A negative *fix* specifies removal of elements.

The number of elements added to each respective end is *end* or *beg*
**increased** to an integral multiple of *X*, where *X* is 32 for
`PRIMTYPE` `VECTOR` and 64 for `PRIMTYPE` `UVECTOR` (`1` produces 32
or 64; `-1` produces 0). The elements added will be `LOSE`s if *vu* is
of `PRIMTYPE` `VECTOR`, and "empty" whatever-they-are's if *vu* is of
`PRIMTYPE` `UVECTOR`. An "empty" object of `PRIMTYPE` `WORD` contains
zero. An "empty" object of any other `PRIMTYPE` has zero in its "value
word" (appendix 1) and is not safe to play with: it should be replaced
via `PUT`.

Note that, if elements are added to the beginning of *vu*,
previously-existing references to *vu* will have to use `TOP` or
`BACK` to get at the added elements.

**Caution:** `GROW` is a **very** expensive operation; it **requires**
a garbage collection (section 22.4) **every** time it is used. It
should be reserved for **very special** circumstances, such as where
the pattern of shared elements is terribly important.

Example:

    <SET A '![1]>$
    ![1!]
    <GROW .A 0 1>$
    ![0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1!]
    .A$
    ![1!]

#### 7.6.3.2. SORT

This `SUBR` will sort `PRIMTYPE`s `VECTOR`, `UVECTOR` and `TUPLE`
(section 9.2). It works most efficiently if the sort keys are of
`PRIMTYPE` `WORD`, `ATOM` or `STRING`. However, the keys may be of any
`TYPE`, and `SORT` will still work. `SORT` acts on fixed-length
records which consist of one or more contiguous elements in the
structure being sorted. One element in the record is declared to be
the sort key. Also, any number of additional structures can be
rearranged based on how the main structure is sorted.

    <SORT pred s1 l1 off s2 l2 s3 l3 sN lN>

where:

*pred* is either (see chapter 8 for information about predicates):

1.  `TYPE` `FALSE`, in which case the `TYPE`s of all the sort keys
    must be the same; they must be of `PRIMTYPE` `WORD`, `STRING` or
    `ATOM`; and a radix-exchange sort is used; or
2.  something applicable to two sort keys which returns `TYPE` `FALSE`
    if the first is not bigger than the second, in which case a shell
    sort is used. For example, `,G?` sorts numbers in ascending order,
    `,L?` in descending order. Note: if your *pred* is buggy, the
    `SORT` may never terminate.

*s1* ... *sN* are the (`PRIMTYPE`) `VECTOR`s, `UVECTOR`s or `TUPLE`s
being sorted, and *s1* contains the sort keys;

*l1* ... *lN* are the corresponding lengths of sort records (optional,
one by default); and

*off* is the offset from start of record to sort key (optional, zero
by default).

`SORT` returns the sorted *s1* as a value.

Note: the `SUBR` `SORT` calls the `RSUBR` (chapter 19) `SORTX`; if the
`RSUBR` must be loaded, you may see some output from the loader on
your terminal.

Examples:

    <SORT <> <SET A <IUVECTOR 500 '<RANDOM>>>>$
    ![...!]

sorts a `UVECTOR` of random integers.

    <SET V [1 MONEY 2 SHOW 3 READY 4 GO]>$
    [...]
    <SORT <> .V 2 1>$
    [4 GO 1 MONEY 3 READY 2 SHOW]

    <SORT ,L? .V 2>$
    [4 GO 3 READY 2 SHOW 1 MONEY]
    .V$
    [4 GO 3 READY 2 SHOW 1 MONEY]

    <SORT <> ![2 1 4 3 6 5 8 7] 1 0 .V>$
    ![1 2 3 4 5 6 7 8!]
    .V$
    [GO 4 READY 3 SHOW 2 MONEY 1]

The first sort was based on the `ATOM`s' `PNAME`s, considering records
to be two elements. The second one sorted based on the `FIX`es. The
third interchanged pairs of elements of each of its structured
arguments.

### 7.6.4. VECTOR (the PRIMTYPE) \[1\]

Any Muddle object may be an element of a `PRIMTYPE` `VECTOR`. A
`PRIMTYPE` `VECTOR` takes two words of storage more than an equivalent
`PRIMTYPE` `LIST`, but takes it all in a contiguous chunk, whereas a
`PRIMTYPE` `LIST` may be physically spread out in storage (appendix
1). There are no `SUBR`s or `FSUBR`s which operate only on `PRIMTYPE`
`VECTOR`.

### 7.6.5. UVECTOR (the PRIMTYPE) \[1\]

The difference between `PRIMTYPE`s `UVECTOR` and `VECTOR` is that
every element of a `PRIMTYPE` `UVECTOR` must be of the same `TYPE`. A
`PRIMTYPE` `UVECTOR` takes approximately half the storage of a
`PRIMTYPE` `VECTOR` or `PRIMTYPE` `LIST` and, like a `PRIMTYPE`
`VECTOR`, takes it in a contiguous chunk (appendix 1).

\[Note: due to an implementation restriction (appendix 1), `PRIMTYPE`
`STRING`s, `BYTES`es, `LOCD`s (chapter 12), and objects on the control
stack (chapter 22) may **not** be elements of `PRIMTYPE` `UVECTOR`s.\]

The "same `TYPE`" restriction causes an equivalent restriction to
apply to `EVAL` of the arguments to either of the `SUBR`s `UVECTOR` or
`IUVECTOR`. Note that attempting to say

    ![1 .A!]

will cause `READ` to produce an error, since you're attempting to put
a `FORM` and a `FIX` into the same `UVECTOR`. On the other hand,

    <UVECTOR 1 .A>

is legal, and will `EVAL` to the appropriate `UVECTOR` without error
if `.A` `EVAL`s to a `TYPE` `FIX`.

The following `SUBR`s work on `PRIMTYPE` `UVECTOR`s along.

#### 7.6.5.1. UTYPE \[1\]

    <UTYPE primtype-uvector>

("uniform type") evaluates to the `TYPE` of every element in its
argument. Example:

    <UTYPE '![A B C]>$
    ATOM

#### 7.6.5.2. CHUTYPE \[1\]

    <CHUTYPE uv:primtype-uvector type>

("change uniform type") changes the `UTYPE` of *uv* to *type*,
simultaneously changing the `TYPE` of all elements of *uv*, and
returns the new, changed, *uv*. This works only when the `PRIMTYPE` of
the elements of *uv* can remain the same through the whole procedure.
(Exception: a *uv* of `UTYPE` `LOSE` can be `CHUTYPE`d to any *type*
(legal in a `UVECTOR` of course); the resulting elements are "empty",
as for `GROW`.)

`CHUTYPE` actually changes *uv*; hence **all** references to that
object will reflect the change. This is quite different from `CHTYPE`.

Examples:

    <SET LOST <IUVECTOR 2>>$
    ![#LOSE *000000000000* #LOSE *000000000000*!]
    <UTYPE .LOST>$
    LOSE
    <CHUTYPE .LOST FORM>$
    ![<> <>!]
    .LOST$
    ![<> <>!]
    <CHUTYPE .LOST LIST>$
    ![() ()!]

### 7.6.6. STRING (the PRIMTYPE) and CHARACTER \[1\]

The best mental image of a `PRIMTYPE` `STRING` is a `PRIMTYPE`
`UVECTOR` of `CHARACTER`s -- where `CHARACTER` is the Muddle `TYPE`
for a single ASCII character. The representation of a `CHARACTER`, by
the way, is

    !\any-ASCII-character

That is, the characters `!\` (exclamation-point backslash) preceding a
single ASCII character represent the corresponding object of `TYPE`
`CHARACTER` (`PRIMTYPE` `WORD`). (The characters `!"`
(exclamation-point double-quote) preceding a character are also
acceptable for inputting a `CHARACTER`, for historical reasons.)

The `SUBR` `ISTRING` will produce an error if you give it an argument
that produces a non-`CHARACTER`. `STRING` can take either `CHARACTER`s
or `STRING`s.

There are no `SUBR`s which uniquely manipulate `PRIMTYPE` `STRING`s,
but some are particularly useful in connection with them:

#### 7.6.6.1. ASCII \[1\]

    <ASCII fix-or-character>

If its argument is of `TYPE` `FIX`, `ASCII` evaluates to the
`CHARACTER` with the 7-bit ASCII code of its argument. Example:
`<ASCII 65>` evaluates to `!\A`.

If its argument is of `TYPE` `CHARACTER`, `ASCII` evaluates to the
`FIX`ed-point number which is its argument's 7-bit ASCII code.
Example: `<ASCII !\Z>` evaluates to `90`.

\[Actually, a `FIX` can be `CHTYPE`d to a `CHARACTER` (or vice versa)
directly, but `ASCII` checks in the former case that the `FIX` is
within the permissible range.\]

#### 7.6.6.2. PARSE \[1\]

    <PARSE string radix:fix>

`PARSE` applies to its argument `READ`'s algorithm for converting
ASCII representations to Muddle objects and returns the **first**
object created. The remainder of *string*, after the first object
represented, is ignored. *radix* (optional, ten by default) is used
for converting any `FIX`es that occur. \[See also sections 15.7.2 and
17.1.3 for additional arguments.\]

#### 7.6.6.3. LPARSE \[1\]

`LPARSE` ("list parse") is exactly like `PARSE` (above), except that
it parses the **entire** *string* and returns a `LIST` of **all**
objects created. If given an empty `STRING` or one containing only
separators, `LPARSE` returns an empty `LIST`, whereas `PARSE` gets an
error.

#### 7.6.6.4. UNPARSE \[1\]

    <UNPARSE any radix:fix>

`UNPARSE` applies to its argument `PRINT`'s algorithm for converting
Muddle objects to ASCII representations and returns a `STRING` which
contains the `CHARACTER`s `PRINT` would have typed out. \[However,
this `STRING` will **not** contain any of the gratuitous
carriage-returns `PRINT` adds to accommodate a `CHANNEL`'s finite
line-width (section 11.2.8).\] *radix* (optional, ten by default) is
used for converting any `FIX`es that occur.

### 7.6.7. BYTES

A (`PRIMTYPE`) `BYTES` is a string of uniformly-sized bytes. The bytes
can be any size between 1 and 36 bits inclusive. A `BYTES` is similar
in some ways to a `UVECTOR` of `FIX`es and in some ways to a `STRING`
of non-seven-bit bytes. The elements of a `BYTES` are always of `TYPE`
`FIX`.

The `SUBR`s `BYTES` and `IBYTES` are similar to `STRING` and
`ISTRING`, respectively, except that each of the former takes a first
argument giving the size of the bytes in the generated `BYTES`.
`BYTES` takes one required argument which is a `FIX` specifying a byte
size and any number of `PRIMTYPE` `WORD`s. It returns an object of
`TYPE` `BYTES` with that byte size containing the objects as elements.
These objects will be `ANDB`ed with the appropriate mask of 1-bits to
fit in the byte size. `IBYTES` takes two required `FIX`es and one
optional argument. It uses the first `FIX` to specify the byte size
and the second to specify the number of elements. The third argument
is repeatedly evaluated to generate `FIX`es that become elements of
the `BYTES` (if it is omitted, bytes filled with zeros are generated).
The analog to `UTYPE` is `BYTE-SIZE`. Examples:

    <BYTES 3 <+ 2 2> 9 -1>$
    #3 {4 1 7}
    <SET A 0>$
    0
    <IBYTES 3 9 '<SET A <+ .A 1>>>$
    #3 {1 2 3 4 5 6 7 0 1}
    <IBYTES 3 4>$
    #3 {0 0 0 0}
    <BYTE-SIZE <BYTES 1>>$
    1

### 7.6.8. TEMPLATE

A `TEMPLATE` is similar to a PL/I "structure" of one level: the
elements are packed together and reduced in size to save storage
space, while an auxiliary internal data structure describes the
packing format and the elements' real `TYPE`s (appendix 1). The
interpreter is not able to create objects of `PRIMTYPE` `TEMPLATE`
(Lebling, 1979); however, it can apply the standard built-in
Subroutines to them, with the same effects as with other "arrays".

7.7. SEGMENTs \[1\]
-------------------

Objects of `TYPE` `SEGMENT` (whose `TYPEPRIM` is `LIST`) look very
much like `FORM`s. `SEGMENT`s, however, undergo a non-standard
evaluation designed to ease the construction of structured objects
from elements of other structured objects.

### 7.7.1. Representation \[1\]

The representation of an object of `TYPE` `SEGMENT` is the following:

    !< func arg-1 arg-2 ... arg-N !>

where the second `!` (exclamation-point) is optional, and *fun* and
*arg-1* through *arg-N* are any legal constituents of a `FORM` (that
is, anything). The pointed brackets can be implicit, as in the period
and comma notation for `LVAL` and `GVAL`.

All of the following are `SEGMENT`s:

    !<3 .FOO>    !.FOO    !,FOO

### 7.7.2. Evaluation \[1\]

A `SEGMENT` is evaluated in exactly the same manner as a `FORM`, with
the following three exceptions:

1.  It had better be done inside an `EVAL` of a structure; otherwise
    an error occurs. (See special case of `FORM`s in section 7.7.5.)
2.  It had better `EVAL` to a structured object; otherwise an error
    occurs.
3.  What actually gets inserted into the structure being built are the
    elements of the structure returned by the `FORM`-like evaluation.

### 7.7.3 Examples \[1\]

    <SET ZOP '![2 3 4]>$
    ![2 3 4!]
    <SET ARF (B 3 4)>$
    (B 3 4)
    (.ARF !.ZOP)$
    ((B 3 4) 2 3 4)
    ![!.ZOP !<REST .ARF>!]$
    ![2 3 4 3 4!]

    <SET S "STRUNG.">$
    "STRUNG."
    (!.S)$
    (!\S !\T !\R !\U !\N !\G !\.)

    <SET NIL ()>$
    ()
    [!.NIL]$
    []

### 7.7.4. Note on Efficiency \[1\]

Most of the cases in which is is possible to use `SEGMENT`s require
`EVAL` to generate an entire new object. Naturally, this uses up both
storage and time. However, there is one case which it is possible to
handle without copying, and `EVAL` uses it. When the structure being
built is a `PRIMTYPE` `LIST`, and the segment value of a `PRIMTYPE`
`LIST` is the last (rightmost) element being concatenated, that last
`PRIMTYPE` `LIST` is not copied. This case is similar to `CONS` and is
the principle reason why `PRIMTYPE` `LIST`s have their structures more
easily varied than `PRIMTYPE` `VECTOR` or `UVECTOR`.

Examples:

    .ARF$
    (B 3 4)

This does not copy ARF:

    (1 2 !.ARF)$
    (1 2 B 3 4)

These do:

    (1 !.ARF 2)              ;"not last element"$
    (1 B 3 4 2)
    [1 2 !.ARF]              ;"not PRIMTYPE LIST"$
    [1 2 B 3 4]
    (1 2 !.ARF !<REST '(1)>) ;"still not last element"$
    (1 2 B 3 4)

Note the following, which occurs because copying does **not** take
place:

    <SET DOG (A !.ARF)>$
    (A B 3 4)
    <PUT .ARF 1 "BOWOW">$
    ("BOWOW" 3 4)
    .DOG$
    (A "BOWOW" 3 4)
    <PUT .DOG 3 "WOOF">$
    (A "BOWOW" "WOOF" 4)
    .ARF$
    ("BOWOW" "WOOF" 4)

Since `ARF` was not copied, it was literally part of `DOG`. Hence,
when an element of `ARF` was changed, `DOG` was changed. Similarly,
when an element of `DOG` which `ARF` shared was changed, `ARF` was
changed too.

### 7.7.5. SEGMENTs in FORMs \[1\]

When a `SEGMENT` appears as an element of a `FORM`, the effect is
approximately the same as if the elements of the `EVAL` of the
`SEGMENT` were in the `FORM`. Example:

    <SET A '![1 2 3 4]>$
    ![1 2 3 4!]
    <+ !.A 5>$
    15

Note: the elements of the structure segment-evaluated in a `FORM` are
**not** re-evaluated if the thing being applied is a `SUBR`. Thus if
`.A` were `(1 2 <+ 3 4> 5)`, the above example would produce an error:
you can't add up `FORM`s.

You could perform the same summation of `5` and the elements of `A` by
using

    <EVAL <CHTYPE (+ !.A 5) FORM>>

(Note that `EVAL` must be explicitly called as a `SUBR`; if it were
not so called, you would just get the `FORM` `<+ 1 2 3 4 5>` -- not
its "value".) However, the latter is more expensive both in time and
in storage: when you use the `SEGMENT` directly in the `FORM`, a new
`FORM` is, in fact, **not** generated as it is in the latter case.
(The elements are put on "the control stack" with the other
arguments.)

7.8. Self-referencing Structures
--------------------------------

It is possible for a structured object to "contain" itself, either as
a subset or as an element, as an element of a structured element, etc.
Such an object cannot be `PRINT`ed, because recursion begins and never
terminates. Warning: if you try the examples in this section with a
live Muddle, be sure you know how to use `^S` (section 1.2) to save
`PRINT` from endless agony. (Certain constructs with `ATOM`s can give
`PRINT` similar trouble: see chapters 12 and 15.)

### 7.8.1. Self-subset

    <PUTREST head:primtype-list tail:primtype-list>

If *head* is a subset of *tail*, that is, if `<REST tail fix>` is the
same object as `<REST head 0>` for some *fix*, then both *head* and
*tail* will be "circular" (and this self-referencing) after the
`PUTREST`. Example:

    <SET WALTZ (1 2 3)>$
    (1 2 3)
    <PUTREST <REST .WALTZ 2> .WALTZ>$
    (3 1 2 3 1 2 3 1 2 3 1 2 3 ...

### 7.8.2. Self-element

    <PUT s1:structured fix s2:structured>

If *s1* is the same object as *s2*, then it will "contain" itself (and
thus be self-referencing) after the `PUT`. Examples:

    <SET S <LIST 1 2 3>>        ;"or VECTOR"$
    (1 2 3)
    <PUT .S 3 .S>$
    (1 2 (1 2 (1 2 (1 2 ...
    <SET U ![![]]>$
    ![![!]!]
    <PUT .U 1 .U>$
    ![![![![![![...

Test your reaction time or your terminal's bracket-maker. Amaze your
friends.

Chapter 8. Truth
================

8.1. Truth Values \[1\]
-----------------------

Muddle represents "false" with an object of a particular `TYPE`:
`TYPE` `FALSE` (unsurprisingly). `TYPE` `FALSE` is structured: its
`PRIMTYPE` is `LIST`. Thus, you can give reasons or excuses by making
them elements of a `FALSE`. (Again, `EVAL`ing a `FALSE` neither copies
it nor `EVAL`s its elements, so it is not necessary to `QUOTE` a
`FALSE` appearing in a program.) Objects of `TYPE` `FALSE` are
represented in "\# notation":

    #FALSE list-of-its-elements

The empty `FORM` evaluates to the empty `FALSE`:

    <>$
    #FALSE ()

Anything which is not `FALSE`, is, reasonably enough, true. In this
document the "data type" *false-or-any* in metasyntactic variables
means that the only significant attribute of the object in that
context is whether its `TYPE` is `FALSE` or not.

8.2. Predicates \[1\]
---------------------

There are numerous Muddle F/SUBRs which can return a `FALSE` or a
true. See appendix 2 to find them all. Most return either `#FALSE ()`
or the `ATOM` with `PNAME` `T`. (The latter is for historical reasons,
namely Lisp (Moon, 1974).) Some predicates which are meaningful now
are described next.

### 8.2.1. Arithmetic \[1\]

    <0? fix-or-float>

evaluates to `T` only if its argument is identically equal to `0` or
`0.0`.

    <1? fix-or-float>

evaluates to `T` only if its argument is identically equal to `1` or
`1.0`.

    <G? n:fix-or-float m:fix-or-float>

evaluates to `T` only if *n* is algebraically greater than *m*. `L=?`
is the Boolean complement of `G?`; that is, it is `T` only if *n* is
not algebraically greater than *m*.

    <L? n:fix-or-float m:fix-or-float>

evaluates to `T` only if *n* is algebraically less than *m*. `G=?` is
the Boolean complement of `L?`.

### 8.2.2. Equality and Membership \[1\]

    <==? e1:any e2:any>

evaluates to `T` only if *e1* is the **same object** as *e2* (appendix
1). Two objects that look the same when `PRINT`ed may not be `==?`.
Two `FIX`es of the same "value" are "the same object"; so are two
`FLOAT`s of **exactly** the same "value". Empty objects of `PRIMTYPE`
`LIST` (and no other structured `PRIMTYPE`) are `==?` if their `TYPE`s
are the same. Example:

    <==? <SET X "RANDOM STRING"> <TOP <REST .X 6>>>$
    T
    <==? .X "RANDOM STRING">$
    #FALSE ()

`N==?` is the Boolean complement of `==?`.

    <=? e1:any e2:any>

evaluates to `T` if *e1* and *e2* have the same `TYPE` and are
structurally equal -- that is, they "look the same", their printed
representations are the same. `=?` is much slower than `==?`. `=?`
should be used only when its characteristics are necessary: they are
not in any comparisons of unstructured objects. `==?` and `=?` always
return the same value for `FIX`es, `FLOAT`s, `ATOM`s, etc.
(Mnemonically, `==?` tests for "more equality" than `=?`; in fact, it
tests for actual physical identity.)

Example, illustrating non-copying of a `SEGMENT` in Direct
Representation of a `LIST`:

    <SET A '(1 2 3)>$
    (1 2 3)
    <==? .A (!.A)>$
    T
    <==? .A <SET B <LIST !.A>>>$
    #FALSE ()
    <=? .A .B>$
    T

`N=?` is the Boolean complement of `=?`.

    <MEMBER object:any structured>

runs down *structured* from first to last element, comparing each
element of *structured* with *object*. If it finds an element of
*structured* which is `=?` to *object*, it returns
`<REST structured i>` (which is of `TYPE` `<PRIMTYPE structured>`),
where the (*i*+1)th element of *structured* is `=?` to *object*. That
is, the first element of what it returns is the **first** element of
*structured* that is `=?` to *object*.

If no element of *structured* is `=?` to *object*, `MEMBER` returns
`#FALSE ()`.

The search is more efficient if *structured* is of `PRIMTYPE` `VECTOR`
(or `UVECTOR`, if possible) than if it is of `PRIMTYPE` `LIST`. As
usual, if *structured* is constant, it should be `QUOTE`d.

If *object* and *structured* are of `PRIMTYPE` `STRING` \[or
`BYTES`\], `MEMBER` does a substring search. Example:

    <MEMBER "PART" "SUM OF PARTS">$
    "PARTS"

`<MEMQ object:any structured>` ("member quick") is exactly the same as
`MEMBER`, except that the comparison test is `==?`.

    <STRCOMP s1 s2>

("string comparison") can be given either two `STRING`s or two `ATOM`s
as arguments. In the latter case the `PNAME`s are used. It actually
isn't a predicate, since it can return three possible values: `0` if
*s1* is `=?` to *s2*; `1` if *s1* sorts alphabetically after *s2*; and
`-1` if *s1* sorts alphabetically before *s2*. "Alphabetically" means,
in this case, according to the numeric order of ASCII, with the
standard alphabetizing rules.

\[A predicate suitable for an ascending `SORT` (which see) is
`<G? <STRCOMP .ARG1 .ARG2> 0>`.\]

### 8.2.3. Boolean Operators \[1\]

    <NOT e:false-or-any>

evaluates to `T` only if *e* evaluates to a `FALSE`, and to
`#FALSE ()` otherwise.

    <AND e1 e2 ... eN>

`AND` is an `FSUBR`. It evaluates its arguments from first to last as
they appear in the `FORM`. As soon as one of them evaluates to a
`FALSE`, it returns that `FALSE`, ignoring any remaining arguments. If
none of them evaluate to `FALSE`, it returns `EVAL` of its last
argument. `<AND>` returns `T`. `AND?` is the `SUBR` equivalent to
`AND`, that is, all its arguments are evaluated before any of them is
tested.

    <OR e1 e2 ... eN>

`OR` is an `FSUBR`. It evaluates its arguments from first to last as
they appear in the `FORM`. As soon as one of them evaluates to a
non-`FALSE`, it returns that non-`FALSE` value, ignoring any remaining
arguments. If this never occurs, it returns the last `FALSE` it saw.
`<OR>` returns `#FALSE ()`. `OR?` is the `SUBR` equivalent to `OR`.

### 8.2.4. Object Properties \[1\]

    <TYPE? any type-1 ... type-N>

evaluates to *type-i* only if `<==? type-i <TYPE any>>` is true. It is
faster and gives more information than `OR`ing tests for each `TYPE`.
If the test fails for all *type-i*'s, `TYPE?` returns `#FALSE ()`.

    <APPLICABLE? e>

evaluates to `T` only if *e* is of a `TYPE` that can legally be
applied to arguments in a `FORM`, that is, be (`EVAL` of) the first
element of a `FORM` being evaluated (appendix 3).

    <MONAD? e>

evaluates to `#FALSE ()` only if `NTH` and `REST` (with non-zero
second argument) can be performed on its argument without error. An
unstructured or empty structured object will cause `MONAD?` to return
`T`.

    <STRUCTURED? e>

evaluates to `T` only if *e* is a structured object. It is **not** the
inverse of `MONAD?`, since each returns `T` if its argument is an
empty structure.

    <EMPTY? structured>

evaluates to `T` only if its argument, which must be a structured
object, has no elements.

    <LENGTH? structured fix>

evaluates to `<LENGTH structured>` only if that is less than or equal
to *fix*; otherwise, it evaluates to `#FALSE ()`. Mnemonically, you
can think of the first two letters of `LENGTH?` as signifying the
"less than or equal to" sense of the test.

This `SUBR` was invented to use on lists, because Muddle can determine
their lengths only by stepping along the list, counting the elements.
If a program needs to know only how the length compares with a given
number, `LENGTH?` will tell without necessarily stepping all the way
to the end of the list, in contrast to `LENGTH`.

\[If *structured* is a circular `PRIMTYPE` `LIST`, `LENGTH?` will
return a value, whereas `LENGTH` will execute forever. To see if you
can do `<REST structured <+ 1 fix>>` without error, do the test
`<NOT <LENGTH? structured fix>>`.\]

8.3. COND \[1\]
---------------

The Muddle Subroutine which is most used for varying evaluation
depending on a truth value is the `FSUBR` `COND` ("conditional"). A
call to `COND` has this format:

    <COND clause-1:list ... clause-N:list>

where *N* is at least one.

`COND` always returns the result of the **last** evaluation it
performs. The following rules determine the order of evaluations
performed.

1.  Evaluate the first element of each clause (from first to last)
    until either a non-`FALSE` object results or the clauses are
    exhausted.
2.  If a non-`FALSE` object is found in (1), immediately evaluate the
    remaining elements (if any) of that clause and ignore any
    remaining clauses.

In other words, `COND` goes walking down its clauses, `EVAL`ing the
first element of each clause, looking for a non-`FALSE` result. As
soon as it finds a non-`FALSE`, it forgets about all the other clauses
and evaluates, in order, the other elements of the current clause and
returns the last thing it evaluates. If it can't find a non-`FALSE`,
it returns the last `FALSE` it saw.

### 8.3.1. Examples

    <SET F '(1)>$
    (1)
    <COND (<EMPTY? .F> EMP) (<1? <LENGTH .F>> ONE)>$
    ONE
    <SET F ()>$
    ()
    <COND (<EMPTY? .F> EMP) (<1? <LENGTH .F>> ONE)>$
    EMP
    <SET F '(1 2 3)>$
    (1 2 3)
    <COND (<EMPTY? .F> EMP) (<1? <LENGTH .F>> ONE)>$
    #FALSE ()
    <COND (<LENGTH? .F 2> SMALL) (BIG)>$
    BIG

    <DEFINE FACT (N)        ;"the standard recursive factorial"
            <COND (<0? .N> 1)
                  (ELSE <* .N <FACT <- .N 1>>>)>>$
    FACT
    <FACT 5>$
    120

8.4. Shortcuts with Conditionals
--------------------------------

### 8.4.1. AND and OR as Short CONDs

Since `AND` and `OR` are `FSUBR`s, they can be used as miniature
`COND`s. A construct of the form

    <AND pre-conditions action(s)>

or

    <OR pre-exclusions action(s)>

will allow *action(s)* to be evaluated only if all the
*pre-conditions* are true or only if all the *pre-exclusions* are
false, respectively. By nesting and using both `AND` and `OR`, fairly
powerful constructs can be made. Of course, if *action(s)* are more
than one thing, you must be careful that none but the last returns
false or true, respectively. Watch out especially for `TERPRI`
(chapter 11). Examples:

    <AND <ASSIGNED? FLAG> .FLAG <FCN .ARG>>

applies `FCN` only if someone else has `SET` `FLAG` to true.
(`ASSIGNED?` is true if its argument `ATOM` has an `LVAL`.) No error
can occur in the testing of `FLAG` because of the order of evaluation.

    <AND <SET C <OPEN "READ" "A FILE">> <LOAD .C> <CLOSE .C>>

effectively `FLOAD`s the file (chapter 11) without the possibility of
getting an error if the file cannot be opened.

### 8.4.2. Embedded Unconditionals

One of the disadvantages of `COND` is that there is no straightforward
way to do things unconditionally in between tests. One way around this
problem is to insert a dummy clause that never succeeds, because its
only `LIST` element is an `AND` that returns a `FALSE` for the test.
Example:

    <COND   (<0? .N> <F0 .N>)
            (<1? .N> <F1 .N>)
            (<AND <SET N <* 2 <FIX </ .N 2>>>>
                            ;"Round .N down to even number."
                  <>>)
            (<LENGTH? .VEC .N> '[])
            (T <REST .VEC <+ 1 .N>>)>

A variation is to make the last `AND` argument into the test for the
`COND` clause. (That is, the third and fourth clauses in the above
example can be combined.) Of course, you must be careful that no other
`AND` argument evaluates to a `FALSE`; most Subroutines do not return
a `FALSE` without a very good reason for it. (A notable exception is
`TERPRI` (which see).) Even safer is to use `PROG` (section 10.1)
instead of `AND`.

Another variation is to increase the nesting with a new `COND` after
the unconditional part. At least this method does not make the code
appear to a human reader as though it does something other than what
it really does. The above example could be done this way:

    <COND   (<0? .N> <F0 .N>)
            (<1? .N> <F1 .N>)
            (T
             <SET N <* 2 <FIX </ .N 2>>>>
             <COND  (<LENGTH? .VEC .N> '[])
                    (T <REST .VEC <+ 1 .N>>)>)>

Chapter 9. Functions
====================

This chapter could be named "fun and games with argument `LIST`s". Its
purpose is to explain the more complicated things which can be done
with `FUNCTION`s, and this involves, basically, explaining all the
various tokens which can appear in the argument `LIST` of a
`FUNCTION`. Topics are covered in what is approximately an order of
increasing complexity. This order has little to do with the order in
which tokens can actually appear in an argument `LIST`, so what an
argument `LIST` "looks like" overall gets rather lost in the shuffle.
To alleviate this problem, section 9.9 is a summary of everything that
can go into an argument `LIST`, in the correct order. If you find
yourself getting lost, please refer to that summary.

9.1. "OPTIONAL" \[1\]
---------------------

Muddle provides very convenient means for allowing optional arguments.
The `STRING` `"OPTIONAL"` (or `"OPT"` -- they're totally equivalent)
in the argument `LIST` allows the specification of optional arguments
with values to be assigned by default. The syntax of the `"OPTIONAL"`
part of the argument `LIST` is as follows:

    "OPTIONAL" al-1 al-2 ... al-N

First, there is the `STRING` `"OPTIONAL"`. Then there is any number of
either `ATOM`s or two-element `LIST`s, intermixed, one per optional
argument. The first element of each two-element `LIST` must be an
`ATOM`; this is the dummy variable. The second element is an arbitrary
Muddle expression. If there are required arguments, they must come
before the `"OPTIONAL"`.

When `EVAL` is binding the variables of a `FUNCTION` and sees
`"OPTIONAL"`, the following happens:

-   If an explicit argument was given in the position of an optional
    one, the explicit argument is bound to the corresponding dummy
    `ATOM`.
-   If there is no explicit argument and the `ATOM` stands alone, that
    is, it is not the first element of a two-element `LIST`, that
    `ATOM` becomes "bound", but no local value is assigned to it \[see
    below\]. A local value can be assigned to it by using `SET`.
-   If there is no explicit argument and the `ATOM` is the first
    element of a two-element `LIST`, the Muddle expression in the
    `LIST` with the `ATOM` is evaluated and bound to the `ATOM`.

\[Until an `ATOM` is assigned, any attempt to reference its `LVAL`
will produce an error. The predicate `SUBR`s `BOUND?` and `ASSIGNED?`
can be used to check for such situations. `BOUND?` returns `T` if its
argument is currently bound via an argument `LIST` or has ever been
`SET` while not bound via an argument `LIST`. The latter kind of
binding is called "top-level binding", because it is done outside all
active argument-`LIST` binding. `ASSIGNED?` will return `#FALSE ()` if
its argument is **either** unassigned **or** unbound. By the way,
there are two predicates for global values similar to `BOUND?` and
`ASSIGNED?`, namely `GBOUND?` and `GASSIGNED?`. Each returns `T` only
if its argument, which (as in `BOUND?` and `ASSIGNED?`) must be an
`ATOM`, has a global value "slot" (chapter 22) or a global value,
respectively.\]

Example:

    <DEFINE INC1 (A "OPTIONAL" (N 1)) <SET .A <+ ..A .N>>>$
    INC1
    <SET B 0>$
    0
    <INC1 B>$
    1
    <INC1 B 5>$
    0

Here we defined another (not quite working) increment `FUNCTION`. It
now takes an optional argument specifying how much to increment the
`ATOM` it is given. If not given, the increment is `1`. Now, `1` is a
pretty simple Muddle expression: there is no reason why the optional
argument cannot be complicated -- for example, a call to a `FUNCTION`
which reads a file on an I/O device.

9.2. TUPLEs
-----------

### 9.2.1. "TUPLE" and TUPLE (the TYPE) \[1\]

There are also times when you want to be able to have an arbitrary
number of arguments. You can always do this by defining the `FUNCTION`
as having a structure as its argument, with the arbitrary number of
arguments as elements of the structure. This can, however, lead to
inelegant-looking `FORM`s and extra garbage to be collected. The
`STRING` `"TUPLE"` appearing in the argument `LIST` allows you to
avoid that. It must follow explicit and optional dummy arguments (if
there are any of either) and must be followed by an `ATOM`.

The effect of `"TUPLE"` appearing in an argument `LIST` is the
following: any arguments left in the `FORM`, after satisfying explicit
and optional arguments, are `EVAL`ed and made sequential elements of
an object of `TYPE` and `PRIMTYPE` `TUPLE`. The `TUPLE` is the bound
to the `ATOM` following `"TUPLE"` in the argument `LIST`. If there
were no arguments left by the time the `"TUPLE"` was reached, an empty
`TUPLE` is bound to the `ATOM`.

An object of `TYPE` `TUPLE` is exactly the same as a `VECTOR` except
that a `TUPLE` is not held in garbage-collected storage. It is instead
held with `ATOM` bindings in a control stack. This does not affect
manipulation of the `TUPLE` within the function generating it or any
function called within that one: it can be treated just like a
`VECTOR`. Note, however, that a `TUPLE` ceases to exist when the
function which generated it returns. Returning a `TUPLE` as a value is
a good way to generate an error. (A copy of a `TUPLE` can easily be
generated by segment-evaluating the `TUPLE` into something; that copy
can be returned.) The predicate `LEGAL?` returns `#FALSE ()` if it is
given a `TUPLE` generated by an `APPLICABLE` object which has already
returned, and `T` if it is given a `TUPLE` which is still "good".

Example:

    <DEFINE NTHARG (N "TUPLE" T)
                    ;"Get all but first argument into T."
        <COND (<==? 1 .N> 1)
                    ;"If N is 1, return 1st arg, i.e., .N,
                      i.e., 1.  Note that <1? .N> would be
                      true even if .N were 1.0."
              (<L? <LENGTH .T> <SET N <- .N 1>>>
               #FALSE ("DUMMY"))
                    ;"Check to see if there is an Nth arg,
                      and make N a good index into T while
                      you're at it.
                      If there isn't an Nth arg, complain."
              (ELSE <NTH .T .N>)>>

`NTHARG`, above, takes any number of arguments. Its first argument
must be of `TYPE` `FIX`. It returns `EVAL` of its Nth argument, if it
has an Nth argument. If it doesn't, it returns `#FALSE ("DUMMY")`.
(The `ELSE` is not absolutely necessary in the last clause. If the Nth
argument is a `FALSE`, the `COND` will return that `FALSE`.) Exercise
for the reader: `NTHARG` will generate an error if its first argument
is not `FIX`. Where and why? (How about `<NTHARG 1.5 2 3>`?) Fix it.

### 9.2.2. TUPLE (the SUBR) and ITUPLE

These `SUBR`s are the same as `VECTOR` and `IVECTOR`, except that they
build `TUPLE`s (that is, vectors on the control stack). They can be
used only at top level in an `"OPTIONAL"` list or `"AUX"` list (see
below). The clear advantage of `TUPLE` and `ITUPLE` ("implicit tuple")
is in storage-management efficiency. They produce no garbage, since
they are flushed automatically upon function return.

Examples:

    <DEFINE F (A B "AUX" (C <ITUPLE 10 3>)) ...>

creates a 10-element `TUPLE` and `SET`s `C` to it.

    <DEFINE H ("OPTIONAL" (A <ITUPLE 10 '<I>>)
                    "AUX" (B <TUPLE !.A 1 2 3>))
                    ...>

These are valid uses of `TUPLE` and `ITUPLE`. However, the following
is **not** a valid use of `TUPLE`, because it is not called at top
level of the `"AUX"`:

    <DEFINE NO (A B "AUX" (C <REST <TUPLE !.A>>)) ...>

However, the desired effect could be achieved by

    <DEFINE OK (A B "AUX" (D <TUPLE !.A>) (C <REST .D>)) ...>

9.3 "AUX" \[1\]
---------------

`"AUX"` (or `"EXTRA"` -- they're totally equivalent) are `STRING`s
which, placed in an argument `LIST`, serve to dynamically allocate
temporary variables for the use of a Function.

`"AUX"` must appear in the argument `LIST` after any information about
explicit arguments. It is followed by `ATOM`s or two-element `LIST`s
as if it were `"OPTIONAL"`. `ATOM`s in the two-element `LIST`s are
bound to `EVAL` of the second element in the `LIST`. Atoms not in such
`LIST`s are initially **unassigned**: they are explicitly given "no"
`LVAL`.

All binding specified in an argument `LIST` is done sequentially from
first to last, so initialization expressions for `"AUX"` (or
`"OPTIONAL"`) can refer to objects which have just been bound. For
example, this works:

    <DEFINE AUXEX ("TUPLE" T
                     "AUX" (A <LENGTH .T>) (B <* 2 .A>))
            ![.A .B]>$
    AUXEX
    <AUXEX 1 2 "FOO">$
    ![3 6!]

9.4. QUOTEd arguments
---------------------

If an `ATOM` in an argument `LIST` which is to be bound to a required
or optional argument is surrounded by a call to `QUOTE`, that `ATOM`
is bound to the **unevaluated** argument. Example:

    <DEFINE Q2 (A 'B) (.A .B)>$
    Q2
    <Q2 <+ 1 2> <+ 1 2>>$
    (3 <+ 1 2>)

It is not often appropriate for a function to take its arguments
unevaluated, because such a practice makes it less modular and harder
to maintain: it and the programs that call it tend to need to know
more about each other, and a change in its argument structure would
tend to require more changes in the programs that call it. And, since
few functions, in practice, do take unevaluated arguments, users tend
to assume that no functions do (except `FSUBR`s of course), and
confusion inevitably results.

9.5. "ARGS"
-----------

The indicator `"ARGS"` can appear in an argument `LIST` with precisely
the same syntax as `"TUPLE"`. However, `"ARGS"` causes the `ATOM`
following it to be bound to a `LIST` of the remaining **unevaluated**
arguments.

`"ARGS"` does not cause any copying to take place. It simply gives you

    <REST application:form fix>

with an appropriate *fix*. The `TYPE` change to `LIST` is a result of
the `REST`. Since the `LIST` shares all its elements with the original
`FORM`, `PUT`s into the `LIST` will change the calling program,
however dangerous that may be.

Examples:

    <DEFINE QIT (N "ARGS" L) <.N .L>>$
    QIT
    <QIT 2 <+ 3 4 <LENGTH ,QALL> FOO>$
    <LENGTH ,QALL>

    <DEFINE FUNCT1 ("ARGS" ARGL-AND-BODY)
            <CHTYPE .ARGL-AND-BODY FUNCTION>>$
    FUNCT1
    <FUNCT1 (A B) <+ .A .B>>$
    #FUNCTION ((A B) <+ .A .B>)

The last example is a perfectly valid equivalent of the `FSUBR`
`FUNCTION`.

9.6. "CALL"
-----------

The indicator `"CALL"` is an ultimate `"ARGS"`. If it appears in an
argument `LIST`, it must be followed by an `ATOM` and must be the only
thing used to gather arguments. `"CALL"` causes the `ATOM` which
follows it to become bound to the actual `FORM` that is being
evaluated -- that is, you get the "function call" itself. Since
`"CALL"` binds to the `FORM` itself, and not a copy, `PUT`s into that
`FORM` will change the calling code.

`"CALL"` exists as a Catch-22 for argument manipulation. If you can't
do it with `"CALL"`, it can't be done.

9.7. EVAL and "BIND"
--------------------

Obtaining unevaluated arguments, for example, for `QUOTE` and
`"ARGS"`, very often implies that you wish to evaluate them at some
point. You can do this by explicitly calling `EVAL`, which is a
`SUBR`. Example:

    <SET F '<+ 1 2>>$
    <+ 1 2>
    <EVAL .F>$
    3

`EVAL` can take a second argument, of `TYPE` `ENVIRONMENT` (or others,
see section 20.8). An `ENVIRONMENT` consists basically of a state of
`ATOM` bindings; it is the "world" mentioned in chapter 5. Now, since
binding changes the `ENVIRONMENT`, if you wish to use `EVAL` within a
`FUNCTION`, you probably want to get hold of the environment which
existed **before** that `FUNCTION`'s binding took place. The indicator
`"BIND"`, which must, if it is used, be the first thing in an argument
`LIST`, provides this information. It binds the `ATOM` immediately
following it to the `ENVIRONMENT` existing "at call time" -- that is,
just before any binding is done for its `FUNCTION`. Example:

    <SET A 0>$
    0
    <DEFINE WRONG ('B "AUX" (A 1)) <EVAL .B>>$
    WRONG
    <WRONG .A>
    1
    <DEFINE RIGHT ("BIND" E 'B "AUX" (A 1)) <EVAL .B .E>>$
    RIGHT
    <RIGHT .A>$
    0

### 9.7.1. Local Values versus ENVIRONMENTs

`SET`, `LVAL`, `VALUE`, `BOUND?`, `ASSIGNED?`, and `UNASSIGN` all take
a final optional argument which has not previously been mentioned: an
`ENVIRONMENT` (or other `TYPE`s, see section 20.8). If this argument
is given, the `SET` or `LVAL` is done in the `ENVIRONMENT` specified.
`LVAL` cannot be abbreviated by `.` (period) if it is given an
explicit second argument.

This feature is just what is needed to cure the `INC` bug mentioned in
chapter 5. A "correct" `INC` can be defined as follows:

    <DEFINE INC ("BIND" OUTER ATM)
            <SET .ATM <+ 1 <LVAL .ATM .OUTER>> .OUTER>>

9.8. ACTIVATION, "NAME", "ACT", "AGAIN", and RETURN \[1\]
---------------------------------------------------------

`EVAL`uation of a `FUNCTION`, after the argument `LIST` has been taken
care of, normally consists of `EVAL`uating each of the objects in the
body in the order given, and returning the value of the last thing
`EVAL`ed. If you want to vary this sequence, you need to know, at
least, where the `FUNCTION` begins. Actually, `EVAL` normally hasn't
the foggiest idea of where its current `FUNCTION` began. "Where'd I
start" information is bundled up with a `TYPE` called `ACTIVATION`. In
"normal" `FUNCTION` `EVAL`uation, `ACTIVATION`s are not generated: one
can be generated, and bound to an `ATOM`, in either of the two
following ways:

1.  Put an `ATOM` immediately before the argument `LIST`. The
    `ACTIVATION` of the Function will be bound to that `ATOM`.
2.  As the last thing in the argument `LIST`, insert either of the
    `STRING`s `"NAME"` or `"ACT"` and follow it with an `ATOM`. The
    `ATOM` will be bound to the `ACTIVATION` of the Function.

In this document "Function" (capitalized) will designate anything that
can generate an `ACTIVATION`; besides `TYPE` `FUNCTION`, this class
includes the `FSUBR`s `PROG`, `BIND`, and `REPEAT`, yet to be
discussed.

Each `ACTIVATION` refers explicitly to a particular evaluation of a
Function. For example, if a recursive `FUNCTION` generates an
`ACTIVATION`, a new `ACTIVATION` referring explicitly to each
recursion step is generated on every recursion.

Like `TUPLE`s, `ACTIVATION`s are held in a control stack. Unlike
`TUPLE`s, there is **no way** to get a copy of an `ACTIVATION` which
can usefully be returned as a value. (This is a consequence of the
fact that `ACTIVATION`s refer to evaluations; when the evaluation is
finished, the `ACTIVATION` no longer exists.) `ACTIVATION`s can be
tested, like `TUPLE`s, by `LEGAL?` for legality. They are used by the
`SUBR`s `AGAIN` and `RETURN`.

`AGAIN` can take one argument: an `ACTIVATION`. It means "start doing
this again", where "this" is specified by the `ACTIVATION`.
Specifically, `AGAIN` causes `EVAL` to return to where it started
working on the **body** of the Function in the evaluation specified by
the `ACTIVATION`. The evaluation is not redone completely: in
particular, no re-binding (of arguments, `"AUX"` variables, etc.) is
done.

`RETURN` can take two arguments: an arbitrary expression and an
`ACTIVATION`, in that order. It causes the Function evaluation whose
`ACTIVATION` it is given to terminate and return `EVAL` of `RETURN`'s
first argument. That is, `RETURN` means "quit doing this and return
that", where "this" is the `ACTIVATION` -- its second argument -- and
"that" is the expression -- its first argument. Example:

    <DEFINE MY+ ("TUPLE" T "AUX" (M 0) "NAME" NM)
            <COND (<EMPTY? .T> <RETURN .M .NM>)>
            <SET M <+ .M <1 .T>>>
            <SET T <REST .T>>
            <AGAIN .NM>>$
    MY+
    <MY+ 1 3 <LENGTH "FOO">>$
    7
    <MY+>$
    0

Note: suppose an `ACTIVATION` of one Function (call it `F1`) is passed
to another Function (call it `F2`) -- for example, via an application
of `F2` within `F1` with `F1`'s `ACTIVATION` as an argument. If `F2`
`RETURN`s to `F1`'s `ACTIVATION`, `F2` **and** `F1` terminate
immediately, and **`F1`** returns the `RETURN`'s first argument. This
technique is suitable for error exits. `AGAIN` can clearly pull a
similar trick. In the following example, `F1` computes the sum of `F2`
applied to each of its arguments; `F2` computes the product of the
elements of its structured argument, but it aborts if it finds an
element that is not a number.

    <DEFINE F1 ACT ("TUPLE" T "AUX" (T1 .T))
            <COND (<NOT <EMPTY? .T1>>
                   <PUT .T1 1 <F2 <1 .T1> .ACT>>
                   <SET T1 <REST .T1>>
                   <AGAIN .ACT>)
                  (ELSE <+ !.T>)>>$
    F1
    <DEFINE F2 (S A "AUX" (S1 .S))
            <REPEAT MY-ACT ((PRD 1))
               <COND (<NOT <EMPTY? .S1>>
                      <COND (<NOT <TYPE? 1 .S1> FIX FLOAT>>
                             <RETURN #FALSE ("NON-NUMBER") .A>)
                            (ELSE <SET PRD <* .PRD <1 .S1>>>)>
                      <SET S1 <REST .S1>>)
                     (ELSE <RETURN .PRD>)>>>$
    F2

    <F1 '(1 2) '(3 4)>$
    14
    <F1 '(T 2) '(3 4)>$
    #FALSE ("NON-NUMBER")

9.9. Argument List Summary
--------------------------

The following is a listing of all the various tokens which can appear
in the argument `LIST` of a `FUNCTION`, in the order in which they can
occur. Short descriptions of their effects are included. **All** of
them are **optional** -- that is, any of them (in any position) can be
left out or included -- but the order in which they appear **must** be
that of this list. "`QUOTE`d `ATOM`", "matching object", and "2-list"
are defined below.

(1) `"BIND"`

must be followed by an `ATOM`. It binds that `ATOM` to the
`ENVIRONMENT` which existed when the `FUNCTION` was applied.

(2) `ATOM`s and `QUOTE`d `ATOM`s (any number)

are required arguments. `QUOTE`d `ATOM`s are bound to the matching
object. `ATOM`s are bound to `EVAL` of the matching object in the
`ENVIRONMENT` existing when the `FUNCTION` was applied.

(3) `"OPTIONAL"` or `"OPT"` (they're equivalent)

is followed by any number of `ATOM`s, `QUOTE`d `ATOM`s, or 2-lists.
These are optional arguments. If a matching object exists, an `ATOM`
-- either standing alone or the first element of a 2-list -- is bound
to `EVAL` of the object, performed in the `ENVIRONMENT` existing when
the `FUNCTION` was applied. A `QUOTE`d `ATOM` -- alone or in a 2-list
-- is bound to the matching object itself. If no such object exists,
`ATOM`s and `QUOTE`d `ATOM`s are left unbound, and the first element
of each 2-list is bound to `EVAL` of the corresponding second element.
(This `EVAL` is done in the new `ENVIRONMENT` of the Function as it is
being constructed.)

(4) `"ARGS"` (and **not** `"TUPLE"`)

must be followed by an `ATOM`. The `ATOM` is bound to a `LIST` of
**all** the remaining arguments, **unevaluated**. (If there are no
more arguments, the `LIST` is empty.) This `LIST` is actually a `REST`
of the `FORM` applying the `FUNCTION`. If `"ARGS"` appears in the
argument `LIST`, `"TUPLE"` should not appear.

(4) `"TUPLE"` (and **not** `"ARGS"`)

must be followed by an `ATOM`. The `ATOM` is bound to a `TUPLE`
("`VECTOR` on the control stack") of all the remaining arguments,
**evaluated** in the environment existing when the `FUNCTION` was
applied. (If no arguments remain, the `TUPLE` is empty.) If `"TUPLE"`
appears in the argument `LIST`, `"ARGS"` should not appear.

(5) `"AUX"` or `"EXTRA"` (they're equivalent)

is followed by any number of `ATOM`s or 2-lists. These are auxiliary
variables, bound away from the previous environment for the use of
this Function. `ATOM`s are bound in the `ENVIRONMENT` of the Function,
but they are unassigned; the first element of each 2-list is both
bound and assigned to `EVAL` of the corresponding second element.
(This `EVAL` is done in the new `ENVIRONMENT` of the Function as it is
being constructed.)

(6) `"NAME"` or `"ACT"` (they're equivalent)

must be followed by an `ATOM`. The `ATOM` is bound to the `ACTIVATION`
of the current evaluation of the Function.

**ALSO** -- in place of sections (2) (3) **and** (4), you can have

(2-3-4) `"CALL"`

which must be followed by an `ATOM`. The `ATOM` is bound to the `FORM`
which caused application of this `FUNCTION`.

The special terms used above mean this:

"`QUOTE`d `ATOM`" -- a two-element `FORM` whose first element is the
`ATOM` `QUOTE`, and whose second element is any `ATOM`. (Can be typed
-- and will be `PRINT`ed -- as `'atom`.)

"Matching object" -- that element of a `FORM` whose position in the
`FORM` matches the position of a required or optional argument in an
argument `LIST`.

"2-list" -- a two-element `LIST` whose first element is an `ATOM` (or
`QUOTE`d `ATOM`: see below) and whose second element can be anything
but a `SEGMENT`. `EVAL` of the second element is assigned to a new
binding of the first element (the `ATOM`) as the "value by default" in
`"OPTIONAL"` or the "initial value" in `"AUX"`. In the case of
`"OPTIONAL"`, the first element of a 2-list can be a `QUOTE`d `ATOM`;
in this case, an argument which is supplied is not `EVAL`ed, but if it
is not supplied the second element of the `LIST` **is** `EVAL`ed and
assigned to the `ATOM`.

9.10. APPLY \[1\]
-----------------

Occasionally there is a valid reason for the first element of a `FORM`
not to be an `ATOM`. For example, the object to be applied to
arguments may be chosen at run time, or it may depend on the arguments
in some way. While `EVAL` is perfectly happy in this case to
`EVAL`uate the first element and go on from there, the compiler
(Lebling, 1979) can generate more efficient code if it knows whether
the result of the evaluation will (1) always be of `TYPE` `FIX`, (2)
always be an applicable non-`FIX` object that evaluates all its
arguments, or (3) neither. The easiest way to tell the compiler if (1)
or (2) is true is to use the `ATOM` `NTH` (section 7.1.2) or `PUT`
(section 7.1.4) in case (1) or `APPLY` in case (2) as the first
element of the `FORM`. (Note: case (1) can compile into in-line code,
but case (2) compiles into a fully mediated call into the
interpreter.)

    <APPLY object arg-1 ... arg-N>

evaluates *object* and all the *arg-i*'s and then applies the former
to all the latter. An error occurs if *object* evaluates to something
not applicable, or to an `FSUBR`, or to a `FUNCTION` (or user
Subroutine -- chapter 19) with `"ARGS"` or `"CALL"` or `QUOTE`d
arguments.

Example:

    <APPLY <NTH .ANALYZERS
                <LENGTH <MEMQ <TYPE .ARG> .ARGTYPES>>>
           .ARG>

calls a function to analyze `.ARG`. Which function is called depends
on the `TYPE` of the argument; this represents the idea of a dispatch
table.

9.11. CLOSURE
-------------

    <CLOSURE function a1 ... aN>

where *function* is a `FUNCTION`, and *a1* through *aN* are any number
of `ATOM`s, returns an object of `TYPE` `CLOSURE`. This can be applied
like any other function, but, whenever it is applied, the `ATOM`s
given in the call to `CLOSURE` are **first** bound to the `VALUE`s
they had when the `CLOSURE` was generated, then the *function* is
applied as normal. This is a "poor man's `funarg`".

A `CLOSURE` is useful when a `FUNCTION` must have state information
remembered between calls to it, especially in these two cases: when
the `LVAL`s of external state `ATOM`s might be compromised by other
programs, or when more than one distinct sequence of calls are active
concurrently. Example of the latter: each object of a structured
`NEWTYPE` might have an associated `CLOSURE` that coughs up one
element at a time, with a value in the `CLOSURE` that is a structure
containing all the relevant information.

Chapter 10. Looping
===================

10.1. PROG and REPEAT \[1\]
---------------------------

`PROG` and `REPEAT` are almost identical `FSUBR`s which make it
possible to vary the order of `EVAL`uation arbitrarily -- that is, to
have "jumps". The syntax of `PROG` ("program") is

    <PROG act:atom aux:list body>

where

-   *act* is an optional `ATOM`, which is bound to the `ACTIVATION` of
    the `PROG`.
-   *aux* is a `LIST` which looks exactly like that part of a
    `FUNCTION`'s argument `LIST` which follows an `"AUX"`, and serves
    exactly the same purpose. It is not optional. If you need no
    temporary variables of `"ACT"`, make it `()`.
-   *body* is a non-zero number of arbitrary Muddle expressions.

The syntax of `REPEAT` is identical, except that, of course, `REPEAT`
is the first element of the `FORM`, not `PROG`.

### 10.1.1. Basic EVALuation \[1\]

Upon entering a `PROG`, an `ACTIVATION` is **always** generated. If
there is an `ATOM` in the right place, the `ACTIVATION` is also bound
to that `ATOM`. The variables in the *aux* (if any) are then bound as
indicated in the *aux*. All of the expressions in *body* are then
`EVAL`uated in their order of occurrence. If nothing untoward happens,
you leave the `PROG` upon evaluating the last expression in *body*,
returning the value of that last expression.

`PROG` thus provides a way to package together a group of things you
wish to do, in a somewhat more limited way than can be done with a
`FUNCTION`. But `PROG`s are generally used for their other properties.

`REPEAT` acts in all ways **exactly** like a `PROG` whose last
expression is `<AGAIN>`. The only way to leave a `REPEAT` is to
explicitly use `RETURN` (or `GO` with a `TAG` -- section 10.4).

### 10.1.2. AGAIN and RETURN in PROG and REPEAT \[1\]

Within a `PROG` or `REPEAT`, you always have a defined `ACTIVATION`,
whether you bind it to an `ATOM` or not. \[In fact the interpreter
binds it to the `ATOM` `LPROG\ !-INTERRUPTS` ("last PROG"). The
`FSUBR` `BIND` is identical to `PROG` except that `BIND` does not bind
that `ATOM`, so that `AGAIN` and `RETURN` with no `ACTIVATION`
argument will not refer to it. This feature could be useful within
`MACRO`s.\]

If `AGAIN` is used with no arguments, it uses the `ACTIVATION` of the
closest surrounding `PROG` or `REPEAT` **within the current function**
(an error occurs if there is none) and re-starts the `PROG` or
`REPEAT` without rebinding the *aux* variables, just the way it works
in a `FUNCTION`. With an argument, it can of course re-start any
Function (`PROG` or `REPEAT` or `FUNCTION`) within which it is
embedded at run time.

As with `AGAIN`, if `RETURN` is given no `ACTIVATION` argument, it
uses the `ACTIVATION` of the closest surrounding `PROG` or `REPEAT`
within the current function and causes that `PROG` or `REPEAT` to
terminate and return `RETURN`'s first argument. If `RETURN` is given
**no** arguments, it causes the closest surrounding `PROG` or `REPEAT`
to return the `ATOM` `T`. Also like `AGAIN`, it can, with an
`ACTIVATION` argument, terminate any Function within which it is
embedded at run time.

### 10.1.3. Examples \[1\]

Examples of the use of `PROG` are difficult to find, since it is
almost never necessary, and it slows down the interpreter (chapter
24). `PROG` can be useful as a point of return from the middle of a
computation, or inside a `COND` (which see), but we won't exemplify
those uses. Instead, what follows is an example of a typically poor
use of `PROG` which has been observed among Lisp (Moon, 1974)
programmers using Muddle. Then, the same thing is done using `REPEAT`.
In both cases, the example `FUNCTION` just adds up all its arguments
and returns the sum. (The `SUBR` `GO` is discussed in section 10.4.)

    ;"Lisp style"
        <DEFINE MY+ ("TUPLE" TUP)
                <PROG (SUM)
                        <SET SUM 0>
                  LP    <COND (<EMPTY? .TUP> <RETURN .SUM>)>
                        <SET SUM <+ .SUM <1 .TUP>>>
                        <SET TUP <REST .TUP>>
                        <GO LP>>>

    ;"Muddle style"
        <DEFINE MY+ ("TUPLE" TUP)
                <REPEAT ((SUM 0))
                        <COND (<EMPTY? .TUP> <RETURN .SUM>)>
                        <SET SUM <+ .SUM <1 .TUP>>
                        <SET TUP <REST .TUP>>>>

Of course, neither of the above is optimal Muddle code for this
problem, since `MY+` can be written using `SEGMENT` evaluation as

    <DEFINE MY+ ("TUPLE" TUP) <+ !.TUP>>

There are, of course, lots of problems which can't be handled so
simply, and lots of uses for `REPEAT`.

10.2. MAPF and MAPR: Basics \[1\]
---------------------------------

`MAPF` ("map first") and `MAPR` ("map rest") are two `SUBR`s which
take care of a majority of cases which require loops over data. The
basic idea is the following:

Suppose you have a `LIST` (or other structure) of data, and you want
to apply a particular function to each element. That is exactly what
`MAPF` does: you give it the function and the structure, and it
applies the function to each element of the structure, starting with
the first.

On the other hand, suppose you want to **change** each element of a
structure according to a particular algorithm. This can be done only
with great pain using `MAPF`, since you don't have easy access to the
**structure** inside the function: you have only the structure's
elements. `MAPR` solves the problem by applying a function to `REST`s
of a structure: first to `<REST structure 0>`, then to
`<REST structure 1>`, etc. Thus, the function can change the structure
by changing its argument, for example, by a
`<PUT argument 1 something>`. It can even `PUT` a new element farther
down the structure, which will be seen by the function on subsequent
applications.

Now suppose, in addition to applying a function to a structure, you
want to record the results -- the values returned by the function --
in another structure. Both `MAPF` and `MAPR` can do this: they both
take an additional function as an argument, and, when the looping is
over, apply the additional function to **all** the results, and then
return the results of that application. Thus, if the additional
function is `,LIST`, you get a `LIST` of the previous results; if it
is `.VECTOR`, you get a `VECTOR` of results; etc.

Finally, it might be the case that you really want to loop a function
over more than one structure simultaneously. For instance, consider
creating a `LIST` whose elements are the element-by-element sum of the
contents of two other `LIST`s. Both `MAPF` and `MAPR` allow this; you
can, in fact, give each of them any number of structures full of
arguments for your looping function.

This was all mentioned because `MAPF` and `MAPR` appear to be complex
when seen baldly, due to the fact that the argument descriptions must
take into account the general case. Simpler, degenerate cases are
usually the ones used.

### 10.2.1. MAPF \[1\]

    <MAPF finalf loopf s1 s2 ... sN>

where (after argument evaluation)

-   *finalf* is something applicable that evaluates all its arguments,
    or a `FALSE`;
-   *loopf* is something applicable to *N* arguments that evaluates
    all its arguments; and
-   *s1* through *sN* are structured objects (any `TYPE`)

does the following:

1.  First, it applies *loopf* to *N* arguments: the first element of
    each of the structures. Then it `REST`s each of the structures,
    and does the application again, looping until **any** of the
    structures runs out of elements. Each of the values returned by
    *loopf* is recorded in a `TUPLE`.
2.  Then, it applies *finalf* to all the recorded values
    simultaneously, and returns the result of that application. If
    *finalf* is a `FALSE`, the recorded values are "thrown away"
    (actually never recorded in the first place) and the `MAPF`
    returns only the last value returned by *loopf*. If any of the
    *si* structures is empty, to that *loopf* is never invoked,
    *finalf* is applied to **no** arguments; if *finalf* is a `FALSE`,
    `MAPF` returns `#FALSE ()`.

### 10.2.2 MAPR \[1\]

    <MAPR finalf loopf s1 s2 ... sN>

acts just like `MAPF`, but, instead of applying *loopf* to `NTH`s of
the structures -- that is, `<NTH si 1>`, `<NTH si 2>`, etc. -- it
applies it to `REST`s of the structures -- that is, `<REST si 0>`,
`<REST si 1>`, etc.

### 10.2.3. Examples \[1\]

Make the element-wise sum of two `LIST`s:

    <MAPF .LIST .+ '(1 2 3 4) '(10 11 12 13)>$
    (11 13 15 17)

Change a `UVECTOR` to contain double its values:

    <SET UV '![5 6 7 8 9]>$
    ![5 6 7 8 9!]
    <MAPR <>
           #FUNCTION ((L) <PUT .L 1 <* <1 .L> 2>>)
           .UV>$
    ![18!]
    .UV$
    ![10 12 14 16 18!]

Create a `STRING` from `CHARACTER`s:

    <MAPF ,STRING 1 '["MODELING" "DEVELOPMENT" "LIBRARY"]>$
    "Muddle"

Sum the squares of the elements of a `UVECTOR`:

    <MAPF ,+ #FUNCTION ((N) <* .N .N>) '![3 4]>$
    25

A parallel assignment `FUNCTION` (Note that the arguments to `MAPF`
are of different lengths.):

    <DEFINE PSET ("TUPLE" TUP)
            <MAPF <>
                  ,SET
                  .TUP
                  <REST .TUP </ <LENGTH .TUP> 2>>>>$
    PSET
    <PSET A B C 1 2 3>$
    3
    .A$
    1
    .B$
    2
    .C$
    3

Note: it is easy to forget that *finalf* **must** evaluate its
arguments, which precludes the use of an `FSUBR`. It is primarily for
this reason that the `SUBR`s `AND?` and `OR?` were invented. As an
example, the predicate `=?` could have been defined this way:

    <DEFINE =? (A B)
            <COND (<MONAD? .A> <==? .A .B>)
                  (<AND <NOT <MONAD? .B>>
                        <==? <TYPE .A> <TYPE .B>>
                        <==? <LENGTH .A> <LENGTH .B>>>
                   <MAPF ,AND? ,=? .A .B>)>>

\[By the way, the following shows how to construct a value that has
the same `TYPE` as an argument.

    <DEFINE MAP-NOT (S)
     <COND (<MEMQ <PRIMTYPE .S> '![LIST VECTOR UVECTOR STRING]>
            <CHTYPE <MAPF ,<PRIMTYPE .S> ,NOT .S>
                    <TYPE .S>>)>>

It works because the `ATOM`s that name the common `STRUCTURED`
`PRIMTYPS`s (`LIST`, `VECTOR`, `UVECTOR` and `STRING`) have as `GVAL`s
the corresponding `SUBR`s to build objects of those `TYPE`s.\]

10.3. More on MAPF and MAPR
---------------------------

### 10.3.1. MAPRET

`MAPRET` is a `SUBR` that enables the *loopf* being used in a `MAPR`
or `MAPF` (and lexically within it, that is, not separated from it by
a function call) to return from zero to any number of values as
opposed to just one. For example, suppose a `MAPF` of the following
form is used:

    <MAPF ,LIST <FUNCTION (E) ...> ...>

Now suppose that the programmer wants to add no elements to the final
`LIST` on some calls to the `FUNCTION` and add many on other calls to
the `FUNCTION`. To accomplish this, the `FUNCTION` simply calls
`MAPRET` with the elements it wants added to the `LIST`. More
generally, `MAPRET` causes its arguments to be added to the final
`TUPLE` of arguments to which the *finalf* will be applied.

Warning: `MAPRET` is guaranteed to work only if it is called from an
explicit `FUNCTION` which is the second argument to a `MAPF` or
`MAPR`. In other words, the second argument to `MAPF` or `MAPR` must
be `#FUNCTION (...)` or `<FUNCTION ...>` if `MAPRET` is to be used.

Example: the following returns a `LIST` of all the `ATOM`s in an
`OBLIST` (chapter 15):

    <DEFINE ATOMS (OB)
            <MAPF .LIST
                  <FUNCTION (BKT) <MAPRET !.BKT>>
                  .OB>>

### 10.3.2. MAPSTOP

`MAPSTOP` is the same as `MAPRET`, except that, after adding its
arguments, if any, to the final `TUPLE`, it forces the application of
*finalf* to occur, whether or not the structured objects have run out
of objects. Example: the following copies the first ten (or all)
elements of its argument into a `LIST`:

    <DEFINE FIRST-TEN (STRUC "AUX" (I 10))
     <MAPF ,LIST
          <FUNCTION (E)
              <COND (<0? <SET I <- .I 1>>> <MAPSTOP .E>)>
              .E>
          .STRUC>>

### 10.3.3. MAPLEAVE

`MAPLEAVE` is analogous to `RETURN`, except that it works in
(lexically within) `MAPF` or `MAPR` instead of `PROG` or `REPEAT`. It
flushes the accumulated `TUPLE` of results and returns its argument
(optional, `T` by default) as the value of the `MAPF` or `MAPR`. (It
finds the MAPF/R that should returns in the current binding of the
`ATOM` `LMAP\ !-INTERRUPTS` ("last map").) Example: the following
finds and returns the first non-zero element of its argument, or
`#FALSE ()` if there is none:

    <DEFINE FIRST-N0 (STRUC)
            <MAPF <>
                  <FUNCTION (X)
                    <COND (<N==? .X 0> <MAPLEAVE .X>)>>
                  .STRUC>>

### 10.3.4. Only two arguments

If `MAPF` or `MAPR` is given only two arguments, the iteration
function *loopf* is applied to no arguments each time, and the looping
continues indefinitely until a `MAPLEAVE` or `MAPSTOP` is invoked.
Example: the following returns a `LIST` of the integers from one less
than its argument to zero.

    <DEFINE LNUM (N)
            <MAPF ,LIST
                  <FUNCTION ()
                    <COND (<=? <SET N <- .N 1>>> <MAPSTOP 0>)
                          (ELSE .N)>>>>

One principle use of this form of MAPF/R involves processing input
characters, in cases where you don't know how many characters are
going to arrive. The example below demonstrates this, using `SUBR`s
which are more fully explained in chapter 11. Another example can be
found in chapter 13.

Example: the following `FUNCTION` reads characters from the current
input channel until an `$` (`ESC`) is read, and then returns what was
read as one `STRING`. (The `SUBR` `READCHR` reads one character from
the input channel and returns it. `NEXTCHR` returns the next
`CHARACTER` which `READCHR` will return -- chapter 11.)

    <DEFINE RDSTR ()
      <MAPF .STRING
            <FUNCTION () <COND (<NOT <==? <NEXTCHR> <ASCII 27>>>
                                <READCHR>)
                               (T
                                <MAPSTOP>)>>>>$
    RDSTR

    <PROG () <READCHR> ;"Flush the ESC ending this input."
                 <RDSTR>>$
    ABC123<+ 3 4>$"ABC123<+ 3 4>"

### 10.3.5. STACKFORM

The `FSUBR` `STACKFORM` is archaic, due to improvements in the
implementation of MAPF/R, and it should not be used in new programs.

    <STACKFORM function arg pred>

is exactly equivalent to

    <MAPF function
          <FUNCTION () <COND (pred arg) (T <MAPSTOP>)>>>

In fact MAPF/R is more powerful, because `MAPRET`, `MAPSTOP`, and
`MAPLEAVE` provide flexibility not available with `STACKFORM`.

10.4. GO and TAG
----------------

`GO` is provided in Muddle for people who can't recover from a
youthful experience with Basic, Fortran, PL/I, etc. The `SUBR`s
previously described in this chapter are much more tasteful for making
good, clean, "structured" programs. `GO` just bollixes things.

`GO` is a `SUBR` which allows you to break the normal order of
evaluation and re-start just before any top-level expression in a
`PROG` or `REPEAT`. It can take two `TYPE`s of arguments: `ATOM` or
`TAG`.

Given an `ATOM`, `GO` searches the *body* of the immediately
surrounding `PROG` or `REPEAT` within the current Function, starting
after *aux*, for an occurrence of that `ATOM` at the top level of
*body*. (This search is effectively a `MEMQ`.) If it doesn't find the
`ATOM`, an error occurs. If it does, evaluation is resumed at the
expression following the `ATOM`.

The `SUBR` `TAG` generates and returns objects of `TYPE` `TAG`. This
`SUBR` takes one argument: an `ATOM` which would be a legal argument
for a `GO`. An object of `TYPE` `TAG` contains sufficient information
to allow you to `GO` to any top-level position in a `PROG` or `REPEAT`
from within any function called inside the `PROG` or `REPEAT`. `GO`
with a `TAG` is vaguely like `AGAIN` with an `ACTIVATION`; it allows
you to "go back" to the middle of any `PROG` or `REPEAT` which called
you. Also like `ACTIVATION`s, `TAG`s into a `PROG` or `REPEAT` can no
longer be used after the `PROG` or `REPEAT` has returned. `LEGAL?` can
be used to see if a `TAG` is still valid.

10.5. Looping versus Recursion
------------------------------

Since any program in Muddle can be called recursively, champions of
"pure Lisp" (Moon, 1974) or somesuch may be tempted to implement any
repetitive algorithm using recursion. The advantage of the looping
techniques described in this chapter over recursion is that the
overhead of calls is eliminated. However, a long program (say, bigger
than half a printed page) may be more difficult to write iteratively
than recursively and hence more difficult to maintain. A program whose
repetition is controlled by a structured object (for example, "walking
a tree" to visit each monad in the object) often should use looping
for covering one "level" of the structure and recursion to change
"levels".

Chapter 11. Input/Output
========================

The Muddle interpreter can transmit information between an object in
Muddle and an external device in three ways. Historically, the first
way was to **convert** an object into a string of characters, or vice
versa. The transformation is nearly one-to-one (although some Muddle
objects, for example `TUPLE`s, cannot be input in this way) and is
similar in style to Fortran's formatted I/O. It is what `READ` and
`PRINT` do, and it is the normal method for terminal I/O.

The second way is used for the contents of Muddle objects rather than
the objects themselves. Here an **image** of numbers or characters
within an object is transmitted, similar in style to Fortran's
unformatted I/O.

The third way is to **dump** an object in a clever format so that it
can be reproduced exactly when input the next time. Exact reproduction
means that any sharing between structures or self-reference is
preserved: only the garbage collector itself can do I/O in this way.

11.1. Conversion I/O
--------------------

All conversion-I/O `SUBR`s in Muddle take an optional argument which
directs their attention to a specific I/O channel. This section will
describe `SUBR`s without their optional arguments. In this situation,
they all refer to a particular channel by default, initially the
terminal running the Muddle. When given an optional argument, that
argument follows any arguments indicated here. Some of these `SUBR`s
also have additional optional arguments, relevant to conversion,
discussion of which will be deferred until later.

### 11.1.1. Input

All of the following input Subroutines, when directed at a terminal,
hang until `$` (`ESC`) is typed and allow normal use of `rubout`,
`^D`, `^L` and `^@`.

#### 11.1.1.1. READ

    <READ>

This returns the entire Muddle object whose character representation
is next in the input stream. Successive `<READ>`s return successive
objects. This is precisely the `SUBR` `READ` mentioned in chapter 2.
See also sections 11.3, 15.7.1, and 17.1.3 for optional arguments.

#### 11.1.1.2. READCHR

    <READCHR>

("read character") returns the next `CHARACTER` in the input stream.
Successive `<READCHR>`s return successive `CHARACTER`s.

#### 11.1.1.3. NEXTCHR

    <NEXTCHR>

("next character") returns the `CHARACTER` which `READCHR` will return
the next time `READCHR` is called. Multiple `<NEXTCHR>`s, with no
input operations between them, all return the same thing.

### 11.1.2. Output

If an object to be output requires (or can tolerate) separators within
it (for example, between the elements in a structured object or after
the `TYPE` name in "\# notation"), these conversion-output `SUBR`s
will use a carriage-return/line-feed separator to prevent overflowing
a line. Overflow is detected in advance from elements of the `CHANNEL`
in use (section 11.2.8).

#### 11.1.2.1. PRINT

    <PRINT any>

This outputs, in order,

1.  a carriage-return line-feed,
2.  the character representation of `EVAL` of its argument (`PRINT` is
    a `SUBR`), and
3.  a space

and then returns `EVAL` of its argument. This is precisely the `SUBR`
`PRINT` mentioned in chapter 2.

#### 11.1.2.2. PRIN1

    <PRIN1 any>

outputs just the representation of, and returns, `EVAL` of *any*.

#### 11.1.2.3. PRINC

    <PRINC any>

("print characters") acts exactly like `PRIN1`, except that

1.  if its argument is a `STRING` or a `CHARACTER`, it suppresses the
    surrounding `"`s or initial `!\` respectively; or
2.  if its argument is an `ATOM`, it suppresses any `\`s or `OBLIST`
    trailers (chapter 15) which would otherwise be necessary.

If `PRINC`'s argument is a structure containing `STRING`s,
`CHARACTER`s, or `ATOM`s, the service mentioned will be done for all
of them. Ditto for the `ATOM` used to name the `TYPE` in "\#
notation".

#### 11.1.2.4. TERPRI

    <TERPRI>

("terminate printing") outputs a carriage-return line-feed and then
returns `# FALSE ()`!

#### 11.1.2.5. CRLF

("carriage-return line-feed") outputs a carriage-return line-feed and
then returns `T`.

#### 11.1.2.6. FLATSIZE

    <FLATSIZE any max:fix radix:fix>

does not actually cause any output to occur and does not take a
`CHANNEL` argument. Instead, or compares *max* with the number of
characters `PRIN1` would take to print *any*. If *max* is less than
the number of characters needed (including the case where *any* is
self-referencing, `FLATSIZE` returns `#FALSE ()`; otherwise, it
returns the number of characters needed by `PRIN1` *any*. *radix*
(optional, ten by default) is used for converting any `FIX`es that
occur.

This `SUBR` is especially useful in conjunction with (section 11.2.8)
those elements of a `CHANNEL` which specify the number of characters
per output line and the current position on an input line.

CHANNEL (the TYPE)
------------------

I/O channels are dynamically assigned in Muddle, and are represented
by an object of `TYPE` `CHANNEL`, which is of `PRIMTYPE` `VECTOR`. The
format of a `CHANNEL` will be explained later, in section 11.2.8.
First, how to generate and use them.

### 11.2.1. OPEN

    <OPEN mode file-spec>

or

    <OPEN mode name1 name2 device dir>

`OPEN` is a `SUBR` which creates and returns a `CHANNEL`. All its
arguments must be of `TYPE` `STRING`, and **all** are optional. The
preceding statement is false when the *device* is `"INT"` or `"NET"`;
see sections 11.9 and 11.10. If the attempted opening of an
operating-system I/O channel fails, `OPEN` returns
`#FALSE (reason:string file-spec:string status:fix)`, where the
*reason* and the *status* are supplied by the operating system, and
the `file-spec` is the standard name of the file (after any name
transformations by the operating system) that Muddle was trying to
open.

The choice of *mode* is usually determined by which `SUBR`s will be
used on the `CHANNEL`, and whether or not the *device* is a terminal.
The following table tells which `SUBR`s can be used with which modes,
where `OK` indicates an allowed use:

  -------------------------------------------------------------------------------------------------------------------------------------------------
  "READ"               "PRINT"               "READB"               "PRINTB", "PRINTO"                           mode / SUBRs
  -------------------- --------------------- --------------------- -------------------------------------------- -----------------------------------
  OK                                         OK                                                                 `READ` `READCHR` `NEXTCHR`
                                                                                                                `READSTRING` `FILECOPY`
                                                                                                                `FILE-LENGTH LOAD`

                       OK                                          OK\*                                         `PRINT` `PRIN1` `PRINC` `IMAGE`
                                                                                                                `CRLF` `TERPRI` `FILECOPY`
                                                                                                                `PRINTSTRING` `BUFOUT` `NETS`
                                                                                                                `RENAME`

                                             OK                                                                 `READB` `GC-READ`

                                                                   OK                                           `PRINTB` `GC-DUMP`

  OK                                         OK                    OK                                           `ACCESS`

  OK                   OK                    OK                    OK                                           `RESET`

  OK                   OK                                                                                       `ECHOPAIR`

  OK                                                                                                            `TTYECHO` `TYI`
  -------------------------------------------------------------------------------------------------------------------------------------------------

`*` PRINTing (or `PRIN1`ing) an `RSUBR` (chapter 19) on a `"PRINTB"`
or `"PRINTO"` `CHANNEL` has special effects.

`"PRINTB"` differs from `"PRINTO"` in that the latter mode is used to
update a `"DSK"` file without copying it. `"READB"` and `"PRINTB"` are
not used with terminals. `"READ"` is the mode used by default.

The next one to four arguments to `OPEN` specify the file involved. If
only one `STRING` is used, it can contain the entire specification,
according to standard operating-system syntax. Otherwise, the
string(s) are interpreted as follows:

*name1* is the first file name, that part to the left of the space (in
the ITS version) or period (in the Tenex and Tops-20 versions). The
name used by default is `<VALUE NM1>`, if any, otherwise `"INPUT"`.

*name2* is the second fail name, that part to the right of the space
(ITS) or period (Tenex and Tops-20). The name used by default is
`<VALUE NM2>`, if any, otherwise `">"` or `"MUD"` and highest version
number (Tenex) or generation number (Tops-20).

*device* is the device name. The name used by default is
`<VALUE DEV>`, if any, otherwise `"DSK"`. (Devices about which Muddle
has no special knowledge are assumed to behave like `"DSK"`.)

*dir* is the disk-directory name. The name used by default is
`<VALUE SNM>`, if any, otherwise the "working-directory" name as
defined by her operating system.

Examples:

`<OPEN "PRINT" "TPL:">` opens a conversion-output channel to the TPL
device.

`<OPEN "PRINT" "DUMMY" "NAMES" "IPL">` does the same.

`<OPEN "PRINT" "TPL">` opens a `CHANNEL` to the file `DSK:TPL >` (ITS
version) or `DSK:TPL.MUD` (Tenex and Tops-20 versions).

`<OPEN "READ" "FOO" ">" "DSK" "GUEST">` opens up a conversion-input
`CHANNEL` to the given file.

`<OPEN "READ" "GUEST;FOO">` does the same in the ITS version.

### 11.2.2. OPEN-NR

`OPEN-NR` is the same as `OPEN`, except that the date and time of last
reference of the opened file are not changes.

### 11.2.3. CHANNEL (the SUBR)

`CHANNEL` is called exactly like `OPEN`, but it **always** return an
unopened `CHANNEL`, which can later be opened by `RESET` (below) just
as if it had once been open.

### 11.2.4. FILE-EXISTS?

`FILE-EXISTS?` tests for the existence of a file without creating a
`CHANNEL`, which occupies about a hundred machine words of storage. It
takes file-name arguments just like `OPEN` (but no *mode* argument)
and returns either T, \`\#FALSE (reason:string status:fix),

### 11.2.5. CLOSE

    <CLOSE channel>

closes *channel* and returns its argument, with its "state" changed to
"closed". If *channel* is for output, all buffered output is written
out first. No harm is done if *channel* is already `CLOSE`d.

### 11.2.6. CHANLIST

    <CHANLIST>

returns a `LIST` whose elements are all the currently open `CHANNEL`s.
The first two elements are usually `.INCHAN` and `.OUTCHAN` (see
below). A `CHANNEL` not referenced by anything except `<CHANLIST>`
will be `CLOSEd` during garbage collection.

### 11.2.7. INCHAN and OUTCHAN

The channel used by default for input `SUBR`s is the local value of
the `ATOM` `INCHAN`. The channel used by default for output SUBRs is
the local value of the `ATOM` `OUTCHAN`.

You can direct I/O to a `CHANNEL` by `SET`ting `INCHAN` or `OUTCHAN`
(remembering their old values somewhere), or by giving the `SUBR` you
with to use an argument of `TYPE` `CHANNEL`. (These actually have the
same effect, because `READ` binds `INCHAN` to an explicit argument,
and `PRINT` binds `OUTCHAN` similarly. Thus the `CHANNEL` being used
is available for `READ` macros (section 17.1), or by giving the `SUBR`
you wish to use an argument of `TYPE` `CHANNEL`. Thus the `CHANNEL`
being used is available for `READ` macros (section 17.1) and
`PRINTTYPE`s (section 6.4.4).)

By the way, a good trick for playing with `INCHAN` and `OUTCHAN`
values within a function is to use the `ATOM`s `INCHAN` and `OUTCHAN`
as `"AUX"` variables, re-binding their local values to the `CHANNEL`
you want. When you leave , of course, the old `LVAL`s are expanded
(which is the whole point). The `ATOM`s must be declared `SPECIAL`
(chapter 14) for this trick to compile correctly.

`INCHAN` and `OUTCHAN` also have global values, initially the
`CHANNEL`s directed at the terminal running `Muddle`. Initially,
`INCHAN`'s and `OUTCHAN`s local and global values are the same.

### 11.2.8. Contents of CHANNELs

The contents of an object of `TYPE` `CHANNEL` are referred to by the
I/O `SUBR`s each time such a `SUBR` is used. If you change the
contents of a `CHANNEL` (for example, with `PUT`), the next use of
that `CHANNEL` will be changed accordingly. Some elements of
`CHANNEL`s, however, should be played with seldom, if ever, and only
at your own peril. These are marked below with an `*` (asterisk).
Caveat user.

There follows a table of the contents of a `CHANNEL`, the `TYPE` of
each element, and an interpretation. The format used is the following:

*element-number: type interpretation*

  ----------------------------------------------------------------------------------------------------
  element-number             type               interpretation
  -------------------------- ------------------ ------------------------------------------------------
  -1                         `LIST`             transcript channel(s) (see below)

  \* 0                       varies             device-dependent information

  \* 1                       `FIX`              channel number (ITS) or JFN (Tenex and Tops-20), `0`
                                                for internal or closed

  \* 2                       `STRING`           mode

  \* 3                       `STRING`           first file name argument

  \* 4                       `STRING`           second file name argument

  \* 5                       `STRING`           device name argument

  \* 6                       `STRING`           directory name argument

  \* 7                       `STRING`           real first file name

  \* 8                       `STRING`           real second file name

  \* 9                       `STRING`           real device name

  \* 10                      `STRING`           real directory name

  \* 11                      `FIX`              various status bits

  \* 12                      `FIX`              PDP-10 instruction used to do one I/O operation

  13                         `FIX`              number of characters per line of output

  14                         `FIX`              current character position on a line

  15                         `FIX`              number of lines per page

  16                         `FIX`              current line number on a page

  17                         `FIX`              access pointer for file-oriented devices

  18                         `FIX`              radix for `FIX` conversion

  19                         `FIX`              sink for an internal `CHANNEL`
  ----------------------------------------------------------------------------------------------------

N.B.: The elements of a `CHANNEL` below number 1 are usually invisible
but are obtainable via `<NTH <TOP channel> fix>`, for some appropriate
*fix*.

The transcript-channels slot has this meaning: if this slot contains a
`LIST` of `CHANNEL`s, then anything input or output on the original
`CHANNEL` is output on these `CHANNEL`s. Caution: do not use a
`CHANNEL` as its own transcript channel; you probably won't live to
tell about it.

#### 11.2.8.2. Input CHANNELs

The contents of the elements up to number 12 of a `CHANNEL` used for
input are the same as that for output. The remaining elements are as
follows ((same) indicates that the use is the same as that for
output):

  element-number   type       interpretation
  ---------------- ---------- ---------------------------------------------------
  13               varies     object evaluated when end of file is reached
  \* 14            `FIX`      one "look-ahead" character, used by `READ`
  \* 15            `FIX`      PDP-10 instruction executed waiting for input
  16               `LIST`     queue of buffers for input from a terminal
  17               `FIX`      access pointer for file-oriented devices (same)
  18               `FIX`      radix for `FIX` conversion (same)
  19               `STRING`   buffer for input or source for internal `CHANNEL`

11.3. End-of-File "Routine"
---------------------------

As mentioned above, an explicit `CHANNEL` is the first optional
argument of all `SUBR`s used for conversion I/O. The second optional
argument for conversion-**input** `SUBR`s is an "end-of-file routine"
-- that is, something for the input `SUBR` to `EVAL` and return, if it
reaches the end of the file it is reading. A typical end-of-file
argument is a `QUOTE`d `FORM` which applies a function of yours. The
value of this argument used by default is a call to `ERROR`. Note: the
`CHANNEL` has been `CLOSE`d by the time this argument is evaluated.

Example: the following `FUNCTION` counts the occurrences of a
character in a file, according to its arguments. The file names,
device, and directory are optional, with the usual names used by
default.

    <DEFINE COUNT-CHAR
            (CHAR "TUPLE" FILE "AUX" (CNT 0) (CHN <OPEN "READ" !.FILE>))
        <COND (.CHN                 ;"If CHN is FALSE, bad OPEN: return the FALSE
                                    so result can be tested by another FUNCTION."
               <REPEAT ()
                    <AND <==? .CHAR <READCHR .CHN '<RETURN>>>
                         <SET CNT <+ 1 .CNT>>>>
                    ;"Until EOF, keep reading and testing a character at a time."
                .CNT                ;"Then return the count.")>>

11.4. Imaged I/O
----------------

### 11.4.1. Input

#### 11.4.1.1. READB

    <READB buffer:uvector-or-storage channel eof:any>

The *channel* must be open in `"READB"` mode. `READB` will read as
many 36-bit binary words as necessary to fill the *buffer* (whose
`UTYPE` must be of `PRIMTYPE` `WORD`), unless it hits the end of the
file. `READB` returns the number of words actually read, as a
`FIX`ed-point number. This will normally be the length of the
*buffer*, unless the end of file was read, in which case it will be
less, and only the beginning of *buffer* will have been filled
(`SUBSTRUC` may help). An attempt to `READB` again, after *buffer* is
not filled, will evaluate the end-of-file routine *eof*, which is
optional, a call to `ERROR` by default.

#### 11.4.1.2. READSTRING

    <READSTRING buffer:string channel stop:fix-or-string eof>

is the `STRING` analog to `READB`, where *buffer* and *eof* are as in
`READB`, and *channel* is any input `CHANNEL` (`.INCHAN` by default).
*stop* tells when to stop inputting: if a `FIX`, read this many
`CHARACTER`s (fill up *buffer* by default); if a `STRING`, stop
reading if any `CHARACTER` in this `STRING` is read (don't include
this `CHARACTER` in final `STRING`).

### 11.4.2. Output

#### 11.4.2.1. PRINTB

    <PRINTB buffer:uvector-or-storage channel>

This call writes the entire contents of the *buffer* into the
specified channel open in `"PRINTB"` or `"PRINTO"` mode. It returns
*buffer*.

#### 11.4.2.2. PRINTSTRING

    <PRINTSTRING buffer:string channel count:fix>

is analogous to `READSTRING`. It outputs *buffer* on *channel*, either
the whole thing or the first *count* characters, and returns the
number of characters output.

#### 11.4.2.3. IMAGE

    <IMAGE fix channel>

is a rather special-purpose `SUBR`. When any conversion-output routine
outputs an ASCII control character (with special exceptions like
carriage-returns, line-feeds, etc.), it actually outputs two
characters: `^` (circumflex), followed by the upper-case character
which has been control-shifted. `IMAGE`, on the other hand, always
outputs the real thing: that ASCII character whose ASCII 7-bit code is
*fix*. It is guaranteed not to give any gratuitous linefeeds or such.
*channel* is optional, `.OUTCHAN` by default, and its slots for
current character position (number 14) and current line number (16)
are not updated. `IMAGE` returns *fix*.

11.5 Dumped I/O
---------------

### 11.5.1. Output: GC-DUMP

    <GC-DUMP any printb:channel-or-false>

dumps *any* on *printb* in a clever format so that `GC-READ` (below)
can reproduce *any* exactly, including sharing. *any* cannot live on
the control stack, not can it be of `PRIMTYPE` `PROCESS` or `LOCD` or
`ASOC` (which see). *any* is returned as a value.

If *printb* is a `CHANNEL`, it must be open in `"PRINTB"` or
`"PRINTO"` mode. If *printb* is a `FALSE`, `GC-DUMP` instead returns a
`UVECTOR` (of `UTYPE` `PRIMTYPE` `WORD`) that contains what it would
have output on a `CHANNEL`. This `UVECTOR` can be `PRINTB`ed anywhere
you desire, but, if it is changed **in any way**, `GC-READ` will not
be able to input it. Probably the only reason to get it is to check
its length before output.

Except for the miniature garbage collection required, `GC-DUMP` is
about twice as fast as `PRINT`, but the amount of external storage
used is two or three times as much.

### 11.5.2. Input: GC-READ

    <GC-READ readb:channel eof:any>

returns one object from the *channel*, which must be open in `"READB"`
mode. The file must have been produced by `GC-DUMP`. *eof* is
optional. `GC-READ` is about ten times faster than `READ`.

11.6. SAVE Files
----------------

The entire state of Muddle can be saved away in a file for later
restoration: this is done with the `SUBR`s `SAVE` and `RESTORE`. This
is a very different form of I/O from any mentioned up to now; the file
used contains an actual image of your Muddle address space and is not,
in general, "legible" to other Muddle routines. `RESTORE`ing a `SAVE`
file is **much** faster than re-`READ`ing the objects it contains.

Since a `SAVE` file does not contain all extant Muddle objects, only
the impure and `PURIFY`ed (section 22.9.2) ones, a change to the
interpreter has the result of making all previous `SAVE` files
unusable. To prevent errors from arising from this, the interpreter
has a release number, which is incremented whenever changes are
installed. The current release number is printed out on initially
starting up the program and is available as the `GVAL` of the `ATOM`
`MUDDLE`. This release number is written out as the very first part of
each `SAVE` file. If `RESTORE` attempts to re-load a `SAVE` file whose
release number is not the same as the interpreter being used, an error
is produced. If desired, the release number of a `SAVE` file can be
obtained by doing a `READ` of that file. Only that initial `READ` will
work; the rest of the file is not ASCII.

### 11.6.1. SAVE

    <SAVE file-spec:string gc?:false-or-any>

or

    <SAVE name1 name2 device dir gc?:false-or-any>

saves the entire state of your Muddle away in the file specified by
its arguments, and then returns `"SAVED"`. All `STRING` arguments are
optional, with `"MUDDLE"`, `"SAVE"`, `"DSK"`, and `<VALUE SNM>` used
by default. *gc?* is optional and, if supplied and of `TYPE` `FALSE`,
causes no garbage collection to occur before `SAVE`ing. (`FSAVE` is an
alias for `SAVE` that may be seen in old programs.)

If, after restoring, `RESTORE` finds that `<VALUE SNM>` is the null
`STRING` (`""`), it will ask the operating system for the name of the
"working directory" and call `SNAME` with the result. This mechanism
is handy for "public" `SAVE` files, which should not point the user at
a particular disk directory.

In the ITS version, the file is actually written with the name
`_MUDS_ >` and renamed to the argument(s) only when complete, to
prevent losing a previous `SAVE` file if a crash occurs. In the Tenex
and Tops-20 versions, version/generation numbers provide the same
safety.

Example:

    <DEFINE SAVE-IT ("OPTIONAL"
                     (FILE '("PUBLIC" "SAVE" "DSK" "GUEST"))
                     "AUX" (SNM ""))
            <SETUP>
            <COND (<=? "SAVED" <SAVE !.FILE>>   ;"See below."
                   <CLEANUP>
                   "Saved.")
                  (T
                   <CRLF>
                   <PRINC "Amazing program at your service.">
                   <CRLF>
                   <START-RUNNING>)>>

### 11.6.2. RESTORE

    <RESTORE file-spec>

or

    <RESTORE name1 name2 device dir>

**replaces** the entire current state of your Muddle with that `SAVE`d
in the file specified. All arguments are optional, with the same
values used by default as by `SAVE`.

`RESTORE` completely replaces the contents of the Muddle, including
the state of execution existing when the `SAVE` was done and the state
of all open I/O `CHANNEL`s. If a file which was open when the `SAVE`
was done does not exist when the `RESTORE` is done, a message to that
effect will appear on the terminal.

A `RESTORE` **never** returns (unless it gets an error): it causes a
`SAVE` done some time ago to return **again** (this time with the
value `"RESTORED"`), even if the `SAVE` was done in the midst of
running a program. In the latter case, the program will continue its
execution upon `RESTORE`ation.

11.7. Other I/O Functions
-------------------------

### 11.7.1. LOAD

    <LOAD input:channel look-up>

eventually returns `"DONE"`. First, however, it `READ`s and `EVAL`s
every Muddle object in the file pointed to by *input*, and then
`CLOSE`s *input*. Any occurrences of `rubout`, `^@`, `^D`, `^L`, etc.,
in the file are given no special meaning; they are simply `ATOM`
constituents.

*look-up* is optional, used to specify a `LIST` of `OBLIST`s for the
`READ`. `.OBLIST` is used by default (chapter 15).

### 11.7.2. FLOAD

    <FLOAD file-spec look-up>

or

    <FLOAD name1 name2 device dir look-up>

("file load") acts just like `LOAD`, except that it takes arguments
(with values used by default) like `OPEN`, `OPEN`s the `CHANNEL`
itself for reading, and `CLOSE`s the `CHANNEL` when done. *look-up* is
optional, as in `LOAD`. If the `OPEN` fails, an error occurs, giving
the reason for failure.

### 11.7.3. SNAME

`<SNAME string>` ("system name", a hangover from ITS) is identical in
effect with `<SETG SNM string>`, that is, it causes *string* to become
the *dir* argument used by default by all `SUBR`s which want file
specifications (in the absence of a local value for `SNM`). `SNAME`
returns its argument.

`<SNAME>` is identical in effect with `<GVAL SNM>`, that is, it
returns the current *dir* used by default.

### 11.7.4. ACCESS

    <ACCESS channel fix>

returns *channel*, after making the next character or binary word
(depending on the mode of *channel*, which should not be `"PRINT"`)
which will be input from or output to *channel* the (*fix*+1)st one
from the beginning of the file. *channel* must be open to a randomly
accessible device (`"DSK"`, `"USR"`, etc.). A *fix* of `0` positions
*channel* at the beginning of the file.

### 11.7.5. FILE-LENGTH

    <FILE-LENGTH input:channel>

returns a `FIX`, the length of the file open on *input*. This
information is supplied by the operating system, and it may not be
available, for example, with the `"NET"` device (section 11.10). If
*input*'s mode is `"READ"`, the length is in characters (rounded up to
a multiple of five); if `"READB"`, in binary words. If `ACCESS` is
applied to *input* and this length or more, then the next input
operation will detect the end of file.

### 11.7.6. FILECOPY

    <FILECOPY input:channel output:channel>

copies characters from *input* to *output* until the end of file on
*input* (thus closing *input*) and returns the number of characters
copied. Both arguments are optional, with `.INCHAN` and `.OUTCHAN`
used by default, respectively. The operation is essentially a
`READSTRING` -- `PRINTSTRING` loop. Neither `CHANNEL` need be freshly
`OPEN`ed, and *output* need not be immediately `CLOSE`d. Restriction:
internally a `<FILE-LENGTH input>` is done, which must succeed; thus
`FILECOPY` might lose if *input* is a `"NET"` `CHANNEL`.

### 11.7.7. RESET

    <RESET channel>

returns *channel*, after "resetting" it. Resetting a `CHANNEL` is like
`OPEN`ing it afresh, with only the file-name slots preserved. For an
input `CHANNEL`, this means emptying all input buffers and, if it is a
`CHANNEL` to a file, doing an `ACCESS` to `0` on it. For an output
`CHANNEL`, this means returning to the beginning of the file -- which
implies, if the mode is not `"PRINTO"`, destroying any output done to
it so far. If the opening fails (for example, if the mode slot of
*channel* says input, and if the file specified in its real-name slots
does not exist), `RESET` (like `OPEN`) returns
`#FALSE (reason:string file-spec:string status:fix)`.

### 11.7.8. BUFOUT

    <BUFOUT output:channel>

causes all internal Muddle buffers for *output* to be written out and
returns its argument. This is helpful if the operating system or
Muddle is flaky and you want to attempt to minimize your losses. The
output may be padded with up to four extra spaces, if *output*'s mode
is `"PRINT"`.

### 11.7.9. RENAME

`RENAME` is for renaming and deleting files. It takes three kinds of
arguments:

-   (a) two file names, in either single- or multi-`STRING` format,
        separated by the `ATOM` `TO`,

-   (b) one file name in either format, or

-   (c) a `CHANNEL` and a file name in either format (only in the ITS
        version).

Omitted file-name parts use the same values by default as does `OPEN`.
If the operation is successful, `RENAME` returns `T`, otherwise
`#FALSE (reason:string status:fix)`.

In case (a) the file specified by the first argument is renamed to the
second argument. For example:

    <RENAME "FOO 3" TO "BAR">       ;"Rename FOO 3 to BAR >."

In case (b) the single file name specifies a file to be deleted. For
example:

    <RENAME "FOO FOO DSK:HARRY;">  ;"Rename FOO 3 to BAR >."

In case (c) the `CHANNEL` must be open in either `"PRINT"` or
`"PRINTB"` mode, and a rename while open for writing is attempted. The
real-name slots in the `CHANNEL` are updated to reflect any successful
change.

11.8. Terminal CHANNELs
-----------------------

Muddle behaves like the ITS version of the text editor Teco with
respect to typing in carriage-return, in that it automatically adds a
line-feed. In order to type in a lone carriage-return, a
carriage-return followed by a rubout must be typed. Also `PRINT`,
`PRINT1` and `PRINC` do not automatically add a line-feed when a
carriage-return is output. This enables overstriking on a terminal
that lacks backspacing capability. It also means that what goes on a
terminal and what goes in a file are more likely to look the same.

In the ITS version, Muddle's primary terminal output channel (usually
`,OUTCHAN`) is normally not in "display" mode, except when `PRINC`ing
a `STRING`. Thus errors will rarely occur when a user is typing in
text containing display-mode control codes.

In the ITS version, Muddle can start up without a terminal, give
control of the terminal away to an inferior operating-system process
or get it back while running. Doing a `RESET` on either of the
terminal channels causes Muddle to find out if it now has the
terminal; if it does, the terminal is reopened and the current screen
size and device parameters are updated. If it doesn't have the
terminal, an internal flag is set, causing output to the terminal to
be ignored and attempted input from the terminal to make the
operating-system process go to sleep.

In the ITS version, there are some peculiarities associated with
pseudo-terminals (`"STY"` and `"STn"` devices). If the `CHANNEL` given
to `READCHR` is open in `"READ"` mode to a pseudo-terminal, and if no
input is available, `READCHR` returns `-1`, `TYPE` `FIX`. If the
`CHANNEL` given to `READSTRING` is open in `"READ"` mode to a
pseudo-terminal, reading also stops if and when no more characters are
available, that is, when `READCHR` would return `-1`.

11.8.1. ECHOPAIR
----------------

    <ECHOPAIR terminal-in:channel terminal-out:channel>

returns its first argument, after making the two `CHANNEL`s "know
about each other" so that `rubout`, `^@`, `^D` and `^L` on
*terminal-in* will cause the appropriate output on *terminal-out*.

### 11.8.2. TTYECHO

    <TTYECHO terminal-input:channel pred>

turns the echoing of typed characters on *channel* off or on,
according to whether or not *pred* is `TYPE` `FALSE`, and returns
*channel*. It is useful in conjunction with `TYI` (below) for a
program that wants to do character input and echoing in its own
fashion.

### 11.8.3. TYI

    <TYI terminal-input:channel>

returns one `CHARACTER` from *channel* (optional, `.INCHAN` by
default) when it is typed, rather than after `$` (`ESC`) is typed, as
is the case with `READCHR`. The following example echos input
characters as their ASCII values, until a carriage-return is typed:

    <REPEAT ((FOO <TTYECHO .INCHAN <>>))
       <AND <==? 13 <PRINC <ASCII <TYI .INCHAN>>>>
            <RETURN <TTYECHO .INCHAN T>>>>

11.9. Internal CHANNELs
-----------------------

If the *device* specified in an `OPEN` is `"INT"`, a `CHANNEL` is
created which does not refer to any I/O device outside Muddle. In this
case, the mode must be `"READ"` or `"PRINT"`, and there is another
argument, which must be a function.

For a `"READ"` `CHANNEL`, the function must take no arguments.
Whenever a `CHARACTER` is desired from this `CHANNEL`, the function
will be applied to no arguments and must return a `CHARACTER`. This
will occur once per call to `READCHR` using this `CHANNEL`, and
several times per call to `READ`. In the ITS version, the function can
signal that its "end-of-file" has been reached by returning
`<CHTYPE *777777000003* CHARACTER>` (-1 in left half, control-C in
right), which is the standard ITS end-of-file signal. In the Tenex and
Tops-20 versions, the function should return either that or
`<CHTYPE *777777000032* CHARACTER>` (-1 and control-Z), the latter
being their standard end-of-file signal.

For a `"PRINT"` `CHANNEL`, the function must take one argument, which
will be a `CHARACTER`. It can dispose of its argument in any way it
pleases. The value returned by the function is ignored.

Example: `<OPEN "PRINT" "INT:" ,FCN>` opens an internal output
`CHANNEL` with `,FCN` as its character-gobbler.

11.10. The "NET" Device: the ARPA Network
-----------------------------------------

The `"NET"` device is different in many ways from conventional
devices. In the ITS version, it is the only device besides `"INT"`
that does not take all strings as its arguments to `OPEN`, and it must
take an additional optional argument to specify the byte size of the
socket. The format of a call to open a network socket is

    <OPEN mode:string local-socket:fix "NET" foreign-host:fix byte-size:fix>

where:

-   *mode* is the mode of the desired `CHANNEL`. This must be either
    `"READ"`, `"PRINT"`, `"READB"` or `"PRINTB"`.
-   *local-socket* is the local socket number. If it is `-1`, the
    operating system will generate a unique local socket number. If it
    is not, in the Tenex and Tops-20 versions, the socket number is
    "fork-relative".
-   *foreign-socket* is the foreign socket number. If it is `-1`, this
    is an `OPEN` for "listening".
-   *foreign-host* is the foreign host number. If it is an `OPEN` for
    listening, this argument is ignored.
-   *byte-size* is the optional byte size. For `"READ"` or `"PRINT"`
    this must be either `7` (used by default) or `8`. For `"READB"` or
    `"PRINTB"`, it can be any integer from `1` to `36` (used by
    default).

In the Tenex and Tops-20 versions, `OPEN` can instead be given a
`STRING` argument of the form `"NET:..."`. In this case the local
socket number can be "directory-relative".

Like any other `OPEN`, either a `CHANNEL` or a `FALSE` is returned.
Once open, a network `CHANNEL` can be used like any other `CHANNEL`,
except that `FILE-LENGTH`, `ACCESS`, `RENAME`, etc., cannot be done.
The "argument" first-name, second-name, and directory-name slots in
the `CHANNEL` are used for local socket, foreign socket, and foreign
host (as specified in the call to `OPEN`), respectively. The
corresponding "real" slots are used somewhat differently. If a channel
is `OPEN`ed with local socket `-1`, the "real" first-name slot will
contain the unique socket number generated by the operating system. If
a listening socket is `OPEN`ed, the foreign socket and host numbers of
the answering host are stored in the "real" second-name and
directory-name slots of the `CHANNEL` when the Request For Connection
is received.

An interrupt (chapter 21) can be associated with a `"NET"`-device
`CHANNEL`, so that a program will know that the `CHANNEL` has or needs
data, according to its *mode*.

There also exist several special-purpose `SUBR`s for the `"NET"`
device. These are described next.

### 11.10.1. NETSTATE

    <NETSTATE network:channel>

returns a `UVECTOR` of three `FIX`es. The first is the state of the
connection, the second is a code specifying why a connection was
closed, and the last is the number of bits available on the connection
for input. The meaning of the state and close codes are
installation-dependent and so are not included here.

### 11.10.2. NETACC

    <NETACC network:channel>

accepts a connection to a socket that is open for listening and
returns its argument. It will return a `FALSE` if the connection is in
the wrong state.

### 11.10.3. NETS

    <NETS network:channel>

returns its argument, after forcing any system-buffered network output
to be sent. ITS normally does this every half second anyway. Tenex and
Tops-20 do not do it unless and until `NETS` is called. `NETS` is
similar to `BUFOUT` for normal `CHANNEL`s, except that even
operating-system buffers are emptied **now**.

Chapter 12. Locatives
=====================

There is in Muddle a facility for obtaining and working directly with
objects which roughly correspond to "pointers" in assembly language or
"lvals" in BCPL or PAL. In Muddle, these are generically known as
**locatives** (from "location") and are of several `TYPE`s, as
mentioned below. Locatives exist to provide efficient means for
altering structures: direct replacement as opposed to re-copying.

Locatives **always** refer to elements in structures. It is not
possible to obtain a locative to something (for example, an `ATOM`)
which is not part of any structured. It is possible to obtain a
locative to any element in any structured object in Muddle -- even to
associations (chapter 13) and to the values of `ATOM`s, structurings
which are normally "hidden".

In the following, the object occupying the structured position to
which you have obtained a locative will be referred to as the object
**pointed to** by the locative.

12.1. Obtaining Locatives
-------------------------

### 12.1.1. LLOC

    <LLOC atom env>

returns a locative (`TYPE` `LOCD`, "locative to iDentifier") to the
`LVAL` of *atom* in *env*. If *atom* is not bound in *env*, an error
occurs. *env* is optional, with the current `ENVIRONMENT` used by
default. The locative returned by `LLOC` is **independent of future
re-bindings** of *atom*. That is, `IN` (see below) of that locative
will return the same thing even if *atom* is re-bound to something
else; `SETLOC` (see below) will affect only that particular binding of
*atom*.

Since bindings are kept on a stack (tra la), any attempt to use a
locative to an `LVAL` which has become unbound will fetch up an error.
(It breaks just like a `TUPLE`....) `LEGAL?` can, once again, be used
to see if a `LOCD` is valid. Caution: `<SET A <LLOC A>>` creates a
self-reference and can make `PRINT` very unhappy.

### 12.1.2. GLOC

    <GLOC atom pred>

returns a locative (`TYPE` `LOCD`) to the `GVAL` of *atom*. If *atom*
has no `GVAL` **slot**, an error occurs, unless *pred* (optional) is
given and not `FALSE`, in which case a slot is created (chapter 22).
Caution: `<SETG A <GLOC A>>` creates a self-reference and can make
`PRINT` very unhappy.

### 12.1.3. AT

    <AT structured N:fix-or-offset>

returns a locative to the <em>N</em>th element in *structured*. *N* is
optional, `1` by default. The exact `TYPE` of the locative returned
depends on the `PRIMTYPE` of *structured*: `LOCL` for `LIST`, `LOCV`
for `VECTOR`, `LOCU` for `UVECTOR`, `LOCS` for `STRING`, `LOCB` for
`BYTES`, `LOCT` for `TEMPLATE`, and `LOCA` for `TUPLE`. If *N* is
greater than `<LENGTH structured>` or less than `1`, or an `OFFSET`
with a Pattern that doesn't match *structured*, an error occurs. The
locative is unaffected by applications of `REST`, `BACK`, `TOP`,
`GROW`, etc. to *structured*.

### 12.1.4. GETPL and GETL

    <GETPL item:any indicator:any default:any>

returns a locative (`TYPE` `LOCAS`) to the association of *item* under
*indicator*. (See chapter 13 for information about associations.) If
no such association exists, `GETPL` returns `EVAL` of *default*.
*default* is optional, `#FALSE ()` by default.

`GETPL` corresponds to `GETPROP` amongst the association machinery.
There also exists `GETL`, which corresponds to `GET`, returning either
a `LOCAS` or a locative to the *indicator*th element of a structured
*item*. `GETL` is like `AT` if *item* is a structure and *indicator*
is a `FIX` or `OFFSET`, and like `GETPL` if not.

12.2. LOCATIVE?
---------------

This `SUBR` is a predicate that tells whether or not is argument is a
locative. It is cheaper than
`<MEMQ <PRIMTYPE arg> '![LOCD LOCL ...]>`.

12.3. Using Locatives
---------------------

The following two `SUBR`s provide the means for working with
locatives. They are independent of the specific `TYPE` of the
locative. The notation *locative* indicates anything which could be
returned by `LLOC`, `GLOC`, `AT`, `GETPL` or `GETL`.

### 12.3.1. IN

    <IN locative>

returns the object to which *locative* points. The only way you can
get an error using `IN` is when *locative* points to an `LVAL` which
has become unbound from an `ATOM`. This is the same as the problem in
referencing `TUPLE`s as mentioned in section 9.2, and it can be
avoided by first testing `<LEGAL? locd>`.

Example:

    <SET A 1>$
    1
    <IN <LLOC A>>$
    1

### 12.3.2. SETLOC

    <SETLOC locative any>

returns *any*, after having made *any* the contents of that position
in a structure pointed to by *locative*. The structure itself is not
otherwise disturbed. An error occurs if *locative* is to a
non-`LEGAL?` `LVAL` or if you try to put an object of the wrong `TYPE`
into a `PRIMTYPE` `UVECTOR`, `STRING`, `BYTES`, or `TEMPLATE`.

Example:

    <SET A (1 2 3)>$
    (1 2 3)
    <SETLOC <AT .A 2> HI>$
    HI
    .A$
    (1 HI 3)

12.4. Note on Locatives
-----------------------

You may have noticed that locatives are, strictly speaking,
unnecessary; you can do everything locatives allow by appropriate use
of, for example, `SET`, `LVAL`, `PUT`, `NTH`, etc. What locatives
provide is generality.

Basically, how you obtained a locative is irrelevant to `SETLOC` and
`IN`; thus the same program can play with `GVAL`s, `LVAL`s, object in
explicit structures, etc., without being bothered by what function it
should use to do so. This is particularly true with respect to
locatives to `LVAL`s; the fact that they are independent of changes in
binding can save a lot of fooling around with `EVAL` and
`ENVIRONMENT`s.

Chapter 13. Association (Properties)
====================================

There is an "associative" data storage and retrieval system embedded
in Muddle which allows the construction of data structures with
arbitrary selectors. It is used via the `SUBR`s described in this
chapter.

13.1. Associative Storage
-------------------------

### 13.1.1. PUTPROP

    <PUTPROP item:any indicator:any value:any>

("put property") returns *item*, having associated *value* with *item*
under the indicator *indicator*.

### 13.1.2. PUT

    <PUT item:any indicator:any value:any>

is identical to `PUTPROP`, except that, if *item* is structured
**and** *indicator* is of `TYPE` `FIX` or `OFFSET`, it does
`<SETLOC <AT item indicator> value>`. In other words, an element with
an integral selector is stored in the structure itself, instead of in
association space. `PUT` (like `AT`) will get an error if *indicator*
is out of range; `PUTPROP` will not.

### 13.1.3. Removing Associations

If `PUTPROP` is used **without** its *value* argument, it removes any
association existing between its *item* argument and its *indicator*
argument. If an association did exist, using `PUTPROP` in this way
returns the *value* which was associated. If no association existed,
it returns `#FALSE ()`.

`PUT`, with arguments which refer to association, can be used in the
same way.

If either *item* or *indicator* cease to exist (that is, no one was
pointing to them, so they were garbage-collected), and no locatives to
the association exist, then the association between them ceases to
exist (is garbage-collected).

13.2. Associative Retrieval
---------------------------

### 13.2.1. GETPROP

    <GETPROP item:any indicator:any exp:any>

("get property") returns the *value* associated with *item* under
*indicator*, if any. If there is no such association, `GETPROP`
returns `EVAL` of *exp* (that is, *exp* gets `EVAL`ed both at call
time and later).

*exp* is optional. If not given, `GETPROP` returns `#FALSE ()` if it
cannot return a *value*.

Note: *item* and *indicator* in `GETPROP` must be the **same Muddle
objects** used to establish the association; that is, they must be
`==?` to the objects used by `PUTPROP` or `PUT`.

### 13.2.2. GET

    <GET item:any indicator:any exp:any>

is the inverse of `PUT`, using `NTH` or `GETPROP` depending on the
test outlined in section 13.1.2. *exp* is optional and used as in
`GETPROP`.

13.3. Examples of Association
-----------------------------

    <SET L '(1 2 3 4)>$
    (1 2 3 4)
    <PUT .L FOO "L is a list.">$
    (1 2 3 4)
    <GET .L FOO>$
    "L is a list."
    <PUTPROP .L 3 '![4]>$
    (1 2 3 4)
    <GETPROP .L 3>$
    ![4!]
    <GET .L 3>$
    3
    <SET N 0>$
    0
    <PUT .N .L "list on a zero">$
    0
    <GET .N '(1 2 3 4)>$
    #FALSE ()

The last example failed because `READ` generated a new `LIST` -- not
the one which is `L`'s `LVAL`. However,

    <GET 0 .L>$
    "list on a zero"

works because `<==? .N 0>` is true.

To associate something with the Nth **position** in a structure, as
opposed to its Nth **element**, associate it with
`<REST structure N-1>`, as in the following:

    <PUT <REST .L 3> PERCENT 0.3>$
    (3 4)
    <GET <2 .L> PERCENT>$
    #FALSE ()
    <GET <REST .L 2> PERCENT>$
    0.30000000

Remember comments?

    <SET N '![A B C ;"third element" D E]>$
    ![A B C D E!]
    <GET <REST .N 2> COMMENT>$
    "third element"

The `'` in the `<SET N ... >` is to keep `EVAL` from generating a new
`UVECTOR` ("Direct Representation"), which would not have the comment
on it (and which would be a needless duplicate). A "top-level" comment
-- one attached to the entire object returned by `READ` -- is `PUT` on
the `CHANNEL` in use, since there is no position in any structure for
it. If no top-level comment follows the object, `READ` removes the
value (`<PUT channel COMMENT>`); so anybody that wants to see a
top-level comment must look for it after each `READ`.

If you need to have a structure with selectors in more than one
dimension (for example, a sparse matrix that does not deserve to be
linearized), associations can be cascaded to achieve the desired
result. In effect an extra level of indirection maps two indicators
into one. For example, to associate *value* with *item* under
*indicator-1* and *indicator-2* simultaneously:

    <PUTPROP indicator-1 indicator-2 T>
    <PUTPROP item <GETPL indicator-1 indicator-2> value>

13.4. Examining Associations
----------------------------

Associations (created by `PUT` and `PUTPROP`) are chained together in
a doubly-linked list, internal to Muddle. The order of associations in
the chain is their order of creation, newest first. There are several
`SUBR`s for examining the chain of associations. `ASSOCIATIONS`
returns the first association in the chain, or `#FALSE ()` if there
are none. `NEXT` takes an association as an argument and returns the
next association in the chain, or `#FALSE ()` if there are no more.
`ITEM`, `INDICATOR` and `AVALUE` all take an association as an
argument and return the item, indicator and value, respectively.
Associations print as:

    #ASOC (item indicator value)

(sic: only one `S`). Example: the following gathers all the existing
associations into a `LIST`.

    <PROG ((A <ASSOCIATIONS>))
     <COND (<NOT .A> '())
           (T (.A !<MAPF ,LIST
                    <FUNCTION () <COND (<SET A <NEXT .A>> .A)
                                       (T <MAPSTOP>)>>>))>>

Chapter 14. Data-type Declarations
==================================

In Muddle, it is possible to declare the permissible range of "types"
and/or structures that an `ATOM`'s values or a function's arguments or
value may have. This is done using a special `TYPE`, the `DECL`
("declaration"). A `DECL` is of `PRIMTYPE` `LIST` but has a
complicated internal structure. `DECL`s are used by the interpreter to
find `TYPE` errors in function calling and by the compiler to generate
more efficient code.

There are two kinds of `DECL`s. The first kind of `DECL` is the most
common. It is called the `ATOM` `DECL` and is used most commonly to
specify the type/structure of the `LVAL`s of the `ATOM`s in the
argument `LIST` of a `FUNCTION` or *aux* `LIST` of a `PROG` or
`REPEAT`. This `DECL` has the form:

    #DECL (atoms:list Pattern ...)

where the pairing of a `LIST` of `ATOM`s and a "Pattern" can be
repeated indefinitely. This declares the `ATOM`s in a *list* to be of
the type/structure specified in the following *Pattern*. The special
`ATOM` `VALUE`, if it appears, declares the result of a `FUNCTION`
call or `PROG` or `REPEAT` evaluation to satisfy the Pattern
specified. An `ATOM` `DECL` is useful in only one place: immediately
following the argument `LIST` of a `FUNCTION`, `PROG`, or `REPEAT`. It
normally includes `ATOM`s in the argument `LIST` and `ATOM`s whose
`LVAL`s are otherwise used in the Function body.

The second kind of `DECL` is rarely seen by the casual Muddle user,
except in appendix 2. It is called the `RSUBR` `DECL`. It is used to
specify the type/structure of the arguments and result of an `RSUBR`
or `RSUBR-ENTRY` (chapter 19). It is of the following form:

    #DECL ("VALUE" Pattern Pattern ...)

where the `STRING` `"VALUE"` precedes the specification of the
type/structure of the value of the call to the `RSUBR`, and the
remaining *Patterns* specify the arguments to the `RSUBR` in order.
The full specification of the `RSUBR` `DECL` will be given in section
14.9. The `RSUBR` `DECL` is useful in only one place: as an element of
an `RSUBR` or `RSUBR-ENTRY`.

14.1. Patterns
--------------

The simplest possible Pattern is to say that a value is exactly some
other object, by giving that object, `QUOTE`d. For example, to declare
that a variable is a particular `ATOM`:

    #DECL ((X) 'T)

declares that `.X` is always the `ATOM` `T`. When variables are
`DECL`ed as "being" some other object in this way, the test used is
`=?`, not `==?`. The distinction is usually not important, since
`ATOM`s, which are most commonly used in this construction, are `==?`
to each other is `=?` anyway.

It is more common to want to specify that a value must be of a given
`TYPE`. This is done with the simplest non-specific Pattern, a `TYPE`
name. For example,

    #DECL ((X) FIX (Y) FLOAT)

declares `.X` to be of `TYPE` `FIX`, and `.Y` of `TYPE` `FLOAT`. In
addition to the names of all of the built-in and created `TYPE`s, such
as `FIX`, `FLOAT` and `LIST`, a few "compound" type names are allowed:

-   `ANY` allows any `TYPE`.
-   `STRUCTURED` allows any structured `TYPE`, such as `LIST`,
    `VECTOR`, `FALSE`, `CHANNEL`, etc. (appendix 3).
-   `LOCATIVE` allows any locative `TYPE`, such as are returned by
    `LLOC`, `GLOC`, `AT`, and so on (chapter 12).
-   `APPLICABLE` allows any applicable `TYPE`, such as `FUNCTION`,
    `SUBR`, `FIX` (!), etc. (appendix 3).
-   Any other `ATOM` can be used to stand for a more complex
    construct, if an association is established on that `ATOM` and the
    `ATOM` `DECL`. A common example is to
    `<PUT NUMBER DECL '<OR FIX FLOAT>>` (see below), so that `NUMBER`
    can be used as a "compound type name".

The single `TYPE` name can be generalized slightly, allowing anything
of a given `PRIMTYPE`, using the following construction:

    #DECL ((X) <PRIMTYPE WORD> (Y) <PRIMTYPE LIST>)

This construction consists of a two-element `FORM`, where the first
element is the `ATOM` `PRIMTYPE`, and the second the name of a
primitive type.

The next step is to specify the elements of a structure. This is done
in the simplest way as follows:

    < structured:type Pattern Pattern ...>

where there is a one-to-one correspondence between the *Pattern* and
the elements of the structure. For example:

    #DECL ((X) <VECTOR FIX FLOAT>)

declares `.X` to be a `VECTOR` having **at least** two elements, the
first of which is a `FIX` and the second a `FLOAT`. It is often
convenient to allow additional elements, so that only the elements
being used in the local neighborhood of the `DECL` need to be
declared. To disallow additional elements, a `SEGMENT` is used instead
of a `FORM` (the "excl-ed" brackets make it look more emphatic). For
example:

    #DECL ((X) !<VECTOR FIX FLOAT>)

declares `.X` to be a `VECTOR` having **exactly** two elements, the
first of which is a `FIX` and the second a `FLOAT`. Note that the
*Patterns* given for elements can be any legal Pattern:

    #DECL ((X) <VECTOR <VECTOR FIX FLOAT>> (Y) <<PRIMTYPE LIST> LIST>)

declares `.X` to be a `VECTOR` containing another `VECTOR` of at least
two elements, and `.Y` to be of `PRIMTYPE LIST`, containing a `LIST`.
In the case of a `BYTES`, the individual elements cannot be declared
(they must be `FIX`es anyway), only the size and number of the bytes:

    #DECL ((B) <BYTES 7 3>)

declares `.B` to be a `BYTES` with `BYTE-SIZE` 7 and at least three
elements.

It is possible to say that some number of elements of a structure
satisfy a given Pattern (or sequence of Patterns). This is called an
"`NTH` construction".

    [ number:fix Pattern Pattern ... ]

states that the sequence of *Patterns* which is `REST` of the `VECTOR`
is repeated the *number* of times given. For example:

    #DECL ((X) <VECTOR [3 FIX] FLOAT> (Y) <LIST [3 FIX FLOAT]>)

`.X` is declared to contain three `FIX`es and a `FLOAT`, perhaps
followed by other elements. `.Y` is declared to repeat the sequence
`FIX`-`FLOAT` three times. Note that there may be more repetitions of
the sequence in `.Y` (but not in `.X`): the `DECL` specifies only the
first six elements.

For indefinite repetition, the same construction is used, but, instead
of the number of repetitions of the sequence of Patterns, the `ATOM`
`REST` is given. This allows any number of repetitions, from zero on
up. For example:

    #DECL ((X) <VECTOR [REST FIX]> (Y) <LIST [3 FIX] [REST FIX]>)

A "`REST` construction" can contain any number of Patterns, just like
an `NTH` construction:

    #DECL ((X) <VECTOR [REST FIX FLOAT LIST]>)

declares that `.X` is a `VECTOR` wherein the sequence
`FIX`-`FLOAT`-`LIST` repeats indefinitely. It does not declare that
`<LENGTH .X>` is an even multiple of three: the `VECTOR` can end at
any point.

A variation on `REST` is `OPT` (or `OPTIONAL`), which is similar to
`REST` except that the construction is scanned once at most instead of
indefinitely, and further undeclared elements can follow. For example:

    #DECL ((X) <VECTOR [OPT FIX]>)

declares that `.X` is a `VECTOR` which is empty or whose first element
is a `FIX`. Only a `REST` construction can follow an "`OPT`
construction".

Note that the `REST` construction must always be the last element of
the structure declaration, since it gives a Pattern for the rest of
the structure. Thus, the `REST` construction is different from all
others in that it has an unlimited range. No matter how many times the
Pattern it gives is `REST`ed off of the structure, the remainder of
the structure still has that Pattern.

This exhausts the possible single Patterns that can be given in a
declaration. However, there is also a compound Pattern defined. It
allows specification of several possible Patterns for one value:

    <OR Pattern Pattern ... >

Any non-compound Pattern can be included as one of the elements of the
compound Pattern. Finally, compound Patterns can be used as Patterns
for elements of structures, and so on.

    #DECL ((X) <OR FIX FLOAT>
           (Y) <OR FIX <UVECTOR [REST <OR FIX FLOAT>]>>)

The `OR` construction can be extended to any level of ridiculousness,
but the higher the level of complexity and compoundedness the less
likely the compiler will find the `DECL` useful.

At the highest level, any Pattern at top level in an `ATOM` `DECL` can
be enclosed in the construction

    < specialty:atom Pattern >

which explicitly declares the specialty of the `ATOM`(s) in the
preceding `LIST`. *specialty* can be either `SPECIAL` or `UNSPECIAL`.
Specialty is important only when the program is to be compiled. The
word comes from the control stack, which is called "special" in Lisp
(Moon, 1974) because the garbage collector finds objects on it and
modifies their internal pointers when storage is compacted. (An
internal stack is used within the interpreter and is not accessible to
programs -- section 22.1) In an interpreted program all local values
are inherently `SPECIAL`, because all bindings are put on the control
stack (but see `SPECIAL-MODE` below). When the program is compiled,
only values declared `SPECIAL` (which may or may not be the
declaration used by default) remain in bindings on the control stack.
All others are taken care of simply by storing objects on the control
stack: the `ATOM`s involved are not needed and are not created on
loading. So, a program that `SET`s an `ATOM`'s local value for another
program to pick up must declare that `ATOM` to be `SPECIAL`. If it
doesn't, the `ATOM`'s binding will go away during compiling, and the
program that needs to refer to the `ATOM` will either get a no-value
error or refer to an erroneous binding. Usually only `ATOM`s which
have the opposite specialty from that of the current `SPECIAL-MODE`
are explicitly declared. The usual `SPECIAL-MODE` is `UNSPECIAL`, so
typically only `SPECIAL` declarations use this construction:

    #DECL ((ACT)) <SPECIAL ACTIVATION>)

explicitly declares `ACT` to be `SPECIAL`.

Most well-written, modular programs get all their information from
their arguments and from `GVAL`s, and thus they rarely use `SPECIAL`
`ATOM`s, except perhaps for `ACTIVATION`s and the `ATOM`s whose
`LVAL`s Muddle uses by default: `INCHAN`, `OUTCHAN`, `OBLIST`, `DEV`,
`SNM`, `NM1`, `NM2`. `OUTCHAN` is a special case: the compiler thinks
that all conversion-output `SUBR`s are called with an explicit
`CHANNEL` argument, whether or not the program being compiled thinks
so. For example, `<CRLF>` is compiled as though it were
`<CRLF .OUTCHAN>`. So you may use (or see) the binding
`(OUTCHAN .OUTCHAN)` in an argument `LIST`, however odd that may
appear, because that -- coupled with the usual `UNSPECIAL` declaration
by default -- makes only one reference to the current binding of
`OUTCHAN` and stuffs the result in a slot on the stack for use within
the Function.

14.2. Examples
--------------

    #DECL ((Q) <OR VECTOR CHANNEL>)

declares .Q to be either a `VECTOR` or a `CHANNEL`.

    #DECL ((P Q R S) <PRIMTYPE LIST>)

declares `.P`, `.Q`, `.R`, and `.S` all to be of `PRIMTYPE` `LIST`.

    #DECL ((F) <FORM [3 ANY]>)

declares `.F` to be a `FORM` whose length is at least three,
containing objects of any old `TYPE`.

    #DECL ((LL) <<PRIMTYPE LIST> [4 <LIST [REST FIX]>]>)

declares `.LL` to be of `PRIMTYPE` `LIST`, and to have at least four
elements, each of which are `LIST`s of unspecified length (possibly
empty) containing `FIX`es.

    #DECL ((VV) <VECTOR FIX ATOM CHARACTER>)

declares `.VV` to be a `VECTOR` with at least three elements. Those
elements are, in order, of `TYPE` `FIX`, `ATOM`, and `CHARACTER`.

    #DECL ((EH) <LIST ATOM [REST FLOAT]>)

declares `.EH` to be a `LIST` whose first element is an `ATOM` and the
rest of whose elements are `FLOAT`s. It also says that `.EH` is at
least one element long.

    #DECL ((FOO) <LIST [REST 'T FIX]>)

declares `.FOO` to be a `LIST` whose odd-positioned elements are the
`ATOM` `T` and whose even-positioned elements are `FIX`es.

    <MAPR <>
          <FUNCTION (X)
            #DECL ((X) <VECTOR [1 FIX]>)
            <PUT .X 1 0>>
          .FOO>

declares `.X` to be a `VECTOR` containing at least one `FIX`. The more
restrictive `[REST FIX]` would take excessive checking time by the
interpreter, because the `REST` of the `VECTOR` would be checked on
each iteration of the `MAPR`. In this case both `DECL`s are equally
powerful, because checking the first element of all the `REST`s of a
structure eventually checks all the elements. Also, since the
`FUNCTION` refers only to the first element of `X`, this is as much
declaration as the compiler can effectively use. (If this `VECTOR`
always contains only `FIX`es, it should be a `UVECTOR` instead, for
space efficiency. Then a `[REST FIX]` `DECL` would make the
interpreter check only the `UTYPE`. If the `FIX`es cover a small
non-negative range, then a `BYTES` might be even better, with a `DECL`
of `<BYTES n 0>`.)

    <DEFINE FACT (N)
            #DECL ((N) <UNSPECIAL FIX>)
            <COND (<0? .N> 1) (ELSE <* .N <FACT <- .N 1>>>)>>

declares `.N` to be of `TYPE` `FIX` and `UNSPECIAL`. This specialty
declaration ensures that, independent of `SPECIAL-MODE` during
compiling, `.N` gets compiled into a fast control-stack reference.

    <PROG ((L (0))
            #DECL ((L VALUE) <UNSPECIAL <LIST [REST FIX]>>
                   (N <UNSPECIAL FIX>))
            <COND (<0? .N> <RETURN .L>)>
            <SET L (<+ .N <1 .L>> !.L)>
            <SET N <- .N 1>>>

The above declares `L` and `N` to be `UNSPECIAL`, says that `.N` is a
`FIX`, and says that `.L`, along with the value returned, is a `LIST`
of any length composed entirely of `FIX`es.

14.3. The DECL Syntax
---------------------

This section gives quasi-BNF productions for the Muddle `DECL` syntax.
In the following table Muddle type-specifiers are distinguished *in
this way*.

    decl    ::=     #DECL (declprs)

    declprs ::=     (atlist) pattern | declprs declprs

    atlist  ::=     atom | atom atlist

    pattern ::=     pat | <UNSPECIAL pat> | <SPECIAL pat>

    pat     ::=     unit | <OR unit ... unit>

    unit    ::=     type | <PRIMTYPE type> | atom | 'any
                    | ANY | STRUCTURED | LOCATIVE |APPLICABLE
                    | <struc elts> | <<OR struc ... struc> elts>
                    | !<struc elts> | !<<OR struc ... struc> elts>
                    | <bstruc fix> | <bstruc fix fix>
                    | !<bstruc fix fix>

    struc   ::=     structured-type | <PRIMTYPE structured-type>

    bstruc  ::=     BYTES | <PRIMTYPE BYTES>

    elts    ::=     pat | pat elts
                    | [fix pat ... pat]
                    | [fix pat ... pat] elts
                    | [opt pat ... pat] | [REST pat ... pat]
                    | [opt pat ... pat] [REST pat ... pat]

    opt     ::=     OPT | OPTIONAL

14.4. Good DECLs
----------------

There are some rules of thumb concerning "good" `DECL`s. A "good"
`DECL` is one that is minimally offensive to the `DECL`-checking
mechanism as the compiler, but that gives the maximum amount of
information. It is simple to state what gives offense to the compiler
and `DECL`-checking mechanism: complexity. For example, a large
compound `DECL` like:

    #DECL ((X) <OR FIX LIST UVECTOR FALSE>)

is a `DECL` that the compiler will find totally useless. It might as
well be `ANY`. The more involved the `OR`, the less information the
compiler will find useful in it. For example, if the function takes
`<OR LIST VECTOR UVECTOR>`, maybe you should really say `STRUCTURED`.
Also, a very general `DECL` indicates a very general program, which is
not likely to be efficient when compiled (of course there is a
trade-off here). Narrowing the `DECL` to one `PRIMTYPE` gives a great
gain in compiled efficiency, to one `TYPE` still more.

Another situation to be avoided is the ordinary large `DECL`, even if
it is perfectly straightforward. If you have created a structure which
has a very specific `DECL` and is used all over your code, it might be
better as a `NEWTYPE` (see below). The advantage of a `NEWTYPE` over a
large explicit `DECL` is twofold. First, the entire structure must be
checked only when it is created, that is, `CHTYPE`d from its
`PRIMTYPE`. As a full `DECL`, it is checked completely on entering
each function and on each reassignment of `ATOM`s `DECL`ed to be it.
Second, the amount of storage saved in the `DECL`s of `FUNCTION`s and
so on is large, not to mention the effort of typing in and keeping up
to date several instances of the full `DECL`.

14.5. Global DECLs
------------------

### 15.4.1. GDECL and MANIFEST

There are two ways to declare `GVAL`s for the `DECL`-checking
mechanism. These are through the `FSUBR` `GDECL` ("global
declaration") and the `SUBR` `MANIFEST`.

    <GDECL atoms:list Pattern ...>

`GDECL` allows the type/structure of global values to be declared in
much the same way as local values. Example:

    <GDECL (X) FIX (Y) <LIST FIX>>

declares `,X` to be a `FIX`, and `,Y` to be a `LIST` containing at
least one `FIX`.

    <MANIFEST atom atom ...>

`MANIFEST` takes as arguments `ATOM`s whose `GVAL`s are declared to be
constants. It is used most commonly to indicate that certain `ATOM`s
are the names of offsets in structures. For example:

    <SETG X 1>
    <MANIFEST X>

allows the compiler to confidently open-compile applications of `X`
(getting the first element of a structure), knowing that `,X` will not
change. Any sort of object can be a `MANIFEST` value: if it does not
get embedded in the compiled code, it is included in the `RSUBR`'s
"reference vector", for fast access. However, as a general rule,
structured objects should not be made `MANIFEST`: the `SETG` will
instead refer to a **distinct** copy of the object in **each** `RSUBR`
that does a `GVAL`. A structured object should instead be `GDECL`ed.

An attempt to `SETG` a `MANIFEST` atom will cause an error, unless
either:

1.  the `ATOM` was previously globally unassigned;
2.  the old value is `==?` to the new value; or
3.  `.REDEFINE` is not `FALSE`.

### 14.5.2. MANIFEST? and UNMANIFEST

    <MANIFEST? atom>

returns `T` if *atom* is `MANIFEST`, `#FALSE ()` otherwise.

    <UNMANIFEST atom atom ...>

removes the `MANIFEST` of the global value of each of its arguments so
that the value can be changed.

### 14.5.3. GBOUND?

    <GBOUND? atom>

("globally bound") returns `T` if *atom* has a global value slot (that
is, if it has ever been `SETG`ed, `MANIFEST`, `GDECL`ed, or `GLOC`ed
(chapter 12) with a true second argument), `#FALSE ()` otherwise.

14.6. NEWTYPE (again)
---------------------

`NEWTYPE` gives the programmer another way to `DECL` objects. The
third (and optional) argument of `NEWTYPE` is a `QUOTE`d Pattern. If
given, it will be saved as the value of an association (chapter 13)
using the name of the `NEWTYPE` as the item and the `ATOM` `DECL` as
the indicator, and it will be used to check any object that is about
to be `CHTYPE`d to the `NEWTYPE`. For example:

    <NEWTYPE COMPLEX-NUMBER VECTOR '<<PRIMTYPE VECTOR> FLOAT FLOAT>>

creates a new `TYPE`, with its first two elements declared to be
`FLOAT`s. If later someone types:

    #COMPLEX-NUMBER [1.0 2]

an error will result (the second element is not a `FLOAT`). The
Pattern can be replaced by doing another `NEWTYPE` for the same
`TYPE`, or by putting a new value in the association. Further
examples:

    <NEWTYPE FOO LIST '<<PRIMTYPE LIST> FIX FLOAT [REST ATOM]>>

causes `FOO`s to contain a `FIX` and a `FLOAT` and any number of
`ATOM`s.

    <NEWTYPE BAR LIST>

    <SET A #BAR (#BAR () 1 1.2 GRITCH)>

    <NEWTYPE BAR LIST '<<PRIMTYPE LIST> BAR [REST FIX FLOAT ATOM]>>

This is an example of a recursively `DECL`ed `TYPE`. Note that
`<1 .A>` does not satisfy the `DECL`, because it is empty, but it was
`CHTYPE`d before the `DECL` was associated with `BAR`. Now, even
`<CHTYPE <1 .A> <TYPE <1 .A>>>` will cause an error.

In each of these examples, the `<<PRIMTYPE ...> ...>` construction was
used, in order to permit `CHTYPE`ing an object into itself. See what
happens otherwise:

    <NEWTYPE OOPS LIST '<LIST ATOM FLOAT>>$
    OOPS
    <SET A <CHTYPE (E 2.71828) OOPS>>$
    #OOPS (E 2.71828)

Now `<CHTYPE .A OOPS>` will cause an error. Unfortunately, you must

    <CHTYPE <CHTYPE .A LIST> OOPS>$
    #OOPS (E 2.71828)

14.7. Controlling DECL Checking
-------------------------------

There are several `SUBR`s and `FSUBR`s in Muddle that are used to
control and interact with the `DECL`-checking mechanism.

### 14.7.1. DECL-CHECK

This entire complex checking mechanism can get in the way during
debugging. As a result, the most commonly used `DECL`-oriented `SUBR`
is `DECL-CHECK`. It is used to enable and disable the entire
`DECL`-checking mechanism.

    <DECL-CHECK false-or-any>

If its single argument is non-`FALSE`, `DECL` checking is turned on;
if it is `FALSE`, `DECL` checking is turned off. The previous state is
returned as a value. If no argument is given, `DECL-CHECK` returns the
current state. In an initial Muddle `DECL` checking is on.

When `DECL` checking is on, the `DECL` of an `ATOM` is checked each
time it is `SET`, the arguments and results of calls to `FUNCTION`s,
`RSUBR`s, and `RSUBR-ENTRY`s are checked, and the values returned by
`PROG` and `REPEAT` are checked. The same is done for `SETG`s and, in
particular, attempts to change `MANIFEST` global values. Attempts to
`CHTYPE` an object to a `NEWTYPE` (if the `NEWTYPE` has the optional
`DECL`) are also checked. When `DECL` checking is off, none of these
checks is performed.

### 14.7.2. SPECIAL-CHECK and SPECIAL-MODE

    <SPECIAL-CHECK false-or-any>

controls whether or not `SPECIAL` checking is performed at run time by
the interpreter. It is initially off. Failure to declare an `ATOM` to
be `SPECIAL` when it should be will produce buggy compiled code.

    <SPECIAL-MODE specialty:atom>

sets the declaration used by default (for `ATOM`s not declared either
way) and returns the previous such declaration, or the current such
declaration if no argument is given. The initial declaration used by
default is `UNSPECIAL`.

### 14.7.3. GET-DECL and PUT-DECL

`GET-DECL` and `PUT-DECL` are used to examine and change the current
`DECL` (of either the global or the local value) of an `ATOM`.

    <GET-DECL locd>

returns the `DECL` Pattern (if any, otherwise `#FALSE ()`) associated
with the global or local value slot of an `ATOM`. For example:

    <PROG (X)
          #DECL ((X) <OR FIX FLOAT>)
          ...
          <GET-DECL <LLOC X>>
          ...>

would return `<OR FIX FLOAT>` as the result of the application of
`GET-DECL`. Note that because of the use of `LLOC` (or `GLOC`, for
global values) the `ATOM` being examined must be bound; otherwise you
will get an error! This can be gotten around by testing first with
`BOUND?` (or `GBOUND?`, or by giving `GLOC` a second argument which is
not `FALSE`).

If the slot being examined is the global slot and the value is
`MANIFEST`, then the `ATOM` `MANIFEST` is returned. If the value being
examined is not `DECL`ed, `#FALSE ()` is returned.

    <PUT-DECL locd Pattern>

makes *Pattern* be the `DECL` for the value and returns *locd*. If
`<DECL-CHECK>` is true, the current value must satisfy the new
Pattern. `PUT-DECL` is normally used in debugging, to change the
`DECL` of an object to correspond to changes in the program. Note that
it is not legal to `PUT-DECL` a "Pattern" of `MANIFEST` or
`#FALSE ()`.

### 14.7.4. DECL?

    <DECL? any Pattern>

specifically checks *any* against *Pattern*. For example:

    <DECL? '[1 2 3] '<VECTOR [REST FIX]>>$
    T
    <DECL? '[1 2.0 3.0] '<VECTOR [REST FIX]>>$
    #FALSE ()

14.8. OFFSET
------------

An `OFFSET` is essentially a `FIX` with a Pattern attached, considered
as an `APPLICABLE` rather than a number. An `OFFSET` allows a program
to specify the type of structure that its `FIX` applies to. `OFFSET`s,
like `DECL`s -- if used properly -- can make debugging considerably
easier; they will eventually also help the compiler generate more
efficient code.

The `SUBR` `OFFSET` takes two arguments, a `FIX` and a Pattern, and
returns an object of `TYPE` and `PRIMTYPE` `OFFSET`. An `OFFSET`, like
a `FIX`, may be given as an argument to `NTH` or `PUT` and may be
applied to arguments. The only difference is that the `STRUCTURED`
argument must match the Pattern contained in the `OFFSET`, or an error
will result. Thus:

    <SETG FOO <OFFSET 1 '<CHANNEL FIX>>>$
    %<OFFSET 1 '<CHANNEL FIX>>
    <FOO ,INCHAN>$
    1
    <FOO <ROOT>>$
    *ERROR*
    ARG-WRONG-TYPE
    NTH
    LISTENING-AT-LEVEL 2 PROCESS 1

Note: when the compiler gets around to understanding `OFFSET`s, it
will not do the right thing with them unless they are `MANIFEST`.
Since there's no good reason not to `MANIFEST` them, this isn't a
problem.

The `SUBR` `INDEX`, given an `OFFSET`, returns its `FIX`:

    <INDEX ,FOO>$
    1

`GET-DECL` of an `OFFSET` returns the associated Pattern; `PUT-DECL`
of an `OFFSET` and a Pattern returns a new `OFFSET` with the same
`INDEX` as the argument, but with a new Pattern:

    <GET-DECL ,FOO>$
    <CHANNEL FIX>
    <PUT-DECL ,FOO OBLIST>$
    %<OFFSET 1 OBLIST>
    ,FOO$
    %<OFFSET 1 '<CHANNEL FIX>>

An `OFFSET` is not a structured object, as this example should make
clear.

14.9. The RSUBR DECL
--------------------

The `RSUBR` `DECL` is similar to the `ATOM` `DECL`, except that the
declarations are of argument positions and value rather than of
specific `ATOM`s. Patterns can be preceded by `STRING`s which further
describe the argument (or value).

The simplest `RSUBR` `DECL` is for an `RSUBR` or `RSUBR-ENTRY`
(chapter 19) which has all of its arguments evaluated and returns a
`DECL`ed value. For example:

    #DECL ("VALUE" FIX FIX FLOAT)

declares that there are two arguments, a `FIX` and a `FLOAT`, and a
result which is a `FIX`. While the `STRING` `"VALUE"` is not
constrained to appear at the front of the `DECL`, it does appear there
by custom. It need not appear at all, if the result is not to be
declared, but (again by custom) in this case it is usually declared
`ANY`.

If any arguments are optional, the `STRING` `"OPTIONAL"` (or `"OPT"`)
is placed before the Pattern for the first optional argument:

    #DECL ("VALUE" FIX FIX "OPTIONAL" FLOAT)

If any of the arguments is not to be evaluated, it is preceded by the
`STRING` `"QUOTE"`:

    #DECL ("VALUE" FIX "QUOTE" FORM)

declares one argument, which is not `EVAL`ed.

If the arguments are to be evaluated and gathered into a `TUPLE`, the
Pattern for it is preceded by the `STRING` `"TUPLE"`:

    #DECL ("VALUE" FIX "TUPLE" <TUPLE [REST FIX]>)

If the arguments are to be unevaluated and gathered into a `LIST`, or
if the calling `FORM` is the only "argument", the Pattern is preceded
by the appropriate `STRING`:

    #DECL ("VALUE" FIX "ARGS" LIST)

    #DECL ("VALUE" FIX "CALL" <PRIMTYPE LIST>)

In every case the special indicator `STRING` is followed by a Pattern
which describes the argument, even though it may sometimes produce
fairly ludicrous results, since the pattern for `"TUPLE"` always must
be a `TUPLE`; for `"ARGS"`, a `LIST`; and for `"CALL"`, a `FORM` or
`SEGMENT`.

Chapter 15. Lexical Blocking
============================

Lexical, or static, blocking is another means of preventing identifier
collisions in Muddle. (The first was dynamic blocking -- binding and
`ENVIRONMENT`s.) By using a subset of the Muddle lexical blocking
facilities, the "block structure" of such languages as Algol, PL/I,
SAIL, etc., can be simulated, should you wish to do so.

15.1. Basic Considerations
--------------------------

Since what follows appears to be rather complex, a short discussion of
the basic problem lexical blocking solves and Muddle's basic solution
will be given first.

`ATOM`s are identifiers. It is thus essential that whenever you type
an `ATOM`, `READ` should respond with the unique identifier you wish
to designate. The problem is that it is unreasonable to expect the
`PNAME`s of all `ATOM`s to be unique. When you use an `ATOM` `A` in a
program, do you mean the `A` you typed two minutes ago, the `A` you
used in another one of your programs, or the `A` used by some library
program?

Dynamic blocking (pushing down of `LVAL`s) solves many such problems.
However, there are some which it does not solve -- such as state
variables (whether they are impure or pure). Major problems with a
system having only dynamic blocking usually arise only when attempts
are made to share large numbers of significant programs among many
people.

The solution used in Muddle is basically as follows: `READ` must
maintain at least one table of `ATOM`s to guarantee any uniqueness.
So, Muddle allows many such tables and makes it easy for the user to
specify which one is wanted. Such a table is an object of `TYPE`
`OBLIST` ("object list"). All the complication which follows arises
out of a desire to provide a powerful, easily used method of working
with `OBLIST`s, with reasonable values used by default.

15.2. OBLISTs
-------------

An `OBLIST` is of `PRIMTYPE` `UVECTOR` with `UTYPE` `LIST`; the `LIST`
holds `ATOM`s. The `ATOM`s are ordered by a hash coding on their
`PNAME`s: each `LIST` is a hashing bucket.) What follows is
information about `OBLIST`s as such.

### 15.2.1. OBLIST Names

Every normally constituted `OBLIST` has a name. The name of an
`OBLIST` is an `ATOM` associated with the `OBLIST` under the indicator
`OBLIST`. Thus,

    <GETPROP oblist OBLIST>

or

    <GET oblist OBLIST>

returns the name of *oblist*.

Similarly, every name of an `OBLIST` is associated with its `OBLIST`,
again under the indicator `OBLIST`, so that

    <GETPROP oblist-name:atom OBLIST>

or

    <GET oblist-name:atom OBLIST>

returns the `OBLIST` whose name is *oblist-name*.

Since there is nothing special about the association of `OBLIST`s and
their names, the name of an `OBLIST` can be changed by the use of
`PUTPROP`, both on the `OBLIST` and its name. It is not wise to change
the `OBLIST` association without changing the name association, since
you are likely to confuse `READ` and `PRINT` terribly.

You can also use `PUT` or `PUTPROP` to remove the association between
an `OBLIST` and its name completely. If you want the `OBLIST` to go
away (be garbage collected), **and** you want to keep its name around,
this must be done: otherwise the association will force it to stay,
even if there are no other references to it. (If you have no
references to either the name or the `OBLIST` (an `ATOM` -- including
a `TYPE` name -- points to its `OBLIST`), both of them -- and their
association -- will go away without your having to remove the
association, of course.) It is not recommended that you remove the
name of an `OBLIST` without having it go away, since then `ATOM`s in
that `OBLIST` will `PRINT` the name as if they were in no `OBLIST` --
which is defeating the purpose of this whole exercise.

### 15.2.2. MOBLIST

    <MOBLIST atom fix>

("make oblist") creates and returns a new `OBLIST`, containing no
`ATOM`s, whose name is *atom*, unless there already exists an `OBLIST`
of that name, in which case it returns the existing `OBLIST`. *fix* is
the size of the `OBLIST` created -- the number of hashing buckets.
*fix* is optional (ignored if the `OBLIST` already exists), 13 by
default. If specified, *fix* should be a prime number, since that
allows the hashing to work better.

### 15.2.3. OBLIST?

    <OBLIST? atom>

returns `#FALSE ()` if *atom* is not in any `OBLIST`. If *atom* is in
an `OBLIST`, it returns that `OBLIST`.

15.3. READ and OBLISTs
----------------------

`READ` can be explicitly told to look up an `ATOM` in a particular
`OBLIST` by giving the `ATOM` a **trailer**. A trailer consists of the
characters `!-` (exclamation-point dash) following the `ATOM`,
immediately followed by the name of the `OBLIST`. For example,

    A!-OB

specifies the unique `ATOM` of `PNAME` `A` which is in the `OBLIST`
whose name is the `ATOM` `OB`.

Note that the name of the `OBLIST` must follow the `!-` with **no**
separators (like space, tab, carriage-return, etc.). There is a name
used by default (section 15.5) which types out and is typed in as
`!-`*separator*.

Trailers can be used recursively:

    B!-A!-OB

specified the unique `ATOM` of `PNAME` `B` which is in the `OBLIST`
whose name is the unique `ATOM` of `PNAME` `A` which is in the
`OBLIST` whose name is `OB`. (Whew!) The repetition is terminated by
the look-up and insertion described below.

If an `ATOM` with a given `PNAME` is not found in the `OBLIST`
specified by a trailer, a new `ATOM` with that `PNAME` is created and
inserted into that `OBLIST`.

If an `OBLIST` whose name is given in a trailer does not exist, `READ`
creates one, of length 13 buckets.

If trailer notation is not used (the "normal" case), and for an `ATOM`
that terminates a trailer, `READ` looks up the `PNAME` of the `ATOM`
in a `LIST` of `OBLIST`s, the `LVAL` of the `ATOM` `OBLIST` by
default. This look-up starts with `<1 .OBLIST>` and continues until
`.OBLIST` is exhausted. If the `ATOM` is not found. `READ` usually
inserts it into `<1 .OBLIST>`. (It is possible to force `READ` to use
a different element of the `LIST` of `OBLIST`s for new insertions. If
the `ATOM` `DEFAULT` is in that `LIST`, the `OBLIST` following that
`ATOM` will be used.)

15.4. PRIN1 and OBLISTs
-----------------------

When `PRINT` is given an `ATOM` to output, it outputs as little of the
trailer as is necessary to specify the `ATOM` uniquely to `READ`. That
is, if the `ATOM` is the **first** `ATOM` of that `PNAME` which `READ`
would find in its normal look-up in the current `.OBLIST`, no trailer
is output. Otherwise, `!-` is output and the name of the `OBLIST` is
recursively `PRIN1`ed.

Warning: there are obscure cases, which do not occur in normal
practice, for which the `PRINT` trailer does not terminate. For
instance, if an `ATOM` must have a trailer printed, and the name of
the `OBLIST` is an `ATOM` in that very same `OBLIST`, death. Any
similar case will also give `PRINT` a hernia.

15.5. Initial State
-------------------

In an initial Muddle, `.OBLIST` contains two `OBLIST`s. `<1 .OBLIST>`
initially contains no `ATOM`s, and `<2 .OBLIST>` contains all the
`ATOM`s whose `GVAL` are `SUBR`s or `FSUBR`s, as well as `OBLIST`,
`DEFAULT`, `T`, etc. It is difficult to lose track of the latter; the
specific trailer `!-`*separator* will **always** cause references to
that `OBLIST`. In addition, the `SUBR` `ROOT`, which takes no
arguments, always returns that `OBLIST`.

The name of `<ROOT>` is `ROOT`; this `ATOM` is in `<ROOT>` and would
cause infinite recursion were it not for the use of `!-`*separator*.
The name of the initial `<1 .OBLIST>` is `INITIAL` (really
`INITIAL!-`).

The `ATOM` `OBLIST` also has a `GVAL`. `,OBLIST` is initially the same
as `.OBLIST`; however, `,OBLIST` is not affected by the `SUBR`s used
to manipulate the `OBLIST` structure. It is instead used only when
errors occur.

In the case of an error, the current `.OBLIST` is checked to see if it
is "reasonable" -- that is, contains nothing of the wrong `TYPE`. (It
is reasonable, but not standard, for `.OBLIST` to be a single `OBLIST`
instead of a `LIST` of them.) If it is reasonable, that value stays
current. Otherwise, `OBLIST` is `SET` to `,OBLIST`. Note that changes
made to the `OBLIST`s on `,OBLIST` -- for example, new `ATOM`s added
-- remain. If even `,OBLIST` is unreasonable, `OBLIST` is `SET` and
`SETG`ed to its initial value. `<ERRET>` (section 16.4) always assumes
that `.OBLIST` is unreasonable.

Three other `OBLIST`s exist in a virgin Muddle: their names and
purposes are as follows:

`ERRORS!-` contains `ATOM`s whose `PNAME`s are used as error messages.
It is returned by `<ERRORS>`.

`INTERRUPTS!-` is used by the interrupt system (section 21.5.1). It is
returned by `<INTERRUPTS>`.

`MUDDLE!-` is used infrequently by the interpreter when loading
compiled programs to fix up references to locations within the
interpreter.

The pre-loading of compiled programs may create other `OBLIST`s in an
initialized Muddle (Lebling, 1979).

15.6. BLOCK and ENDBLOCK
------------------------

These `SUBR`s are analogous to **begin** and **end** in Algol, etc.,
in the way they manipulate static blocking (and in **no** other way.)

    <BLOCK look-up:list-of-oblists>

returns its argument after "pushing" the current `LVAL` of the `ATOM`
`OBLIST` and making its argument the current `LVAL`. You usually want
`<ROOT>` to be an element of *look-up*, normally its last.

    <ENDBLOCK>

"pops" the LVAL of the `ATOM` `OBLIST` and returns the resultant
`LIST` of `OBLIST`s.

Note that this "pushing" and "popping" of `.OBLIST` is entirely
independent of functional application, binding, etc.

15.7. SUBRs Associated with Lexical Blocking
--------------------------------------------

### 15.7.1. READ (again)

    <READ channel eof-routine look-up>

This is a fuller call to `READ`. *look-up* is an `OBLIST` or a `LIST`
of them, used as stated in section 15.3 to look up `ATOM`s and insert
them in `OBLIST`s. If not specified, `.OBLIST` is used. See also
section 11.1.1.1, 11.3, and 17.1.3 for other arguments.

### 15.7.2. PARSE and LPARSE (again)

    <PARSE string radix:fix look-up>

as was previously mentioned, applies `READ`'s algorithm to *string*
and returns the first Muddle object resulting. This **includes**
looking up prospective `ATOM`s on *look-up*, if given, or `.OBLIST`.
`LPARSE` can be called in the same way. See also section 7.6.6.2 and
17.1.3 for other arguments.

### 15.7.3. LOOKUP

    <LOOKUP string oblist>

returns the `ATOM` of `PNAME` *string* in the `OBLIST` *oblist*, if
there is such an `ATOM`; otherwise, it returns `#FALSE ()`. If
*string* would `PARSE` into an `ATOM` anyway, `LOOKUP` is faster,
although it looks in only one `OBLIST` instead of a `LIST` of them.

### 15.7.4. ATOM

    <ATOM string>

creates and returns a spanking new `ATOM` of `PNAME` *string* which is
guaranteed not to be on **any** `OBLIST`.

An `ATOM` which is not on any `OBLIST` is `PRINT`ed with a trailer of
`!-#FALSE ()`.

### 15.7.5. REMOVE

    <REMOVE string oblist>

removes the `ATOM` of `PNAME` *string* from *oblist* and returns that
ATOM. If there is no such `ATOM`, `REMOVE` returns `#FALSE ()`. Also,

    <REMOVE atom>

removes *atom* from its `OBLIST`, if it is on one. It returns *atom*
if it was on an `OBLIST`; otherwise it returns `#FALSE ()`.

### 15.7.6 INSERT

    <INSERT string-or-atom oblist>

creates an `ATOM` of `PNAME` *string*, inserts it into *oblist* and
returns it. If there is already an `ATOM` with the same `PNAME` as
*atom* in *oblist*, an error occurs. The standard way to avoid the
error and always get your *atom* is

    <OR <LOOKUP string oblist> <INSERT string oblist>>

As with `REMOVE`, `INSERT` can also take an `ATOM` as its first
argument; this `ATOM` must not be on any `OBLIST` -- it must have been
`REMOVE`d, or just created by `ATOM` -- else an error occurs. The
`OBLIST` argument is **never** optional. If you would like the new
`ATOM` to live in the `OBLIST` that `READ` would have chosen, you can
`<PARSE string>` instead.

### 15.7.7. PNAME

    <PNAME atom>

returns a `STRING` (newly created) which is *atom*'s `PNAME` ("printed
name"). If trailers are not needed, `PNAME` is much faster than
`UNPARSE` on *atom*. (In fact, `UNPARSE` has to go all the way through
the `PRINT` algorithm **twice**, the first time to see how long a
`STRING` is needed.)

### 15.7.8. SPNAME

`SPNAME` ("shared printed name") is identical to `PNAME`, except that
the `STRING` it returns shares storage with *atom* (appendix 1), which
is more efficient if the `STRING` will not be modified. `PUT`ting into
such a `STRING` will cause an error.

15.8. Example: Another Solution to the INC Problem
--------------------------------------------------

What follows is an example of the way `OBLIST`s are "normally" used to
provide "externally available" `ATOM`s and "local" `ATOM`s which are
not so readily available externally. Lebling (1979) describes a
systematic way to accomplish the same thing and more.

    <MOBLIST INCO 1>
            ;"Create an OBLIST to hold your external symbols.
            Its name is INCO!-INITIAL!- ."

    INC!-INCO
            ;"Put your external symbols into that OBLIST.
        If you have many, just write them successively."

    <BLOCK (<MOBLIST INCI!-INCO 1> <GET INCO OBLIST> <ROOT>)>
        ;"Create a local OBLIST, naming it INCI!-INCO, and set up
        .OBLIST for reading in your program. The OBLIST INCO is
        included in the BLOCK so that as your external symbols are
        used, they will be found in the right place. Note that the
        ATOM INCO is not in any OBLIST of the BLOCK; therefore,
        trailer notation of !-INCO will not work within the current
        BLOCK-ENDBLOCK pair."

    <DEFINE INC ;"INC is found in the INCO OBLIST."
        (A) ;"A is not found and is therefore put into INCI by
    READ."
        #DECL ((VALUE A) <OR FIX FLOAT>)
        <SET .A <+ ..A 1>>> ;"All other ATOMs are found in the
    ROOT."
    <ENDBLOCK>

This example is rather trivial, but it contains all of the issues, of
which there are three.

The first idea is that you should create two `OBLIST`s, one to hold
`ATOM`s which are to be known to other users (`INCO`), and the other
to hold internal `ATOM`s which are not normally of interest to other
(`INCI`). The case above has one `ATOM` in each category.

Second, `INCO` is explicitly used **without** trailers so that
surrounding `BLOCK` and `ENDBLOCK`s will have an effect on it. Thus
`INCO` will be in the `OBLIST` desired by the user; `INC` will be in
`INCO`, and the user can refer to it by saying `INC!-INCO`; `INCI`
will also be in `INCO`, and can be referred to in the same way;
finally, `A` is really `A!-INCI!-INCO`. The point of all this is to
structure the nesting of `OBLIST`s.

Finally, if for some reason (like saving storage space) you wish to
throw `INCI` away, you can follow the `ENDBLOCK` with

    <REMOVE "INCI" <GET INCO OBLIST>>

and thus remove all references to it. The ability to do such pruning
is one reason for structuring `OBLIST` references.

Note that, even after removing `INCI`, you can "get `A` back" -- that
is, be able to type it in -- by saying something of the form

    <INSERT <1 <1 ,INC!-INCO>> <1 .OBLIST>>

thereby grabbing `A` out of the structure of `INC` and re-inserting it
into an `OBLIST`. however, this resurrects the name collision caused
by `<INC!-INCO A>`.

Chapter 16. Errors, Frames, etc.
================================

16.1. LISTEN
------------

This `SUBR` takes any number of arguments. It first checks the `LVAL`s
of `INCHAN`, `OUTCHAN`, and `OBLIST` for reasonability and terminal
usability. In each case, if the value is unreasonable, the `ATOM` is
rebound to the corresponding `GVAL`, if reasonable, or to an invented
reasonable value. `LISTEN` then does `<TTYECHO .INCHAN T>` and
`<ECHOPAIR .INCHAN .OUTCHAN>`. Next, it `PRINT`s its arguments, then
`PRINT`s

    LISTENING-AT-LEVEL i PROCESS p

where *i* is an integer (`FIX`) which is incremented each time
`LISTEN` is called recursively, and *p* is an integer identifying the
`PROCESS` (chapter 20) in which the `LISTEN` was `EVAL`ed. `LISTEN`
then does `<APPLY <VALUE REP>>`, if there is one, and if it is
`APPLICABLE`. If not, it applies the `SUBR` `REP` (without making a
new `FRAME` -- see below). This `SUBR` drops into an infinite
`READ`-`EVAL`-`PRINT` loop, which can be left via `ERRET` (section
16.4).

The standard `LISTEN` loop has two features for getting a handle on
objects that you have typed in and Muddle has typed out. If the `ATOM`
`L-INS` has a local value that is a `LIST`, `LISTEN` will keep recent
inputs (what `READ` returns) in it, most recent first. Similarly, if
the `ATOM` `L-OUTS` has a local value that is a `LIST`, `LISTEN` will
keep recent outputs (what `EVAL` returns) in it, most recent first.
The keeping is done before the `PRINT`ing, so that `^S` does not
defeat its purpose. The user can decide how much to keep around by
setting the length of each `LIST`. Even if `L-OUTS` is not used, the
atom `LAST-OUT` is always `SET` to the last object returned by `EVAL`
in the standard `LISTEN` loop. Example:

    <SET L-INS (NEWEST NEWER NEW)>$
    (NEWEST NEWER NEW)
    .L-INS$
    (.L-INS NEWEST NEWER)
    <SET FOO 69>$
    69
    <SET FIXIT <2 .LINS>>   ;"grab the last input"$
    <SET FOO 69>
    .L-INS$
    (.L-INS <SET FIXIT <2 .L-INS>> <SET FOO 69>)
    <PUT .FIXIT 3 105>$
    <SET FOO 105>
    <EVAL .FIXIT>$
    105
    .L-INS$
    (.L-INS <EVAL .FIXIT> <PUT .FIXIT 3 105>)
    .FOO$
    105

16.2. ERROR
-----------

This `SUBR` is the same as `LISTEN`, except that (1) it generates an
interrupt (chapter 21), if enabled. and (2) it `PRINT`s `*ERROR*`
before `PRINT`ing its arguments.

When any `SUBR` or `FSUBR` detects an anomalous condition (for
example, its arguments are of the wrong `TYPE`), it calls `ERROR` with
at least two arguments, including:

1.  an `ATOM` whose `PNAME` describes the problem, normally from the
    `OBLIST` `ERRORS!-` (appendix 4),
2.  the `ATOM` that names the `SUBR` or `FSUBR`, and
3.  any other information of interest, and **then returns whatever the
    call to `ERROR` returns**. Exception: a few (for example `DEFINE`)
    will take further action that depends on the value returned. This
    nonstandard action is specified in the error message (first
    `ERROR` argument).

16.3. FRAME (the TYPE)

A `FRAME` is the object placed on a `PROCESS`'s control stack (chapter
20) whenever a `SUBR`, `FSUBR`, `RSUBR`, or `RSUBR-ENTRY` (chapter 19)
is applied. (These objects are herein collectively called
"Subroutines".) It contains information describing what was applied,
plus a `TUPLE` whose elements are the arguments to the Subroutine
applied. If any of the Subroutine's arguments are to be evaluated,
they will have been by the time the `FRAME` is generated.

A `FRAME` is an anomalous `TYPE` in the following ways:

1.  It cannot be typed in. It can be generated only by applying a
    Subroutine.
2.  It does not type out in any standard format, but rather as
    `#FRAME` followed by the `PNAME`of the Subroutine applied.

16.3.1. ARGS

    <ARGS frame>

("arguments") returns the argument `TUPLE` of *frame*.

16.3.2. FUNCT

    <FUNCT frame>

("function"} returns the `ATOM` whose `G/LVAL` is being applied in
*frame*.

16.3.3. FRAME (the SUBR)

    <FRAME frame>

returns the `FRAME` stacked **before** *frame* or, if there is none,
it will generate an error. The oldest (lowest) `FRAME` that can be
returned without error has a `FUNCT` of `TOPLEVEL`. If called with no
arguments, `FRAME` returns the topmost `FRAME` used in an application
of `ERROR` or `LISTEN`, which was bound by the interpreter to the
`ATOM` `LERR\ I-INTERRUPTS` ("last error").

16.3.4. Examples

Say you have gotten an error. You can now type at `ERROR`'s `LISTEN`
loop and get things `EVAL`ed. For example,

    <FUNCT <FRAME>>$
    ERROR
    <FUNCT <FRAME <FRAME>>>$
    the-name-of-the-Subroutine-which-called-ERROR:atom
    <ARGS <FRAME <FRAME>>>$
    the-arguments-to-the-Subroutine-which-called-ERROR:tuple

16.4. ERRET

    <ERRET any frame>

This `SUBR` ("error return") (1) causes the control stack to be
stripped down to the level of *frame*, and (2) **then** returns *any*.
The net result is that the application which generated *frame* is
forced to return *any*. Additional side effects that would have
happened in the absence of an error may not have happened.

The second argument to `ERRET` is optional, by default the `FRAME` of
the last invocation of `ERROR` or `LISTEN`.

If `ERRET` is called with **no** arguments, it drops you **all** the
way down to the **bottom** of the control stack -- **before** the
level-1 `LISTEN` loop -- and then calls `LISTEN`. As always, `LISTEN`
first ensures that Muddle is receptive.

Examples:

    <* 3 <+ a 1>>$
    *ERROR*
    ARG-WRONG-TYPE
    +
    LISTENING-AT-LEVEL 2 PROCESS 1
    <ARGS <FRAME <FRAME>>>$
    [a 1]
    <ERRET 5>$  ;"This causes the + to return 5."
    15      ;"finally returned by the *"

Note that when you are in a call to `ERROR`, the most recent set of
bindings is still in effect. This means that you can examine values of
dummy variables while still in the error state. For example,

    <DEFINE F (A "AUX" (B "a string"))
        #DECL ((VALUE) LIST (A) STRUCTURED (B) STRING)
        (.B <REST .A 2>)    ;"Return this LIST.">$
    F
    <F '(1)>$

    *ERROR*
    OUT-OF-BOUNDS
    REST
    LISTENING-AT-LEVEL 2 PROCESS 1
    .A$
    (1)
    .B$
    "a string"
    <ERRET '(5)>    ; "Make the REST return (5)."$
    ("a string" (5))

16.5. RETRY

    <RETRY frame>

causes the control stack to be stripped down just beyond *frame*, and
then causes the Subroutine call that generated *frame* to be done
again. *frame* is optional, by default the `FRAME` of the last
invocation of `ERROR` or `LISTEN`. `RETRY` differs from `AGAIN` in
that (1) it is not intended to be used in programs; (2) it can retry
any old *frame* (any Subroutine call), whereas `AGAIN` requires an
`ACTIVATION` (`PROG` or `REPEAT` or `"ACT"`); and (3) if it retries
the `EVAL` of a `FORM` that makes an `ACTIVATION`, it will cause
rebinding in the argument `LIST`, thus duplicating side effects.

16.6. UNWIND

`UNWIND` is an `FSUBR` that takes two arguments, usually `FORM`s. It
`EVAL`s the first one, and, if the `EVAL` returns normally, the value
of the `EVAL` call is the value of `UNWIND`. If, however, during the
`EVAL` a non-local return attempts to return below the `UNWIND`
`FRAME` in the control stack, the second argument is `EVAL`ed, its
value is ignored, and the non-local return is completed. The second
argument is evaluated in the environment that was present when the
call to `UNWIND` was made. This facility is useful for cleaning up
data bases that are in inconsistent states and for closing temporary
`CHANNEL`s that may be left around. `FLOAD` sets up an `UNWIND` to
close its `CHANNEL` if the user attempts to `ERRET` without finishing
the `FLOAD`. Example:

    <DEFINE CLEAN ACT ("AUX" (C <OPEN "READ" "A FILE">))
        #DECL ((C) <OR CHANNEL FALSE> ...)
        <COND (.C
            <UNWIND <PROG () ... <CLOSE .C>>
                <CLOSE .C>>)>>

16.7. Control-G (\^G)

Typing control-G (`^G`, `<ASCII 7>`) at Muddle causes it to act just
as if an error had occurred in whatever was currently being done. You
can then examine the values of variables as above, continue by
applying `ERRET` to one argument (which is ignored), `RETRY` a `FRAME`
lower on the control stack, or flush everything by applying `ERRET` to
no arguments.

16.8. Control-S (\^S)

Typing control-S (`^S`, `<ASCII 19>`) at Muddle causes it to stop what
is happening and return to the `FRAME` `.LERR\ !-INTERRUPTS`,
returning the `ATOM` `T`. (In the Tenex and Tops-20 versions, `^O`
also has the same effect.)

16.9. OVERFLOW

    <OVERFLOW false-or-any>

There is one error that can be disabled: numeric overflow and
underflow caused by the arithmetic `SUBR`s (`+`, `-`, `*`, `/`). The
`SUBR` `OVERFLOW` takes one argument: if it is of `TYPE` `FALSE`,
under/overflow errors are disabled; otherwise they are enabled. The
initial state is enabled. `OVERFLOW` returns `T` or `#FALSE ()`,
reflecting the previous state. Calling it with no argument returns the
current state.

Chapter 17. Macro-operations
============================

17.1. READ Macros
-----------------

### 17.1.1. % and %%

The tokens `%` and `%%` are interpreted by `READ` in such a way as to
give a "macro" capability to Muddle similar to PL/I's.

Whenever `READ` encounters a single `%` -- anywhere, at any depth of
recursion -- it **immediately**, without looking at the rest of the
input, evaluates the object following the `%`. The result of that
evaluation is used by `READ` in place of the object following the `%`.
That is, `%` means "don't really `READ` this, use `EVAL` of it
instead." `%` is often used in files in front of calls to `ASCII`,
`BITS` (which see), etc., although when the `FUNCTION` is compiled the
compiler will do the evaluation if the arguments are constant. Also
seen is `%.INCHAN`, read as the `CHANNEL` in use during `LOAD` or
`FLOAD`; for example, `<PUT %.INCHAN 18 8>` causes succeeding `FIX`es
to be read as octal.

Whenever `READ` encounters `%%`, it likewise immediately evaluates the
object following the `%%`. However, it completely ignores the result
of that evaluation. Side effects of that evaluation remain, of course.

Example:

    <DEFINE SETUP () <SET A 0>>$
    SETUP
    <DEFINE NXT () <SET A <+ .A 1>>>$
    NXT
    [%%<SETUP> %<NXT> %<NXT> (%%<SETUP>) %<NXT>]$
    [1 2 () 1]

### 17.1.2. LINK

    <LINK exp:any string oblist>

creates an object of `TYPE` `LINK`, `PRIMTYPE` `ATOM`. A `LINK` looks
vaguely like an `ATOM`; it has a `PNAME` (the *string* argument),
resides in an `OBLIST` (the *oblist* argument) and has a "value" (the
*exp* argument). A `LINK` has the strange property that, whenever it
is encountered by `READ` (that is, its `PNAME` is read, just like an
`ATOM`, possibly with `OBLIST` trailers), `READ` substitutes the
`LINK`'s "value" for the `LINK` immediately. The effect of `READ`ing a
`LINK`'s `PNAME` is exactly the same as the effect of reading its
"value".

The *oblist* argument is optional, `<1 .OBLIST>` by default. `LINK`
returns its first argument. The `LINK` is created via `INSERT`, so an
error results if there is already an `ATOM` or `LINK` in *oblist* with
the same `PNAME`.

The primary use of `LINK`s is in interactive work with Muddle:
expressions which are commonly used, but annoyingly long to type, can
be "linked" to `PNAME`s which are shorter. The standard example is the
following:

    <LINK '<ERRET> "^E" <ROOT>>

which links the `ATOM` of `PNAME` `^E` in the `ROOT` `OBLIST` to the
expression `<ERRET>`.

### 17.1.3. Program-defined Macro-characters

During `READ`ing from an input `CHANNEL` or `PARSE`ing a `STRING`, any
character can be made to have a special meaning. A character can cause
an arbitrary routine to be invoked, which can then return any number
of elements to be put into the object being built by `READ`, `PARSE`,
or `LPARSE`. Translation of characters is also possible. This facility
was designed for those persons who want to use Muddle `READ` to do
large parts of their input but have to modify its actions for some
areas: for example, one might want to treat left and right parentheses
as tokens, rather than as delimiters indicating a `LIST`.

#### 17.1.3.1. READ (finally)

Associated with `READ` is an `ATOM`, `READ-TABLE!-`, whose local
value, if any, must be a `VECTOR` of elements, one for each character
up to and including all characters to be treated specially. Each
element indicates, if not `0`, the action to be taken upon `READ`'s
encounter with that character. A similar `VECTOR`, the local value of
`PARSE-TABLE!-`, if any, is used to find the action to take for
characters encountered when `PARSE` or `LPARSE` is applied to a
`STRING`.

These tables can have up to 256 elements, one for each ASCII character
and one for each possible exclamation-point/ASCII-character pair. In
Muddle, the exclamation-point is used as a method of expanding the
ASCII character set, and an exclamation-point/character pair is
treated as one logical character when not reading a `STRING`.

The element corresponding to a character is
`<NTH table <+ 1 <ASCII char>>>`. The element corresponding to an
exclamation-point/ASCII-character pair is
`<NTH table <+ 129 <ASCII char>>>`. The table can be shorter than 256
elements, in which case it is treated as if it were 256 long with `0`
elements beyond its actual length.

An element of the tables must satisfy one of the following `DECL`
Patterns:

> `'0` indicates that no special action is to be taken when this
> character is encountered.
>
> `CHARACTER` indicates that the encountered character is to be
> translated into the given `CHARACTER` whenever it appears, except
> when as an object of `TYPE` `CHARACTER`, or in a `STRING`, or
> immediately following a `\`.
>
> `FIX` indicates that the character is to be given the same treatment
> as the character with the ASCII value of the `FIX`. This allows you
> to cause other characters to be treated in the same way as A-Z for
> example. The same exceptions apply as for a `CHARACTER`.
>
> `<LIST FIX>` indicates the same thing, except that the character
> does not by itself cause a break. Therefore, if it occurs when
> reading an `ATOM` or number, it will be treated as part of that
> `ATOM` or number.
>
> `APPLICABLE` (to one argument) indicates that the character is to be
> a break character. Whenever it is encountered, the reading of the
> current object is finished, and the corresponding element of the
> table is `APPLY`ed to the ASCII `CHARACTER`. (If `READ` is called
> during the application, the end-of-file slot of the `CHANNEL`
> temporarily contains a special kind of `ACTIVATION` (`TYPE` `READA`)
> so that end-of-file can be signalled properly to the original
> `READ`. Isn't that wonderful?) The value returned is taken to be
> what was read, unless an object of `TYPE` `SPLICE` is returned. If
> so, the elements of this object, which is of `PRIMTYPE` `LIST`, are
> spliced in at the point where Muddle is reading. An empty `SPLICE`
> allows one to return nothing. If a structured object is not being
> built, and a `SPLICE` is returned, elements after the first will be
> ignored. A `SPLICE` says "expand me", whereas the structure
> containing a `SEGMENT` says "I will expand you".
>
> `<LIST APPLICABLE>` indicates the same thing, except that the
> character does not by itself cause a break. Therefore, if it occurs
> when reading an `ATOM` or number, it will be treated as part of that
> `ATOM` or number.

`READ` takes an additional optional argument, which is what to use
instead of the local value of the `ATOM` `READ-TABLE` as the `VECTOR`
of read-macro characters. If this argument is supplied, `READ-TABLE`
is rebound to it within the call to `READ`. `READ` takes from zero to
four arguments. The fullest call to `READ` is thus:

    <READ channel eof-routine look-up read-table:vector>

The other arguments are explained in sections 11.1.1.1, 11.3, and
15.7.1.

`ERROR` and `LISTEN` rebind `READ-TABLE` to the `GVAL` of
`READ-TABLE`, if any, else `UNASSIGN` it.

#### 17.1.3.2. Examples

Examples of each of the different kinds of entries in macro tables:

    <SET READ-TABLE <IVECTOR 256 0>>$
    [...]

    <PUT .READ-TABLE <+ 1 <ASCII !\a>> !\A>
                    ;"CHARACTER: translate a to A."$
    [...]
    abc$
    Abc

    <PUT .READ-TABLE <+ 1 <ASCII !\%>> <ASCII !\A>>
            ;"FIX: make % just a normal ASCII character."$
    [...]
    A%BC$
    A\%BC

    <PUT .READ-TABLE <+ 1 <ASCII !\.>> (<ASCII !\.>)>
            ;"<LIST FIX>: make comma no longer a break
              character, but still special if at a break."$
    [...]
    A,B$
    A\,B
    ;"That was an ATOM with PNAME A,B ."
    ',B$
    ,B
    ;"That was the FORM <GVAL B> ."

    <PUT .READ-TABLE <+ 1 <ASCII !\:>>
        #FUNCTION ((X) <LIST COLON <READ>>)>
            ;"APPLICABLE: make a new thing like ( < and [ ."$
    [...]
    B:A$
    B
    (COLON A)
    :::FOO$
    (COLON (COLON (COLON FOO)))

    <PUT .READ-TABLE <+ 1 <ASCII !\:>>
        '(#FUNCTION ((X) <LIST COLON <READ>>))>
            ;"<LIST APPLICABLE>: like above, but not a break
              now."$
    [...]
    B:A$
    B:A
    ;"That was an ATOM."
    :::FOO$
    (COLON (COLON (COLON FOO)))

#### 17.1.3.3. PARSE and LPARSE (finally)

    <PARSE string radix look-up parse-table:vector look-ahead:character>

is the fullest call to `PARSE`. `PARSE` can take from zero to five
arguments. If `PARSE` is given no arguments, it returns the first
object parsed from the local value of the `STRING` `PARSE-STRING` and
additionally `SET`s `PARSE-STRING` to the `STRING` having those
`CHARACTER`s which were parsed `REST`ed off. If `PARSE` is given a
`STRING` to parse, the `ATOM` `PARSE-STRING` is rebound to the
`STRING` within that call. If the *parse-table* argument is given to
`PARSE`, `PARSE-TABLE` is rebound to it within that call to `PARSE`.
Finally, `PARSE` can take a *look-ahead* `CHARACTER`, which is treated
as if it were logically concatenated to the front of the *string*
being parsed. Other arguments are described in sections 7.6.6.2 and
15.7.2.

`LPARSE` is exactly like `PARSE`, except that it tries to parse the
whole `STRING`, returning a `LIST` of the objects created.

17.2. EVAL Macros
-----------------

An `EVAL` macro provides the convenience of a `FUNCTION` without the
overhead of calling, `SPECIAL`s, etc. in the **compiled** version. A
special-purpose function that is called often by `FUNCTION`s that will
be compiled is a good candidate for an `EVAL` macro.

### 17.2.1. DEFMAC and EXPAND

`DEFMAC` ("define macro") is syntactically exactly the same as
`DEFINE`. However, instead of creating a `FUNCTION`, `DEFMAC` creates
a `MACRO`. A `MACRO` is of `PRIMTYPE` `LIST` and in fact has a
`FUNCTION` (or other `APPLICABLE` `TYPE`) as its single element.

A `MACRO` can itself be applied to arguments. A `MACRO` is applied in
a funny way, however: it is `EVAL`ed twice. The first `EVAL` causes
the `MACRO`'s element to be applied to the `MACRO`'s arguments.
Whatever that application returns (usually another `FORM`) is also
`EVAL`ed. The result of the second `EVAL`uation is the result of
applying the `MACRO`. `EXPAND` is used to perform the first `EVAL`
without the second.

To avoid complications, the first `EVAL` (by `EXPAND`, to create the
object to be `EVAL`ed the second time around) is done in a top-level
environment. The result of this policy is that two syntactically
identical invocations of a `MACRO` always return the same expansion to
be `EVAL`ed in the second step. The first `EVAL` generates two extra
`FRAME`s: one for a call to `EXPAND`, and one for a call to `EVAL` the
`MACRO` application in a top-level environment.

Example:

    <DEFMAC INC (ATM "OPTIONAL" (N 1))
            #DECL ((VALUE) FORM (ATM) ATOM (N) <OR FIX FLOAT>)
            <FORM SET .ATM <FORM + <FORM LVAL .ATM> .N>>>$
    INC
    ,INC$
    #MACRO (#FUNCTION ((ATM "OPTIONAL" (N 1)) ...))
    <SET X 1>$
    1
    <INC X>$
    2
    .X$
    2
    <EXPAND '<INC X>>$
    <SET X <+ .X 1>>

Perhaps the intention is clearer if `PARSE` and `%` are used:

    <DEFMAC INC (ATM "OPTIONAL" (N 1))
            #DECL (...)
            <PARSE "<SET %.ATM <+ %.ATM %.N>>">>

`MACRO`s really exhibit their advantages when they are compiled. The
compiler will simply cause the first `EVAL`uation to occur (via
`EXPAND`) and compile the result. The single element of a compiled
`MACRO` is an `RSUBR` or `RSUBR-ENTRY`.

### 17.2.2. Example

Suppose you want to change the following simple `FUNCTION` to a
`MACRO`:

    <DEFINE DOUBLE (X) #DECL ((X) FIX) <+ .X .X>>

You may be tempted to write:

    <DEFMAC DOUBLE (X) #DECL ((X) FIX) <FORM + .X .X>>

This `MACRO` works, but only when the argument does not use temporary
bindings. Consider

    <DEFINE TRIPLE (Y) <+ .Y <DOUBLE .Y>>>

If this `FUNCTION` is applied, the top-level binding of `Y` is used,
not the binding just created by the application. Compilation of this
`FUNCTION` would probably fail, because the compiler probably would
have no top-level binding for `Y`. Well, how about

    <DEFMAC DOUBLE ('X) <FORM + .X .X>>  ;"The DECL has to go."

Now this is more like the original `FUNCTION`, because no longer is
the argument evaluated and the result evaluated again. And `TRIPLE`
works. But now consider

    <DEFINE INC-AND-DOUBLE (Y) <DOUBLE <SET Y <+ 1 .Y>>>>

You might hope that

    <INC-AND-DOUBLE 1> -> <DOUBLE <SET Y <+ 1 1>>>
                       -> <DOUBLE 2>
                       -> <+ 2 2>
                       -> 4

But, when `DOUBLE` is applied to that `FORM`, the argument is
`QUOTE`d, so:

    <INC-AND-DOUBLE 1> -> <DOUBLE <SET Y <+ 1 1>>>
                       -> <FORM + <SET Y <+ 1 .Y>> <SET Y <1 .Y>>>
                       -> <+ 2 3>
                       -> 5

So, since the evaluation of `DOUBLE`'s argument has a side effect, you
should ensure that the evaluation is done exactly once, say by `FORM`:

    <DEFMAC DOUBLE ('ANY)
            <FORM PROG ((X .ANY)) #DECL ((X) FIX) '<+ .X .X>>>

As a bonus, the `DECL` can once more be used.

This example is intended to show that writing good `MACRO`s is a
little trickier than writing good `FUNCTION`s. But the effort may be
worthwhile if the compiled program must be speedy.

Chapter 18. Machine Words and Bits
==================================

The Muddle facility for dealing with uninterpreted machine words and
bits involves two data TYPEs: WORD and BITS. A WORD is simply an
uninterpreted machine word, while a BITS is a "pointer" to a set of
bits within a WORD. Operating on WORDs is usually done only when
compiled programs are used (chapter 19).

18.1. WORDs
-----------

A `WORD` in Muddle is a PDP-10 machine word of 36 bits. A `WORD`
always `PRINT`s in "\# format", and its contents are always printed in
octal (hence preceded and followed by `*`). Examples:

    #WORD 0                  ;"all 0s"$
    #WORD *000000000000*

    #WORD *2000*             ;"one bit 1"$
    #WORD *000000002000*

    #WORD *525252525252*     ;"every other bit 1"$
    #WORD *525252525252*

`WORD` is its own `PRIMTYPE`; it is also the `PRIMTYPE` of `FIX`,
`FLOAT`, `CHARACTER`, and any other `TYPE` which can fit its data into
one machine word.

A `WORD` cannot be an argument to `+`, `-`, or indeed any `SUBR`s
except for `CHTYPE`, `GETBITS`, `PUTBITS` and several bit-manipulating
functions, all to be described below. Thus any arithmetic bit
manipulation must be done by `CHTYPE`ing a `WORD` to `FIX`, doing the
arithmetic, and then `CHTYPE`ing back to `WORD`. However, bit
manipulation can be done without `CHTYPE`ing the thing to be played
with to a `WORD`, so long as it is of `PRIMTYPE` `WORD`; the result of
the manipulation will be of the same `TYPE` as the original object or
can be `CHTYPE`d to it.

18.2. BITS
----------

An object of `TYPE` `BITS` is of `PRIMTYPE` `WORD`, and `PRINT`s just
like a `WORD`. The internal form of a `BITS` is precisely that of a
PDP-10 "byte pointer", which is, in fact, just what a `BITS` is.

For purposes of explaining what a `BITS` is, assume that the bits in a
`WORD` are numbered from **right** to **left**, with the rightmost bit
numbered 0 and the leftmost numbered 35, as in

    35 34 33 ... 2 1 0

(This is not the "standard" ordering: the "standard" one goes from
left to right.)

A `BITS` is most conveniently created via the `SUBR` `BITS`:

    <BITS width:fix right-edge:fix>

returns a `BITS` which "points to" a set of bits *width* wide, with
rightmost bit *right-edge*. Both arguments must be of `TYPE` `FIX`,
and the second is optional, 0 by default.

Examples: the indicated application of `BITS` returns an object of
`TYPE` `BITS` which points to the indicated set of bits in a `WORD`:

  Example         Returns
  --------------- ------------------------------------
  `<BITS 7>`      35 ... 7 **6 ... 0**
  `<BITS 4 18>`   35 ... 22 **21 20 19 18** 17 ... 0
  `<BITS 36>`     ***35 ... 0***

18.3. GETBITS
-------------

    <GETBITS from:primtype-word bits>

where *from* is an object of `PRIMTYPE` `WORD`, returns a **new**
object whose `TYPE` is `WORD`. This object is constructed in the
following way: the set of bits in *from* pointed to by *bits* is
copied into the new object, right-adjusted, that is, lined up against
the right end (bit number 0) of the new object. All those bits of the
new object which are not copied are set to zero. In other words,
`GETBITS` takes bits from an arbitrary place in *from* and puts them
at the right of a new object. The *from* argument to `GETBITS` is not
affected.

Examples:

    <GETBITS #WORD *777777777777* <BITS 3>>$
    #WORD *000000000007*
    <GETBITS *012345670123* <BITS 6 18>>$
    #WORD *000000000045*

18.4. PUTBITS
-------------

    <PUTBITS to:primtype-word bits from:primtype-word>

where *to* and *from* are of `PRIMTYPE` `WORD`, returns a **copy** of
*to*, modified as follows: the set of bits in *to* which are pointed
to by *bits* are replaced by the appropriate number of rightmost bits
copied from *from* (optional, 0 by default). In other words: `PUTBITS`
takes bits from the right of *from* and stuffs them into an arbitrary
position in a copy of *to*. **None** of the arguments to `PUTBITS` is
affected.

Examples:

    <PUTBITS #WORD *777777777777* <BITS 6 3>>$
    #WORD *777777777007*
    <PUTBITS #WORD *666777000111* <BITS 5 15> #WORD *123*>$
    #WORD *666776300111*
    <PUTBITS #WORD *765432107654* <BITS 18>>$
    #WORD *765432000000*

18.5. Bitwise Boolean Operations
--------------------------------

Each of the `SUBR`s `ANDB`, `ORB`, `XORB`, and `EQVB` takes arguments
of `PRIMTYPE` `WORD` and returns a `WORD` which is the bitwise Boolean
"and", inclusive "or", exclusive "or", or "equivalence" (inverse of
exclusive "or"), respectively, of its arguments. Each takes any number
of arguments. If no argument is given, a `WORD` with all bits off
(`ORB` and `XORB`) or on (`ANDB` and `EQVB`) is returned. If only one
argument is given, it is returned unchanged but `CHTYPE`d to a `WORD`.
If more than two arguments are given, the operator is applied to the
first two, then applied to that result and the third, etc. Be sure not
to confuse `AND` and `OR` with `ANDB` and `ORB`.

18.6. Bitwise Shifting Operations
---------------------------------

    <LSH from:primtype-word amount:fix>

returns a **new** `WORD` containing the bits in *from*, shifted the
number of bits specified by *amount* (mod 256, says the hardware).
Zero bits are brought in at the end being vacated; bits shifted out at
the other end are lost. If *amount* is positive, shifting is to the
left; if *amount* is negative, shifting is to the right. Examples:

    <LSH 8 6>$
    #WORD *000000001000*
    <LSH 8 -6>$
    #WORD *000000000000*

    <ROT from:primtype-word amount:fix>

returns a **new** `WORD` containing the bits from *from*, rotated the
number of bits specified by *amount* (mod 256, says the hardware).
Rotation is a cyclic bitwise shift where bits shifted out at one end
are put back in at the other. If *amount* is positive, rotation is to
the left; if *amount* is negative, rotation is to the right. Examples:

    <ROT 8 6>$
    #WORD *000000001000*
    <ROT 8 -6>$
    #WORD *100000000000*

Chapter 19. Compiled Programs
=============================

19.1. RSUBR (the TYPE)
----------------------

`RSUBR`s ("relocatable subroutines") are machine-language programs
written to run in the Muddle environment. They are usually produced by
the Muddle assembler (often from output produced by the compiler)
although this is not necessary. All `RSUBR`s have two components: the
"reference vector" and the "code vector". In some cases the code
vector is in pure storage. There is also a set of "fixups" associated
with every `RSUBR`, although it may not be available in the running
Muddle.

19.2. The Reference Vector
--------------------------

An `RSUBR` is basically a `VECTOR` that has been `CHTYPE`d to `TYPE`
`RSUBR` via the `SUBR` `RSUBR` (see below). This ex-`VECTOR` is the
reference vector. The first three elements of the reference vector
have predefined meanings:

-   The first element is of `TYPE` `CODE` or `PCODE` and is the impure
    or pure code vector respectively.
-   The second element is an `ATOM` and specifies the name of the
    `RSUBR`.
-   The third element is of `TYPE` `DECL` and declares the
    type/structure of the `RSUBR`'s arguments and result.

The rest of the elements of the reference vector are objects in
garbage-collected storage that the `RSUBR` needs to reference and any
impure slots that the `RSUBR` needs to use.

When the `RSUBR` is running, one of the PDP-10 accumulators (with
symbolic name `R`) is always pointing to the reference vector, to
permit rapid access to the various elements.

19.3. RSUBR Linking
-------------------

`RSUBR`s can call any `APPLICABLE` object, all in a uniform manner. In
general, a call to an F/SUBR is linked up at assembly/compile time so
that the calling instruction (UUO) points directly at the code in the
interpreter for the F/SUBR. However, the locations of most other
`APPLICABLE`s are not known at assembly/compile time. Therefore, the
calling UUO is set up to point at a slot in the reference vector (by
indexing off accumulator `R`). This slot initially contains the `ATOM`
whose G/LVAL is the called object. The calling mechanism (UUO handler)
causes control to be transferred to the called object and, depending
on the state of the `RSUBR`-link flag, the `ATOM` will be replaced by
its G/LVAL. (If the call is of the "quick" variety, the called `RSUBR`
or `RSUBR-ENTRY` will be `CHTYPE`d to a `QUICK-RSUBR` or
`QUICK-ENTRY`, respectively, before replacement.) Regardless of the
`RSUBR`-link flag's state, calls to `FUNCTION`s are never permanently
linked. A call to a non-Subroutine generates an extra `FRAME`, whose
`FUNCT` is the dummy `ATOM` `CALLER`.

`RSUBR`s are linked together for faster execution, but linking may not
be desirable if the `RSUBR`s are being debugged, and various revisions
are being re-loaded. A linked call will forever after go to the same
code, regardless of the current G/LVAL of the called `ATOM`. Thus,
while testing `RSUBR`s, you may want to disable linking, by calling
the `RSUBR-LINK` `SUBR` with a `FALSE` argument. Calling it with a
non-`FALSE` argument enables linking thereafter. It returns the
previous state of the link flag, either `T` or `#FALSE ()`. Calling it
with no argument returns the current state.

19.4. Pure and Impure Code
--------------------------

The first element of an `RSUBR` is the code vector, of `TYPE` `CODE`
or `PCODE`. `TYPE` `CODE` is of `PRIMTYPE` `UVECTOR`, and the `UTYPE`
should be of `PRIMTYPE` `WORD`. The code vector is simply a block of
words that are the instructions which comprise the `RSUBR`. Since the
code vector is stored just like a standard `UVECTOR`, it will be moved
around by the garbage collector. Therefore, all `RSUBR` code is
required to be location-insensitive. The compiler guarantees the
location-insensitivity of its output. The assembler helps to make the
code location-insensitive by defining all labels as offsets relative
to the beginning of the code vector and causing instructions that
refer to labels to index automatically off the PDP-10 accumulator
symbolically named `M`. `M`, like `R`, is set up by the UUO handler,
but it points to the code vector instead of the reference vector. The
code vector of an `RSUBR` can be frozen (using the `FREEZE` `SUBR`) to
prevent it from moving during debugging by DDT in the superior
operating-system process.

If the first element of an `RSUBR` is of `TYPE` `PCODE` ("pure code"),
the code vector of the `RSUBR` is pure and sharable. `TYPE` `PCODE` is
of `PRIMTYPE` `WORD`. The left half of the word specifies an offset
into an internal table of pure `RSUBR`s, and the right half specifies
an offset into the block of code where this `RSUBR` starts. The
`PCODE` prints out as:

    %<PCODE name:string offset:fix>

where *name* names the entry in the user's pure-`RSUBR` table, and
*offset* is the offset. (Obviously, `PCODE` is also the name of a
`SUBR`, which generates a pure code vector.) Pure `RSUBR`s may also
move around, but only by being included in Muddle's page map at
different places. Once again `M` can be used exactly as before to do
location-independent address referencing. Individual pure code vectors
can be "unmapped" (marked as being not in primary storage but in their
original pure-code disk files) if the space in storage allocated for
pure code is exhausted. An unmapped `RSUBR` is mapped in again
whenever needed. All pure `RSUBR`s are unmapped before a `SAVE` file
is written, so that the code is not duplicated on disk. A purified
`RSUBR` must use `RGLOC` ("relative GLOC") instead of `GLOC`. `RGLOC`
produces objects of `TYPE` `LOCR` instead of `LOCD`.

19.5. TYPE-C and TYPE-W
=======================

In order to handle user `NEWTYPE`s reasonably, the internal `TYPE`
codes for them have to be able to be different from one Muddle run to
another. Therefore, references to the `TYPE` codes must be in the
reference vector rather than the code vector. To help handle this
problem, two `TYPE`s exist, `TYPE-C` ("type code") and `TYPE-W` ("type
word"), both of `PRIMTYPE` `WORD`. They print as:

    %<TYPE-C type primtype:atom>
    %<TYPE-W type primtype:atom>

The `SUBR` `TYPE-C` produces an internal `TYPE` code for the *type*,
and `TYPE-W` produces a prototype "`TYPE` word" (appendix 1) for an
object of that `TYPE`. The *primtype* argument is optional, included
only as a check against the call to `NEWTYPE`. `TYPE-W` can also take
a third argument, of `PRIMTYPE` `WORD`, whose right half is included
in the generated "`TYPE` word". If *type* is not a valid `TYPE`, a
`NEWTYPE` is automatically done.

To be complete, a similar `SUBR` and `TYPE` should be mentioned here.

    <PRIMTYPE-C type>

produces an internal "storage allocation code" (appendix 1) for the
*type*. The value is of `TYPE` `PRIMTYPE-C`, `PRIMTYPE` `WORD`. In
almost all cases the `SUBR` `TYPEPRIM` gives just as much information,
except in the case of `TEMPLATE`s: all `TYPE`s of `TEMPLATE`s have the
same `TYPEPRIM`, but they all have different `PRIMTYPE-C`s.

19.6. RSUBR (the SUBR)
----------------------

    <RSUBR [code name decl ref ref ...]>

`CHTYPE`s its argument to an `RSUBR`, after checking it for legality.
`RSUBR` is rarely called other than in the Muddle Assembler (Lebling,
1979). It can be used if changes must be made to an `RSUBR` that are
prohibited by Muddle's built-in safety mechanisms. For example, if the
`GVAL` of *name* is an `RSUBR`:

    <SET FIXIT <CHTYPE ,name VECTOR>>$
    [...]

    ...(changes to .FIXIT)...

    <SETG name <RSUBR .FIXIT>>$
    #RSUBR [...]

19.7. RSUBR-ENTRY
-----------------

`RSUBR`s can have multiple entry points. An `RSUBR-ENTRY` can be
applied to arguments exactly like an `RSUBR`.

    <RSUBR-ENTRY [rsubr-or-atom name:atom decl] offset:fix>

returns the `VECTOR` argument `CHTYPE`d to an `RSUBR-ENTRY` into the
*rsubr* at the specified *offset*. If the `RSUBR-ENTRY` is to have a
`DECL` (`RSUBR` style), it should come as shown.

    <ENTRY-LOC rsubr-entry>

("entry location") returns the *offset* into the `RSUBR` of this
entry.

19.8. RSUBRs in Files
---------------------

There are three kinds of files that can contain `RSUBR`s, identified
by second names `BINARY`, `NBIN` and `FBIN`. There is nothing magic
about these names, but they are used by convention.

A `BINARY` file is a completely ASCII file containing complete impure
`RSUBR`s in character representation. Even a code vector appears as
`#CODE` followed by a `UVECTOR` of `PRIMTYPE` `WORD`s. `BINARY` files
are generally slow to load, because of all the parsing that must be
done.

An `NBIN` file contains a mixture of ASCII characters and binary code.
The start of a binary portion is signalled to `READ` by the character
control-C, so naive readers of an `NBIN` file under ITS may
incorrectly assume that it ends before any binary code appears. An
`NBIN` file cannot be edited with a text editor. An `RSUBR` is written
in `NBIN` format by being `PRINT`ed on a `"PRINTB"` `CHANNEL`. The
`RSUBR`s in `NBIN` files are not purified either.

An `FBIN` file is actually part of a triad of files. The `FBIN`
file(s) itself is the impure part of a collection of purified
`RSUBR`s. It is simply ASCII and can be edited at will. (Exception: in
the ITS and Tops-20 versions, the first object in the file should not
be removed or changed in any way, lest a "grim reaper" program for
`FBIN` files think that the other files in the triad are obsolete and
delete them.) The pure code itself resides (in the ITS and Tops-20
versions) in a special large file that contains all currently-used
pure code, or (in the Tenex version) in a file in a special disk
directory with first name the same as the *name* argument to `PCODE`
for the `RSUBR`. The pure-code file is page-mapped directly into
Muddle storage in read-only mode. It can be unmapped when the pure
storage must be reclaimed, and it can be mapped at a different storage
address when pure storage must be compacted. There is also a "fixup"
file (see below) or portion of a file associated with the `FBIN` to
round out the triad.

An initial Muddle can have pure `RSUBR`s in it that were "loaded"
during the initialization procedure. The files are not page-mapped in
until they are actually needed. The "loading" has other side effects,
such as the creation of `OBLIST`s (chapter 15). Exactly what is
pre-loaded is outside the scope of this document.

19.9. Fixups
------------

The purpose of "fixups" is to correct references in the `RSUBR` to
parts of the interpreter that change from one release of Muddle to the
next. The reason the fixups contain a release number is so that they
can be completely ignored when an `RSUBR` is loaded into the same
release of Muddle as that from which it was last written out.

There are three forms of fixups, corresponding to the three kinds of
`RSUBR` files. ASCII `RSUBR`s, found in `BINARY` files, have ASCII
fixups. The fixups are contained in a `LIST` that has the following
format:

    (Muddle-release:fix
        name:atom value:fix (use:fix use:fix ...)
        name:atom value:fix (use:fix use:fix ...)
        ...)

The fixups in `NBIN` files and the fixup files associated with `FBIN`
files are in a fast internal format that looks like a `UVECTOR` of
`PRIMTYPE` `WORD`s.

Fixups are usually discarded after they are used during the loading
procedure. However, if, while reading a `BINARY` or `NBIN` file the
`ATOM` `KEEP-FIXUPS!-` has a non-`FALSE` `LVAL`, the fixups will be
kept, via an association between the `RSUBR` and the `ATOM` `RSUBR`.
It should be noted that, besides correcting the code, the fixups
themselves are corrected when `KEEP-FIXUPS` is bound and true. Also,
the assembler and compiler make the same association when they first
create an `RSUBR`, so that it can be written out with its fixups.

In the case of pure `RSUBR`s (`FBIN` files), things are a little
different. If a pure-code file exists for this release of Muddle, it
is used immediately, and the fixups are completely ignored. If a
pure-code file for this release doesn't exist, the fixup file is used
to create a new copy of the file from an old one, and also a new
version of the fixup file is created to go with the new pure-code
file. This all goes on automatically behind the user's back.

Chapter 20. Coroutines
======================

This chapter purports to explain the coroutine primitives of Muddle.
It does make some attempt to explain coroutines as such, but only as
required to specify the primitives. If you are unfamiliar with the
basic concepts, confusion will probably reign.

A coroutine in Muddle is implemented by an object of `TYPE` `PROCESS`.
In this manual, this use of the word "process" is distinguished by a
capitalization from its normal use of denoting an operating-system
process (which various systems call a process, job, fork, task, etc.).

Muddle's built-in coroutine primitives do not include a "time-sharing
system". Only one `PROCESS` is ever running at a time, and control is
passed back and forth between `PROCESS`es on a coroutine-like basis.
The primitives are sufficient, however, to allow the writing of a
"time-sharing system" **in Muddle**, with the additional use of the
Muddle interrupt primitives. This has, in fact, been done.

20.1. PROCESS (the TYPE)
------------------------

A `PROCESS` is an object which contains the "current state" of a
computation. This includes the `LVAL`s of `ATOM`s ("bindings"),
"depth" of functional application, and "position" within the
application of each applied function. Some of the things which are
**not** part of any specific `PROCESS` are the `GVAL`s of `ATOM`s,
associations (`ASOC`s), and the contents of `OBLIST`s. `GVAL`s (with
`OBLIST`s) are a chief means of communication and sharing between
`PROCESS`es (all `PROCESS`es can refer to the `SUBR` which is the
`GVAL` of `+`, for instance.) Note that an `LVAL` in one `PROCESS`
cannot easily be directly referenced from another `PROCESS`.

A `PROCESS` `PRINT`s as `#PROCESS` *p*, where *p* is a `FIX` which
uniquely identifies the `PROCESS`; *p* is the "`PROCESS` number" typed
out by `LISTEN`. A `PROCESS` cannot be read in by `READ`.

The term "run a `PROCESS`" will be used below to mean "perform some
computation, using the `PROCESS` to record the intermediate state of
that computation".

N.B.: A `PROCESS` is a rather large object; creating one will often
cause a garbage collection.

20.2. STATE of a PROCESS
------------------------

    <STATE process>

returns an `ATOM` (in the `ROOT` `OBLIST`) which indicates the "state"
of the `PROCESS` *process*. The `ATOM`s which `STATE` can return, and
their meanings, are as follows:

-   `RUNABLE` (sic) -- *process* has never ever been run.
-   `RUNNING` -- *process* is currently running, that is, it did the
    application of `STATE`.
-   `RESUMABLE` -- *process* has been run, is not currently running,
    and can run again.
-   `DEAD` -- *process* has been run, but it can **not** run again; it
    has "terminated".

In addition, an interrupt (chapter 21) can be enabled to detect the
time at which a `PROCESS` becomes "blocked" (waiting for terminal
input) or "unblocked" (terminal input arrived). (The `STATE` `BLOCKED`
has not been implemented.)

20.3. PROCESS (the SUBR)
------------------------

    <PROCESS starter:applicable>

creates and returns a new `PROCESS` but does **not** run it; the
`STATE` of the returned `PROCESS` is `RUNABLE` (sic).

*starter* is something applicable to **one** argument, which must be
evaluated. *starter* is used both in starting and "terminating" a
`PROCESS`. In particular, if the *starter* of a `PROCESS` **ever**
returns a value, that `PROCESS` becomes `DEAD`.

20.4. RESUME
------------

The `SUBR` `RESUME` is used to cause a computation to start or to
continue running in another `PROCESS`. An application of `RESUME`
looks like this:

    <RESUME retval:any process>

where *retval* is the "returned value" (see below) of the `PROCESS`
that does the `RESUME`, and *process* is the `PROCESS` to be started
or continued.

The *process* argument to `RESUME` is optional, by default the last
`PROCESS`, if any, to `RESUME` the `PROCESS` in which this `RESUME` is
applied. If and when the current `PROCESS` is later `RESUME`d by
another `PROCESS`, that `RESUME`'s *retval* is returned as the value
of this `RESUME`.

20.5. Switching PROCESSes
-------------------------

### 20.5.1. Starting Up a New PROCESS

Let us say that we are running in some `PROCESS`, and that this
original `PROCESS` is the `GVAL` of `P0`. Somewhere, we have evaluated

    <SETG P1 <PROCESS ,STARTER>>

where `,STARTER` is some appropriate function. Now, **in `,P0`** we
evaluate

    <RESUME .A ,P1>

and the following happens:

1.  **In `,P0`** the arguments of the `RESUME` are evaluated: that is,
    we get that `LVAL` of `A` which is current in `,P0` and the `GVAL`
    of `P1`.
2.  The `STATE` of `,P0` is changed to `RESUMABLE` and `,P0` is
    "frozen" right where it is, in the middle of the `RESUME`.
3.  The `STATE` of `,P1` is changed to `RUNNING`, and `,STARTER` is
    applied to `,P0`'s `LVAL` of `A` **in `,P1`**. `,P1` now continues
    on its way, evaluating the body of `,STARTER.`

The `.A` in the `RESUME` could have been anything, of course. The
important point is that, whatever it is, it is evaluated in `,P0`.

What happens next depends, of course, on what `,STARTER` does.

### 20.5.2. Top-level Return

Let us initially assume that `,STARTER` does nothing relating to
`PROCESS`es, but instead simply returns a value -- say *starval*. What
happens when `,STARTER` returns is this:

1.  The `STATE` of `,P1` is changed to `DEAD`. `,P1` can never again
    be `RESUME`d.
2.  The last `PROCESS` to `RESUME` `,P1` is found, namely `,P0`, and
    its `STATE` is changed to `RUNNING`.
3.  *starval* is returned in `,P0` as the value of the original
    `RESUME`, and `,P0` continues where it left off.

All in all, this simple case looks just like an elaborate version of
applying `,STARTER` to `.A` in `,P0`.

### 20.5.3. Symmetric RESUMEing

Now suppose that while still in `,P1`, the following is evaluated,
either in `,STARTER` or in something called by `,STARTER`:

    <RESUME .BAR ,P0>

This is what happens:

1.  The arguments of the `RESUME` are evaluated **in `,P1`**.
2.  The `STATE` of `,P1` is changed to `RESUMABLE`, and `,P1` is
    "frozen" right in the middle of the `RESUME`.
3.  The `STATE` of `,P0` is changed to `RUNNING`, and `,P1`'s `LVAL`
    of `BAR` is returned as the value of **`,P0'`s** original `RESUME`
    `,P0` then continues right where it left off.

This is **the** interesting case, because `,P0` can now do **another**
`RESUME` of `,P1`; this will "turn off" `,P0`, pass a value to `,P1`
and "turn on" `,P1`. `,P1` can now again `RESUME` `,P0`. which can
`RESUME` `,P1` back again, etc. **ad nauseam**, with everything done
in a perfectly symmetric manner. This can obviously also be done with
three or more `PROCESS`es in the same manner.

Note how this differs from normal functional application: you cannot
"return" from a function without destroying the state that function is
in. The whole point of `PROCESS`es is that you can "return"
(`RESUME`), remembering your state, and later continue where you left
off.

20.6. Example
-------------

    ;"Initially, we are in LISTEN in some PROCESS.
    <DEFINE SUM3 (A)
            #DECL ((A) (OR FIX FLOAT>)
            <REPEAT ((S .A))
                    #DECL ((S) <OR FIX FLOAT>)
                    <SET S <+ .S <RESUME "GOT 1">>>
                    <SET S <+ .S <RESUME "GOT 2">>>
                    <SET S <RESUME .S>>>>$
    SUM3
    ;"SUM3, used as the startup function of another PROCESS,
    gets RESUMEd with numbers. It returns the sum of the last
    three numbers it was given every third RESUME."
    <SETG SUMUP <PROCESS ,SUM3>>$
    ;"Now we start SUMUP and give SUM3 its three numbers."
    <RESUME 5 ,SUMUP>$
    "GOT 1"
    <RESUME 1 ,SUMUP>$
    "GOT 2"
    <RESUME 2 ,SUMUP>$
    8

Just as a note, by taking advantage of Muddle's order of evaluation,
SUM3 could be have been written as:

    <DEFINE SUM3 (A)
            <REPEAT ((S .A))
               #DECL ((A S0 <OR FIX FLOAT>)
               <SET S <RESUME <+ .S <RESUME "GOT 1"> <RESUME "GOT 2">>>>>>

20.7. Other Coroutining Features
--------------------------------

### 20.7.1. BREAK-SEQ

    <BREAK-SEQ any process>

("break evaluation sequence") returns *process*, which must be
`RESUMABLE`, after having modified it so that when it is next
`RESUME`d, it will **first** evaluate *any* and **then** do an
absolutely normal `RESUME`; the value returned by any is thrown away,
and the value given by the `RESUME` is used normally.

If a `PROCESS` is `BREAK-SEQ`ed more than once between `RESUME`s,
**all** of the *any*s `BREAK-SEQ`ed onto it will be remembered and
evaluated when the `RESUME` is finally done. The *any*s will be
evaluated in "last-in first-out" order. The `FRAME` generated by
`EVAL`ing more than one *any* will have as its `FUNCT` the dummy
`ATOM` `BREAKER`.

### 20.7.2. MAIN

When you initially start up Muddle, the `PROCESS` in which you are
running is slightly "special" in these two ways:

1.  Any attempt to cause it become `DEAD` will be met with an error.
2.  `<MAIN>` always returns that `PROCESS`.

The `PROCESS` number of `<MAIN>` is always `1`. The initial `GVAL` of
`THIS-PROCESS` is what `MAIN` always returns, `#PROCESS 1`.

### 20.7.3. ME

    <ME>

returns the `PROCESS` in which it is evaluated. The `LVAL` of
`THIS-PROCESS` in a `RUNABLE` (new) `PROCESS` is what `ME` always
returns.

### 20.7.4. RESUMER

    <RESUMER process>

returns the `PROCESS` which last `RESUME`d *process*. If no `PROCESS`
has ever `RESUME`d process, it returns `#FALSE ()`. *process* is
optional, `<ME>` by default. Note that `<MAIN>` does not ever have any
resumer. Example:

    <PROG ((R <RESUMER>))           ;"not effective in <MAIN>"
       #DECL ((R) <OR PROCESS FALSE>)
       <AND .R
            <==? <STATE .R> RESUMABLE>
            <RESUME T .R>>>

### 20.7.5. SUICIDE

    <SUICIDE retval process>

acts just like `RESUME`, but clobbers the `PROCESS` (which cannot be
`<MAIN>`) in which it is evaluated to the `STATE` `DEAD`.

### 20.7.6. 1STEP

    <1STEP process>

returns *process*, after putting it into "single-step mode".

A `PROCESS` in single-step mode, whenever `RESUME`d, runs only until
an application of `EVAL` in it begins or finishes. At that point in
time, the `PROCESS` that did the `1STEP` is `RESUME`d, with a *retval*
which is a `TUPLE`. If an application of `EVAL` just began, the
`TUPLE` contains the `ATOM` `EVLIN` and the arguments to `EVAL`. If an
application of `EVAL` just finished, the `TUPLE` contains the `ATOM`
`EVLOUT` and the result of the evaluation.

*process* will remain in single-step mode until `FREE-RUN` (below) is
applied to it. Until then, it will stop before and after each `EVAL`
in it. Exception: if it is `RESUME`d from an `EVLIN` break with a
*retval* of `TYPE` `DISMISS` (`PRIMTYPE` `ATOM`), it will leave
single-step mode only until the current call to EVAL is about to
return. Thus lower-level `EVAL`s are skipped over without leaving the
mode. The usefulness of this mode in debugging is obvious.

### 20.7.7. FREE-RUN

    <FREE-RUN process>

takes its argument out of single-step mode. Only the `PROCESS` that
put *process* into single-step mode can take it out of the mode; if
another `PROCESS` tries, `FREE-RUN` returns a `FALSE`.

20.8. Sneakiness with PROCESSes
-------------------------------

`FRAME`s, `ENVIRONMENT`s, `TAG`s, and `ACTIVATION`s are specific to
the `PROCESS` which created them, and each "knows its own father".
**Any** `SUBR` which takes these objects as arguments can take one
which was generated by **any** `PROCESS`, no matter where the `SUBR`
is really applied. This provides a rather sneaky means of crossing
between `PROCESS`es. The various cases are as follows:

`GO`, `RETURN`, `AGAIN`, and `ERRET`, given arguments which lie in
another `PROCESS`, each effectively "restarts" the `PROCESS` of its
argument and acts as if it were evaluated over there. If the `PROCESS`
in which it was executed is later `RESUME`d, it **returns** a value
just like `RESUME`!

`SET`, `UNASSIGN`, `BOUND?`, `ASSIGNED?`, `LVAL`, `VALUE`, and `LLOC`,
given optional `ENVIRONMENT` arguments which lie in another `PROCESS`,
will gleefully change, or return, the local values of `ATOM`s in the
other `PROCESS`. The optional argument can equally well be a
`PROCESS`, `FRAME`, or `ACTIVATION` in another `PROCESS`; in those
cases, each uses the `ENVIRONMENT` which is current in the place
specified.

`FRAME`, `ARGS`, and `FUNCT` will be glad to return the `FRAME`s,
argument `TUPLE`s, and applied Subroutine names of another `PROCESS`.
If one is given a `PROCESS` (including `<ME>`) as an argument instead
of a `FRAME`, it returns all or the appropriate part of the topmost
`FRAME` on that `PROCESS`'s control stack.

If `EVAL` is applied in `PROCESS` `P1` with an `ENVIRONMENT` argument
from a `PROCESS` `P2`, it will do the evaluation **in `P1`** but with
`P2`'s `ENVIRONMENT` (!). That is, the other `PROCESS`'s `LVAL`s, etc.
will be used, but (1) any **new** `FRAME`s needed in the course of the
evaluation will be created in `P1`; and (2) **`P1`** will be `RUNNING`
-- not `P2`. Note the following: if the `EVAL` in `P1` eventually
causes a `RESUME` of `P2`, `P2` could functionally return to below the
point where the `ENVIRONMENT` used in `P1` is defined; a `RESUME` of
`P1` at this point would cause an `ERROR` due to an invalid
`ENVIRONMENT`. (Once again, `LEGAL?` can be used to forestall this.)

20.9. Final Notes
-----------------

1.  A `RESUMABLE` `PROCESS` can be used in place of an `ENVIRONMENT`
    in any application. The "current" `ENVIRONMENT` of the `PROCESS`
    is effectively used.
2.  `FRAME`s and `ENVIRONMENT`s can be `CHTYPE`d arbitrarily to one
    another, or an `ACTIVATION` can be `CHTYPE`d to either of them,
    and the result "works". Historically, these different `TYPE`s were
    first used with different `SUBR`s -- `FRAME` with `ERRET`,
    `ENVIRONMENT` with `LVAL`, `ACTIVATION` with `RETURN` -- hence the
    invention of different `TYPE`s with similar properties.
3.  Bugs in multi-`PROCESS` programs usually exhibit a degree of
    subtlety and nastiness otherwise unknown to the human mind. If
    when attempting to work with multiple processes you begin to feel
    that you are rapidly going insane, you are in good company.

Chapter 21. Interrupts
======================

The Muddle interrupt handling facilities provide the ability to say
the following: whenever "this event" occurs, stop whatever is being
done at the time and perform "this action"; when "this action" is
finished, continue with whatever was originally being done. "This
event" can be things like the typing of a character at a terminal, a
time interval ending, a `PROCESS` becoming blocked, or a
program-defined and -generated "event". "This action" is the
application of a specified `APPLICABLE` object to arguments provided
by the Muddle interrupt system. The sets of events and actions can be
changed in extremely flexible ways, which accounts for both the
variety of `SUBR`s and arguments, and the rich interweaving of the
topics in this chapter. Interrupt handling is a kind of parallel
processing: a program can be divided into a "main-level" part and one
or more interrupt handlers that execute only when conditions are ripe.

21.1. Definitions of Terms
--------------------------

An **interrupt** is not an object in Muddle, but rather a class of
events, for example, "ticks" of a clock, garbage collections, the
typing of a character at a terminal, etc.

An interrupt is said to **occur** when one of the events in its class
takes place.

An **external** interrupt is one whose occurrences are signaled to
Muddle by the operating system, for example, "ticks" of a clock. An
**internal** interrupt is one whose occurrences are detected by Muddle
itself, for example, garbage collections. Muddle can arrange for the
operating system to not signal occurrences of an external interrupt to
it; then, as far as Muddle is concerned, that interrupt does not
occur.

Each interrupt has a **name** which is either a `STRING` (for example,
`"GC"`, `"CHAR"`, `"WRITE"`) or an `ATOM` with that `PNAME` in a
special `OBLIST`, named `INTERRUPTS!-`. (This `OBLIST` is returned by
`<INTERRUPTS>`.) Certain names must always be further specified by a
`CHANNEL` or a `LOCATIVE` to tell **which** interrupt by that name is
meant.

When an interrupt occurs, the interpreter looks for an association on
the interrupt's name. If there is an association, its `AVALUE` should
be an `IHEADER`, which heads a list of actions to be performed. In
each `IHEADER` is the name of the interrupt with which the `IHEADER`
is or was associated.

In each `IHEADER` is an element telling whether it is disabled. If an
`IHEADER` is **disabled**, then none of its actions is performed. The
opposite of disabled is **enabled**. It is sometimes useful to disable
an `IHEADER` temporarily, but removing its association with the
interrupt's name is better than long-term disabling. There are `SUBR`s
for creating an `IHEADER`, associating it with an interrupt, and later
removing the association.

In each `IHEADER` is a **priority**, a `FIX` greater than `0` which
specifies the interrupt's "importance". The processing of a
higher-priority (larger-numbered) interrupt will supersede the
processing of a lower-priority (smaller-numbered) interrupt until the
high-priority interrupt has been handled.

In each `IHEADER` is a (possibly empty) list of `HANDLER`s. (This list
is not a Muddle `LIST`.) Each `HANDLER` corresponds to an action to
perform. There are `SUBR`s for creating a `HANDLER`, adding it to an
`IHEADER`'s list, and later removing it.

In each `HANDLER` is a function that we will call a **handler** (in
lower case), despite possible confusion, because that is really the
best name for it. An **action** consists of applying a handler to
arguments supplied by the interrupt system. The number and meaning of
the arguments depend on the name of the interrupt. In each `HANDLER`
is an element telling in which `PROCESS` the action should be
performed.

21.2. EVENT
-----------

    <EVENT name priority which>

creates and returns an enabled `IHEADER` with no `HANDLER`s. The
*name* may be an `ATOM` in the `INTERRUPTS` `OBLIST` or a `STRING`; if
it is a `STRING`, `EVENT` does a `LOOKUP` or `INSERT` in
`<INTERRUPTS>`. If there already is an `IHEADER` associated with
*name*, `EVENT` just returns it, ignoring the given *priority*.

*which* must be given only for certain *name*s:

-   It must be a `CHANNEL` if and only if *name* is `"CHAR"` (or
    `CHAR!-INTERRUPTS`). In this case it is the input `CHANNEL` from
    the (pseudo-)terminal or Network socket whose received characters
    will cause the interrupt to occur, or the output `CHANNEL` to the
    pseudo-terminal or Network socket whose desired characters will
    cause the interrupt to occur. (See below. Pseudo-terminals are not
    available in the Tenex and Tops-20 versions.)
-   The argument must be a `LOCATIVE` if and only if *name* is
    `"READ"` (or `READ!-INTERRUPTS`) or `"WRITE"` (or
    `WRITE!-INTERRUPTS`). In this case it specifies an object to be
    "monitored" for usage by (interpreted) Muddle programs (section
    21.8.9).

If the interrupt is external, Muddle arranges for the operating system
to signal its occurrences.

21.3. HANDLER (the SUBR)
------------------------

    <HANDLER iheader applicable process>

creates a `HANDLER`, adds it to the front of *iheader*'s `HANDLER`
list (first action to be performed), and returns it as a value.
*applicable* may be any `APPLICABLE` object that takes the proper
number of arguments. (None of the arguments can be `QUOTE`d; they must
all be evaluated at call time.) *process* is the `PROCESS` in which
the handler will be applied, by default whatever `PROCESS` was running
when the interrupt occurred.

The value returned by the handler is ignored, unless it is of `TYPE`
`DISMISS` (`PRIMTYPE` `ATOM`), in which case none of the remaining
actions in the list will be performed.

The processing of an interrupt's actions can terminate prematurely if
a handler calls the `SUBR` `DISMISS` (see below.)

21.4. OFF
---------

    <OFF iheader>

removes the association between *iheader* and the name of its
interrupt, and then disables *iheader* and returns it. (An error
occurs if there is no association.) If the interrupt is external,
Muddle arranges for the operating system not to signal its
occurrences.

    <OFF name which>

finds the `IHEADER` associated with *name* and proceeds as above,
returning the `IHEADER`. *which* must be given only for certain
*names*, as for `EVENT`. Caution: if you `<OFF "CHAR" ,INCHAN>`,
Muddle will become deaf.

    <OFF handler>

returns *handler* after removing it from its list of actions. There is
no effect on any other `HANDLER`s in the list.

Now that you know how to remove `IHEADER`s and `HANDLER`s from their
normal places, you need to know how to put them back:

    <EVENT iheader>

If *iheader* was previously disabled or disassociated from its name,
`EVENT` will associate and enable it.

    <HANDLER iheader handler>

If *handler* was previously removed from its list, `HANDLER` will add
it to the front of *iheader*'s list of actions. Note that *process*
cannot be specified.

21.5. IHEADER and HANDLER (the TYPEs)
-------------------------------------

Both these `TYPE`s are of `PRIMTYPE` `VECTOR`, but they do not `PRINT`
that way, since they are self-referencing. Instead they `PRINT` as

    #type most-interesting-component

The contents of `IHEADER`s and `HANDLER`s can be changed by `PUT`, and
the new values will then determine the behavior of Muddle.

Before describing the elements of these `TYPE`s in detail, here are a
picture and a Pattern, both purporting to show how they look:

    #IHEADER [name:atom or which
              disabled?
              *-----------> #HANDLER [*-----------> #HANDLER [#HANDLER []
              priority] <-------------*                +------*
                                      applicable       |      applicable
                                      process] <-------+      process]

    <IHEADER <OR ATOM CHANNEL LOCATIVE>
             <OR '#LOSE 0 '#LOSE -1>
             <HANDLER HANDLER <OR HANDLER IHEADER> APPLICABLE PROCESS>
             FIX>

### 21.5.1. IHEADER

The elements of an `IHEADER` are as follows:

1.  name of interrupt (`ATOM`, or `CHANNEL` if the name is `"CHAR"`,
    or `LOCATIVE` if the name is `"READ"` or `"WRITE"`)
2.  non-zero if and only if disabled
3.  first `HANDLER`, if any, else a zero-length `HANDLER`
4.  priority

If you lose track of an `IHEADER`, you can get it via the association:

-   For `"CHAR"` interrupts, `<GET channel INTERRUPT>` returns the
    `IHEADER` or `#FALSE ()` if there is no association;
    `<EVENT "CHAR" 0 channel>` returns the `IHEADER`, creating it if
    there is no association.
-   For `"READ"` interrupts, `<GET locative READ!-INTERRUPTS>` returns
    the `IHEADER` or `#FALSE ()` if there is no association;
    `<EVENT "READ" 0 locative>` returns the `IHEADER`, creating it if
    there is no association.
-   For `"WRITE"` interrupts, `<GET locative WRITE!-INTERRUPTS>`
    returns the `IHEADER` or `#FALSE ()` if there is no association:
    `<EVENT "WRITE" 0 locative>` returns the `IHEADER`, creating it if
    there is no association.
-   Otherwise, the `IHEADER` is `PUT` on the name `ATOM` with the
    indicator `INTERRUPT`. Thus, for example,
    `<GET CLOCK!-INTERRUPTS INTERRUPT>` returns the `IHEADER` for the
    clock interrupt or `#FALSE ()` if there is no association;
    `<EVENT "CLOCK" 0>` returns the `IHEADER`, creating it if there is
    no association.

### 21.5.2. HANDLER

A `HANDLER` specifies a **particular** action for a **particular**
interrupt. The elements of a `HANDLER` are as follows:

1.  next `HANDLER` if any, else a zero-length `HANDLER`
2.  previous `HANDLER` or the `IHEADER` (Thus the `HANDLER`s of a
    given interrupt form a "doubly-linked list" chaining between each
    other and back to the `IHEADER`.)
3.  handler to be applied (anything but `APPLICABLE` that evaluates
    its arguments -- the application is done not by `APPLY` but by
    `RUNINT`, which can take a `PROCESS` argument: see next line)
4.  `PROCESS` in which the handler will be applied, or `#PROCESS 0`,
    meaning whatever `PROCESS` was running when the interrupt occurred
    (In the former case, `RUNINT` is applied to the handler and its
    arguments in the currently running `PROCESS`, which causes an
    `APPLY` in the `PROCESS` stored in the `HANDLER`, which `PROCESS`
    must be `RESUMABLE`. The running `PROCESS` becomes `RESUMABLE`,
    and the stored `PROCESS` becomes `RUNNING`, but no other `PROCESS`
    variables (for example `RESUMER`) are changed.)

21.6. Other SUBRs
-----------------

    <ON name applicable priority:fix process which>

is equivalent to

    <HANDLER <EVENT name priority which>
             applicable process>

`ON` is a combination of `EVENT` and `HANDLER`: it creates (or finds)
the `IHEADER`, associates and enables it, adds a `HANDLER` to the
front the list (first to be performed), and returns the `HANDLER`.

    <DISABLE iheader>

is effectively `<PUT iheader 2 #LOSE -1>`. Actually the `TYPE` `LOSE`
is unimportant, but the `-1` signifies that *iheader* is disabled.

    <ENABLE iheader>

is effectively `<PUT iheader 2 #LOSE 0>`. Actually the `TYPE` `LOSE`
is unimportant, but the `0` signfies that *iheader* is enabled.

21.7. Priorities and Interrupt Levels
-------------------------------------

At any given time there is a defined **interrupt level**. This is a
`FIX` which determines which interrupts can really "interrupt" -- that
is, cause the current processing to be suspended while their wants are
satisfied. Normal, non-interrupt programs operate at an interrupt
level of 0 (zero.) An interrupt is processed at an interrupt level
equal to the interrupt's priority.

### 21.7.1. Interrupt Processing

Interrupts "actually" only occur at well-defined points in time:
during a call to a Subroutine, or at critical places within
Subroutines (for example, during each iteration of `MAPF` on a `LIST`,
which may be circular), or while a `PROCESS` is `"BLOCKED"` (see
below). No interrupts can occur during garbage collection.

What actually happens when an enabled interrupt occurs is that the
priority of the interrupt is compared with the current interrupt
level, and the following is done:

If the priority is **greater than** the current interrupt level, the
current processing is "frozen in its tracks" and processing of the
action(s) specified for that interrupt begins.

If the priority is less than or equal to the current interrupt level,
the interrupt occurrence is **queued** -- that is, the fact that it
occurred is saved away for processing when the interrupt level becomes
low enough.

When the processing of an interrupt's actions is completed, Muddle
usually (1) "acts as if" the previously-existing interrupt level is
restored, and processing continues on what was left off (perhaps for
no time duration); and (2) "acts as if" any queued interrupt
occurrences actually occurred right then, in their original order of
occurrence.

### 21.7.2. INT-LEVEL

The `SUBR` `INT-LEVEL` is used to examine and change the current
interrupt level directly.

    <INT-LEVEL>

simply returns the current interrupt level.

    <INT-LEVEL fix>

changes the interrupt level to its argument and returns the
**previously**-existing interrupt level.

If `INT-LEVEL` lowers the priority of the interrupt level, it does not
"really" return until all queued occurrences of interrupts of higher
priority than the target priority have been processed.

Setting the `INT-LEVEL` extremely high (for example,
`<INT-LEVEL <CHTPE <MIN> FIX>>`) effectively disables all interrupts
(but occurrences of enabled interrupts will still be queued).

If `LISTEN` or `ERROR` is called when the `INT-LEVEL` is not zero,
then the typeout will be

    LISTENING-AT-LEVEL I PROCESS p INT-LEVEL i

### 21.7.3. DISMISS

`DISMISS` permits a handler to return an arbitrary value for an
arbitrary `ACTIVATION` at an arbitrary interrupt level. The call is as
follows:

    <DISMISS value:any activation int-level:fix>

where only the *value* is required. If *activation* is omitted, return
is to the place interrupted from, and *value* is ignored. If
*int-level* is omitted, the `INT-LEVEL` prior to the current interrupt
is restored.

21.8. Specific Interrupts
-------------------------

Descriptions of the characteristics of particular "built-in" Muddle
interrupts follow. Each is named by its `STRING` name. Expect this
list to be incomplete yesterday.

`"CHAR"` is currently the most complex built-in interrupt, because it
serves duty in several ways. These different ways will be described in
several different sections. All ways are concerned with characters or
machine words that arrive or depart at unpredictable times, because
Muddle is communicating with a person or another processor. Each
`"CHAR"` `IHEADER` has a `CHANNEL` for the element that names the
interrupt, and the mode of the `CHANNEL` tells what kinds of `"CHAR"`
interrupts occur to be handled through that `IHEADER`.

1.  If the `CHANNEL` is for `INPUT`, "CHAR" occurs every time an
    "interesting" character (see below) is received from the
    `CHANNEL`'s real terminal, or any character is received from the
    `CHANNEL`'s pseudo-terminal, or a character or word is received
    from the `CHANNEL`'s Network socket, or indeed (in the ITS
    version) the operating system generates an interrupt for any
    reason.
2.  If the `CHANNEL` is for output to a pseudo-terminal or Network
    socket, `"CHAR"` occurs every time a character or word is wanted.
3.  If the `CHANNEL` is for output to a terminal, `"CHAR"` occurs
    every time a line-feed character is output or (in the ITS version)
    the operating system generates a screen-full interrupt for the
    terminal.

### 21.8.1. "CHAR" received

A handler for an input `"CHAR"` interrupt on a real terminal must take
two arguments: the `CHARACTER` which was typed, and the `CHANNEL` on
which it was typed.

In the ITS version, the "interesting" characters are those "enabled
for interrupts" on a real terminal, namely `^@` through `^G`, `^K`
through `^_`, and `DEL` (that is, ASCII codes 0-7, 13-37, and 177
octal.)

In the Tenex and Tops-20 versions, the operating system can be told
which characters typed on a terminal should cause this interrupt to
occur, by calling the `SUBR` `ACTIVATE-CHARS` with a `STRING` argument
containing those characters (no more than six, all with ASCII codes
less than 33 octal). If called with no argument, `ACTIVATE-CHARS`
returns a `STRING` containing the characters that currently interrupt.
Initially, only `^G`, `^S`, and `^O` interrupt.

An initial Muddle already has `"CHAR"` enabled on `,INCHAN` with a
priority 8 (eight), the `SUBR` `QUITTER` for a handler to run in
`#PROCESS 0` (the running `PROCESS`); this is how `^G` and `^S` are
processed. In addition, every time a new `CHANNEL` is `OPEN`ed in
`"READ"` mode to a terminal, a similar `IHEADER` and `HANDLER` are
associated with that new `CHANNEL` automatically. These
automatically-generated `IHEADER`s and `HANDLER`s use the standard
machinery, and they can be `DISABLE`d or `OFF`ed at will. **However**,
the `IHEADER` for `,INCHAN` should not be `OFF`ed: Muddle knows that
`$` is typed only by an interrupt!

Example: the following causes the given message to be printed out
whenever a `^Y` is typed on `.INCHAN`:

    <SET H <HANDLER <GET .INCHAN INTERRUPT>
         #FUNCTION ((CHAR CHAN)
          #DECL ((VALUE) ANY (CHAR) CHARACTER (CHAN) CHANNEL)
          <AND <==? .CHAR !\^Y>
               <PRINC " [Some of the best friends are ^Ys.] ">>)>>$
    #HANDLER #FUNCTION **CHAR CHAN) ...)
    <+ 2 ^Y [Some of my best friends are ^Ys.] 2>$
    4
    <OFF .H>$
    #HANDLER #FUNCTION (...)

Note that occurrences of `"CHAR"` do **not** wait for the `$` to be
typed, and the interrupting character is omitted from the input
stream.

A `"CHAR"` interrupt can also be associated with an input `CHANNEL`
open to a Network socket (`"NET"` device). A handler gets applied to a
`NETSTATE` array (which see) and the `CHANNEL`.

In the ITS version, a `"CHAR"` interrupt can also be associated with
an input `CHANNEL` open to a pseudo-terminal ("STY" device and
friends). An interrupt occurs when a character is available for input.
These interrupts are set up in exactly the same way as real-terminal
interrupts, except that a handler gets applied to only **one**
argument, the `CHANNEL`. Pseudo-terminal are not available in the
Tenex and Tops-20 versions.

For any other flavor of ITS channel interrupt, a handler gets applied
to only **one** argument, the `CHANNEL`.

### 21.8.2. "CHAR" wanted

A `"CHAR"` interrupt can be associated with an output `CHANNEL` open
to a Network socket (`"NET"` device). A handlers gets applied to a
`NETSTATE` array (which see) and the `CHANNEL`.

In the ITS version, a `"CHAR"` interrupt can also be associated with
an output `CHANNEL` open to a pseudo-terminal (`"STY"` device and
friends). An interrupt occurs when the program at the other end needs
a character (and the operating-system buffer is empty). A handler gets
applied to one argument, the `CHANNEL`. Pseudo-terminals are not
available in the Tenex and Tops-20 versions.

### 21.8.3. "CHAR" for new line

A handler for an output `"CHAR"` interrupt on a real terminal must
take **one or two** arguments (using `"OPTIONAL"` or `"TUPLE"`): if
two arguments are supplied by the interrupt system, they are the line
number (`FIX`) and the `CHANNEL`, respectively, and the interrupt is
for a line-feed; if only one argument is supplied (only in the ITS
version), it is the `CHANNEL`, and the interrupt is for a full
terminal screen. Note: the supplied line number comes from the
`CHANNEL`, and it may not be accurate if the program alters it in
subtle ways, for example, via `IMAGE` calls or special control
characters. (The program can compensate by putting the proper line
number into the `CHANNEL`.)

### 21.8.4. "GC"

`"GC"` occurs just **after** every garbage collection. Enabling this
interrupt is the only way a program can know that a garbage collection
has occurred. A handler for `"GC"` takes three arguments. The first is
a FLOAT indicating the number of seconds the garbage collection took.
The second argument is a FIX indicating the cause of the garbage
collection, as follows (chapter 22):

0.  Program called GC.
1.  Movable storage was exhausted.
2.  Control stack overflowed.
3.  Top-level LVALs overflowed.
4.  GVAL vector overflowed.
5.  TYPE vector overflowed.
6.  Immovable garbage-collected storage was exhausted.
7.  Internal stack overflowed.
8.  Both control and internal stacks overflowed (rare).
9.  Pure storage was exhausted.
10. Second, exhaustive garbage collection occurred.

The third argument is an ATOM indicating what initiated the garbage
collection: `GC-READ`, `BLOAT`, `GROW`, `LIST`, `VECTOR`, `SET`,
`SETG`, `FREEZE`, `GC`, `NEWTYPE`, `PURIFY`, `PURE-PAGE-LOADER` (pure
storage was exhausted), or `INTERRUPT-HANDLER` (stack overflow,
unfortunately).

### 21.8.5. "DIVERT-AGC"

`"DIVERT-AGC"` ("Automatic Garbage Collection") occurs just **before**
a deferrable garbage collection that is needed because of exhausted
movable garbage-collected storage. Enabling this interrupt is the only
way a program can know that a garbage collection is about to occur. A
handler takes two arguments: A `FIX` telling the number of machine
words needed and an `ATOM` telling what initiated the garbage
collection (see above). If it wishes, a handler can try to prevent a
garbage collection by calling `BLOAT` with the `FIX` argument. If the
pending request for garbage-collected storage cannot then be
satisfied, a garbage collection occurs anyway. `AGC-FLAG` is `SET` to
`T` while the handler is running, so that new storage requests do not
try to cause a garbage collection.

### 21.8.6. "CLOCK"

`"CLOCK"`, when enabled, occurs every half second (the ITS
"slow-clock" tick.) It is not available in the Tenex or Tops-20
versions. It wants handlers which take no arguments. Example:

    <ON "CLOCK" <FUNCTION () <PRINC "TICK ">> 1>

### 21.8.7. "BLOCKED"

`"BLOCKED"` occurs whenever **any** `PROCESS` (not only the `PROCESS`
which may be in a `HANDLER`) starts waiting or terminal input: that
is, an occurrence indicates that somewhere, somebody did a `READ`,
`READCHR`, `NEXTCHR`, `TYI`, etc. to a console. The handler for a
`"BLOCKED"` interrupt should take one argument, namely the `PROCESS`
which started waiting (which will also be the `PROCESS` in which the
handler runs, if no specific one is in the `HANDLER`).

Example: the following will cause Muddle to acquire a `*` prompting
character.

    <ON "BLOCKED" #FUNCTION ((IGNORE) <PRINC !\*>) 5>

### 21.8.8. "UNBLOCKED"

`"UNBLOCKED"` occurs whenever a `$` (`ESC`) is typed on a terminal if
a program was hanging and waiting for input, or when a TYI call (which
see) is satisfied. A handler takes one argument: the `CHANNEL` via
which the `$` or character is input.

### 21.8.9. "READ" and "WRITE"

`"READ"` and `"WRITE"` are associated with read or write references to
Muddle objects. These interrupts are often called "monitors", and
enabling the interrupt is often called "monitoring" the associated
object. A "read reference" to an `ATOM`'s local value includes
applying `BOUND?` or `ASSIGNED?` to the `ATOM`; similarly for a global
value and `GASSIGNED?`. If the `INT-LEVEL` is too high when `"READ"`
or `"WRITE"` occurs, an error occurs, because occurrences of these
interrupts cannot be queued.

Monitors are set up with `EVENT` or `ON`, using a locative to the
object being monitored as the extra *which* argument, just as a
`CHANNEL` is given for `"CHAR"`. A handler for `"READ"` takes two
arguments: the locative and the `FRAME` of the function application
that make the reference. A handler for `"WRITE"` takes three
arguments: the locative, the new value, and the `FRAME`. For example:

    <SET A (1 2 3)>$
    (1 2 3)
    <SET B <AT .A 2>>$
    #LOCL 2
    <ON "WRITE" <FUNCTION (OBJ VAL FRM)
            #DECL ((VALUE VAL ANY (OBJ) LOCATIVE (FRM) FRAME)
            <CRLF>
            <PRINC "Program changed ">
            <PRIN1 .OBJ>
            <PRINC " to ">
            <PRIN1 .VAL>
            <PRINC " via ">
            <PRINC .FRM>
            <CRLF>>
            4 0 .B>$
    #HANDLER FUNCTION (...)
    <1 .A 10>$
    (10 2 3)
    <2 .A 20>$
    Program changed #LOCL 2 to 20 via #FRAME PUT
    (10 20 3)
    <OFF "WRITE" .B>$
    #IHEADER #LOCL 20

### 21.8.10. "SYSDOWN"

`"SYSDOWN"` occurs when a system-going-down or system-revived signal
is received from ITS. It is not available in the Tenex or Tops-20
versions. If no `IHEADER` is associated and enabled, a warning message
is printed on the terminal. A handler takes one argument: a `FIX`
giving the number of thirtieths of a second until the shutdown (-1 for
a reprieve).

### 21.8.11. "ERROR"

In an effort to simplify error handling by programs, Muddle has a
facility allowing errors to be handled like interrupts. `SETG`ing
`ERROR` to a user function is a distasteful method, not safe if any
bugs are around. An `"ERROR"` interrupt wants a handler that takes any
number of arguments, via `"TUPLE"`. When an error occurs, handlers are
applied to the `FRAME` of the `ERROR` call and the `TUPLE` of `ERROR`
arguments. If a given handler "takes care of the error", it can
`ERRET` with a value from the `ERROR` `FRAME`, after having done
`<INT-LEVEL 0>`. If no handler takes care of the error, it falls into
the normal `ERROR`.

If an error occurs at an `INT-LEVEL` greater than or equal to that of
the `"ERROR"` interrupt, real `ERROR` will be called, because
`"ERROR"`interrupts cannot be queued.

### 21.8.12. "IPC"

`"IPC"` occurs when a message is received on the ITS IPC device
(chapter 23). It is not available in the Tenex and Tops-20 versions.

### 21.8.13. "INFERIOR"

`"INFERIOR"` occurs when an inferior ITS process interrupts the Muddle
process. It is not available in the Tenex and Tops-20 versions. A
handler takes one argument: A `FIX` between `0` and `7` inclusive,
telling which inferior process is interrupting.

### 21.8.14. "RUNT and "REALT"

These are not available in the Tenex and Tops-20 versions.

`"RUNT"`, if enabled, occurs **once**, *N* seconds of Muddle running
time (CPU time) after calling `<RUNTIMER N:fix-or-float>`, which
returns its argument. A handler takes no arguments. If `RUNTIMER` is
called with no argument, it returns a `FIX`, the number of run-time
seconds left until the interrupt occurs, or `#FALSE ()` if the
interrupt is not going to occur.

`"REALT"`, if enabled, occurs **every** *N* seconds of real-world time
after calling `<REALTIMER N:fix-or-float>`, which returns its
argument. A handler takes no arguments. `<REALTIMER 0>` tells the
operating system not to generate real-time interrupts. If `REALTIMER`
is called with no argument, it returns a `FIX`, the number of
real-time seconds given in the most recent call to `REALTIMER` with an
argument, or `#FALSE ()` if `REALTIMER` has not been called.

### 21.8.15. "Dangerous" Interrupts

`"MPV"` ("memory protection violation") occurs if Muddle tries to
refer to a storage address not in its address space. `"PURE"` occurs
if Muddle tries to alter read-only storage. `"ILOPR"` occurs if Muddle
executes and illegal instruction ("operator"). `"PARITY"` occurs if
the CPU detects a parity error in Muddle's address space. All of these
require a handler that takes one argument: the address (`TYPE` `WORD`)
following the instruction that was being executed at the time.

`"IOC"` occurs if Muddle tries to deal illegally with an I/O channel.
A handler must take two arguments: a three-element `FALSE` like one
that `OPEN` might return, and the `CHANNEL` that got the error.

Ideally these interrupts should never occur. In fact, in the Tenex and
Tops-20 versions, these interrupts always go to the superior operating
system process instead of to Muddle. In the ITS version, if and when a
"dangerous" interrupt does occur:

-   If no `IHEADER` is associated with the interrupt, then the
    interrupt goes to the superior operating system process.
-   If an `IHEADER` is associated but disabled, the error
    `DANGEROUS-INTERRUPT-NOT-HANDLED` occurs (`FILE-SYSTEM-ERROR` for
    \`"IOC").
-   If an `IHEADER` is associated and enabled, but the `INT-LEVEL` is
    too high, the error `ATTEMPT-TO-DEFER-UNDEFERABLE-INTERRUPT`
    occurs.

21.9. User-Defined Interrupts
-----------------------------

If the interrupt name given to `EVENT` or `ON` is **not** one of the
standard predefined interrupts of Muddle, they will gleefully create
an `ATOM` in `<INTERRUPTS>` and an associated `IHEADER` anyway, making
the assumption that you are setting up a "program-defined" interrupt.

Program-defined interrupts are made to occur by applying the `SUBR`
`INTERRUPT`, as in

    <INTERRUPT name arg1 ... argN>

where *name* is a `STRING`, `ATOM` or `IHEADER`, and *arg1* through
*argN* are the arguments wanted by the handlers for the interrupt.

If the interrupt specified by `INTERRUPT` is enabled, `INTERRUPT`
returns `T`; otherwise it returns `#FALSE ()`. All the usual priority
and queueing rules hold, so that even if `INTERRUPT` returns `T`, it
is possible that nothing "really happened" (yet).

`INTERRUPT` can also be used to cause "artificial" occurrences of
standard predefined Muddle interrupts.

Making a program-defined interrupt occur is similar to calling a
handler directly, but there are differences. The value returned by a
handler is ignored, so side effects must be used in order to
communicate information back to the caller, other than whether any
handler ran or will run. One good use for a program-defined interrupt
is to use the priority and queueing machinery of `INT-LEVEL` to
control the execution of functions that must not run concurrently. For
example, if a `"CHAR"` handler just deposits characters in a buffer,
then a function to process the buffered characters should probably run
at a higher priority level -- to prevent unpredictable changes to the
buffer during the processing -- and it is natural to invoke the
processing with `INTERRUPT`.

In more exotic applications, `INTERRUPT` can signal a condition to be
handled by an unknown number of independent and "nameless" functions.
The functions are "nameless" because the caller doesn't know their
name, only the name of the interrupt. This programming style is
modular and event-driven, and it is one way of implementing
"heuristic" algorithms. In addition, each `HANDLER` has a `PROCESS` in
which to run its handler, and so the different handlers for a given
condition can do their thing in different environments quite easily,
with less explicit control than when using `RESUME`.

21.10. Waiting for Interrupts
-----------------------------

### 21.10.1. HANG

    <HANG pred>

hangs interruptibly, without consuming any CPU time, potentially
forever. `HANG` is nice for a program that cannot do anything until an
interrupt occurs. If the optional *pred* is given, it is evaluated
every time an interrupt occurs and is dismissed back into the `HANG`;
if the result of evaluation is not `FALSE`, `HANG` unhangs and returns
it as a value. If *pred* is not given, there had better be a named
`ACTIVATION` somewhere to which a handler can return.

### 21.10.2. SLEEP

    <SLEEP time:fix-or-float pred>

suspends execution, interruptibly, without consuming any CPU time, for
*time* seconds, where *time* is non-negative, and then returns `T`.
*pred* is the same as for `HANG`.

Chapter 22. Storage Management
==============================

The reason this chapter comes so late in this document is that, except
for special cases, Muddle programs have their storage needs handled
automatically. There is usually no need even to consider storage
management, except as it affects efficiency (chapter 24). This chapter
gives some explanation of why this is so, and covers those special
means by which a program can assume control of storage management.

The Muddle address space is divided into five parts, which are usually
called

1.  movable garbage-collected space,
2.  immovable space (both garbage-collected and not),
3.  user pure/page space,
4.  pure-`RSUBR` mapping space, and
5.  internal storage.

Internal storage occupies both the highest and lowest addresses in the
address space, and its size never changes as Muddle executes. The
other spaces can vary in size according to the needs of the executing
program. Generally the interpreter allocates a contiguous set of
addresses for each space, and each space gradually fills up as new
objects are created and as disk files are mapped in. The action taken
when space becomes full varies, as discussed below.

22.1. Movable Garbage-collected Storage
---------------------------------------

Most storage used explicitly by Muddle programs is obtained from a
pool of free storage managed by a "garbage collector". Storage is
obtained from this pool by the `SUBR`s which construct objects. When a
`SUBR` finds that the pool of available storage is exhausted, it
automatically calls the garbage collector.

The garbage collector has two algorithms available to it: the
"copying" algorithm, which is used by default, and the "mark-sweep"
algorithm. Actually, one often speaks of two separate garbage
collectors, the "copying" one and the "mark-sweep" one, because each
is an independent module that is mapped in to the interpreter's
internal storage from disk only during garbage collection. For
simplicity, this document speaks of "the" garbage collector, which has
two algorithms.

The garbage collector examines the storage pool and **marks** all the
objects there, separating them into two classes: those which cannot
possibly be referenced by a program, and those which can. The
"copying" algorithm then copies the latter into one compact section of
the pool, and the remainder of the pool is made available for newly
constructed objects. The "mark-sweep" algorithm, instead, puts all
objects in the former class (garbage) into "free lists", where the
object-construction `SUBR`s can find them and re-use their storage.

If the request for more storage still cannot be satisfied from
reclaimed storage, the garbage collector will attempt to obtain more
total storage from the operating system under which Muddle runs.
(Also, if there is a gross superfluity of storage space, the garbage
collector will politely return some storage to the operating system.)
Only when the total system resources are exhausted will you finally
lose.

Thus, if you just "forget about" an object, that is, lose all possible
means of referencing it, its storage is automatically reclaimed.
"Object" in this context includes that stack-structured storage space
used in `PROCESS`es for functional application.

### 22.1.1. Stacks and Other Internal Vectors

Control stacks are used in Muddle to control the changes in
environment caused by calling and binding. Each active `PROCESS` has
its own control stack. On this stack are stored `LVAL`s for `ATOM`s;
`PRIMTYPE` `TUPLE`s, which are otherwise like `VECTOR`s; `PRIMTYPE`
`FRAME`s, which are generated by calling Subroutines; and
`ACTIVATION`s, which are generated by calling `FUNCTION`s with named
`ACTIVATION`s, `PROG`, and `REPEAT`. `TAG` and `LLOC` can make `TAG`s
and `LOCD`s (respectively) that refer to a specific place on a
specific control stack. (`LEGAL?` returns `T` if and only if the
portion of the control stack in which its argument is found or to
which its argument refers is still active, or if its argument doesn't
care about the control stack. The garbage collector may change a
non-`LEGAL?` object to `TYPE` `ILLEGAL` before reclaiming it.) As the
word "stack" implies, things can be put on it and removed from it at
only one end, called the top. It has a maximum size (or depth), and
attempting to put too many things on it will cause overflow. A stack
is stored like a `VECTOR`, and it must be `GROW`n if and when it
overflows.

A control stack is actually two stacks in one. One section is used for
"top-level" `LVAL`s -- those `SET` while the `ATOM` is not bound by
any active Function's argument `LIST` or Subroutine's `SPECIAL`
binding -- and the other section is used for everything else. Either
section can overflow, of course. The top-level-`LVAL` section is below
the other one, so that a top-level `LVAL` will be found only if the
`ATOM` is not currently bound elsewhere, namely in the other section.

Muddle also has an internal stack, used for calling and temporary
storage within the interpreter and compiled programs. It too is stored
like a `VECTOR` and can overflow. There are other internal vectors
that can overflow: the "global vector" holds pairs ("slots") of
`ATOM`s and corresponding `GVAL`s ("globally bound" or `GBOUND?` means
that the `ATOM` in question is in this vector, whether or not it
currently has a global value), and the "`TYPE` vector" holds `TYPE`
names (predefined and `NEWTYPE`s) and how they are to be treated.

22.2. Immovable Storage
-----------------------

### 22.2.1. Garbage-collected: FREEZE

In very special circumstances, such as debugging `RSUBR`s, you may
need to prevent an object from being moved by the garbage collector.
`FREEZE` takes one argument, of `PRIMTYPE` `VECTOR`, `UVECTOR`,
`STRING`, `BYTES` or `TUPLE`. It copies its argument into non-moving
garbage-collected space. `FREEZE` returns the copy `CHTYPE`d to its
`PRIMTYPE`, except in the case of a `TUPLE`, which is changed to a
`VECTOR`.

### 22.2.2. Non-garbage-collected: STORAGE (the PRIMTYPE)

An object of `PRIMTYPE` `STORAGE` is really a frozen `UVECTOR` whose
`UTYPE` is of `PRIMTYPE` `WORD`, but it is always pointed to by
something internal to Muddle and thus is never garbage-collectible.
The use of `FREEZE` is always preferable, except when for historical
reasons a `STORAGE` is necessary.

22.3. Other Storage
-------------------

User pure/page space serves two purposes. First, when a user program
`PURIFY`s (see below) Muddle objects, they are copied into this space.
Second, so-called hand-crafted `RSUBR`s (assembled but not compiled)
can call on the interpreter to map pages of disk files into this space
for arbitrary purposes.

Pure-`RSUBR` mapping space is used by the interpreter to dynamically
map pages of pure compiled programs into and out of the Muddle address
space. Pure code can refer to impure storage through the "transfer
vector", another internal vector. This space is the most vulnerable to
being compressed in size by the long-term growth of other spaces.

Internal storage has both pure and impure parts. The interpreter
program itself is pure and sharable, while impure storage is used for
internal pointers, counters, and flags, for example, pointers to the
boundaries of other spaces. In the pure part of this space are most of
the `ATOM`s in an initial Muddle, along with their `OBLIST` buckets
(`LIST`s) and `GVAL` slots (a pure extension of the global vector),
where possible. A `SET` or `SETG` of a pure `ATOM` automatically
impurifies the `ATOM` and as much of its `OBLIST` bucket as needs to
be impure.

22.4. Garbage Collection: Details
---------------------------------

When either of the garbage-collected spaces (movable or immovable)
becomes full, Muddle goes through the following procedure:

1.  A `"DIVERT-AGC"` interrupt occurs if the garbage collection can be
    deferred temporarily by shifting boundaries between storage spaces
    slightly. The interrupt handler may postpone a garbage collection
    by moving boundaries itself with a call to `BLOAT` (below).
2.  The garbage collector begins execution. The "copying" algorithm
    creates an inferior operating-system process (named `AGC` in the
    ITS version) whose address space is used to hold the new copies of
    non-garbage objects. Muddle accesses the inferior's address space
    through two pages ("frontier" and "window") in its internal space
    that are shared with the inferior. If the garbage collection
    occurred because movable garbage-collected space was exhausted,
    then the "mark-sweep" algorithm might be used instead (see below)
    and no inferior process is created.
3.  The garbage collector marks and moves all objects that can
    possibly be referenced hereafter. It begins with the `<MAIN>`
    `PROCESS` and the currently running `PROCESS` `<ME>`, considered
    as vectors containing the control stacks, object pointers in live
    registers, etc. Every object in these "`PROCESS` vectors" is
    marked "accessible", and every element of these objects (bindings,
    etc.), and so on recursively. The "copying" algorithm moves
    objects into the inferior process's address space as it marks
    them.
4.  If the garbage collection is "exhaustive" -- which is possible
    only in the "copying" algorithm -- then both the chain of
    associations and top-level local/global bindings are examined
    thoroughly, which takes more time but is more likely to uncover
    garbage therein. In a normal garbage collection these constructs
    are not treated specially.
5.  Finally, the "mark-sweep" algorithm sweeps through the storage
    space, adding unmarked objects to the internal free lists for
    later re-use. The "copying" algorithm maps the inferior process's
    address space into Muddle's own, replacing old garbagey with the
    new compact storage, and the inferior process is destroyed.

22.5 GC
-------

    <GC min:fix exh?:false-or-any ms-freq:fix>

causes the garbage collector to run and returns the total number of
words of storage reclaimed. All of its arguments are optional: if they
are not supplied, a call to GC simply causes a "copying" garbage
collection.

If *min* is explicitly supplied as an argument, a garbage-collection
parameter is changed permanently before the garbage collector runs.
*min* is the smallest number of words of "free" (unclaimed, available
for use) movable garbage-collected storage the garbage collector will
be satisfied with having after it is done. Initially it is 8192 words.
If the total amount of reclaimed storage is less than *min*, the
garbage collector will ask the operating system for enough storage (in
1024 word blocks) to make it up. N.B.: the system may be incivil
enough not to grant the request; in that case, the garbage collector
will be content with what it has, **unless** that is not enough to
satisfy a **pending** request for storage. Then it will inform you
that it is losing. A large *min* will result in fewer total garbage
collections, but they will take longer since the total quantity of
storage to be dealt with will generally be larger. Smaller *min*s
result in shorter, more frequent garbage collections.

22.6. BLOAT
-----------

`BLOAT` is used to cause a temporary expansion of the available
storage space with or without changing the garbage-collection
parameters. `BLOAT` is particularly useful for avoiding unnecessary
garbage collections when loading a large file. It will cause (at most)
one garbage collection, at the end of which the available storage will
be at least the amount specified in the call to `BLOAT`. (Unless, of
course, the operating system is cranky and will not provide the
storage. Then you will get an error. `<ERRET 1>` from this error will
cause the `BLOAT` to return `1`, which usually just causes you to lose
at a later time -- unless the operating system feels nicer when the
storage is absolutely necessary.)

A call to BLOAT looks like this:

    <BLOAT fre stk lcl glb typ sto pstk
           min plcl pglb ptyp imp pur dpstk dstk>

where all arguments on the first line above are `FIX`, optional (`0`
by default), and indicate the following:

-   *fre*: number of words of free movable storage desired (for
    `LIST`s, `VECTOR`s, `ATOM`s, etc.)
-   *stk*: number of words of free control-stack space desired (for
    functional applications and binding of `ATOM`s)
-   *lcl*: number of new top-level `LVAL`s for which to leave space
    (`SET`s of `ATOM`s which are not currently bound)
-   *glb*: number of new `GVAL`s for which to leave space (in the
    global vector)
-   *typ*: number of new `TYPE` definitions for which to leave space
    (in the `TYPE` vector)
-   *sto*: number of words of immovable garbage-collected storage
    desired
-   *pstk*: number of words of free internal-stack space desired (for
    `READ`ing large `STRING`s, and calling routines within the
    interpreter and compiled programs)

Arguments on the second line are also `FIX` and optional, but they set
garbage-collection parameters permanently, as follows:

-   *min*: as for `GC`
-   *plcl*: number of slots for `LVAL`s added when the space for
    top-level `LVAL`s is expanded (initially 64)
-   *pglb*: number of slots for `GVAL`s added when the global vector
    is grown (initially 64)
-   *ptyp*: number of slots for `TYPE`s added when the `TYPE` vector
    is grown (initially 32)
-   *imp*: number of words of immovable garbage-collected storage
    added when it is expanded (initially 1024)
-   *pur*: number of words reserved for pure compiled programs, if
    possible (initially 0)
-   *dpstk*: most desirable size for the internal stack, to prevent
    repeated shrinking and `GROW`ing (initially 512)
-   *dstk*: most desirable size for the control stack (initially 4096)

`BLOAT` returns the actual number of words of free movable
garbage-collected storage available when it is done.

22.7. BLOAT-STAT
----------------

`BLOAT-STAT` can be used with `BLOAT` to "tune" the garbage collector
to particular program requirements.

    <BLOAT-STAT length-27:uvector>

fills the *uvector* with information about the state of storage of
Muddle. The argument should be a `UVECTOR` of length 27 and `UTYPE`
`FIX`. If `BLOAT-STAT` does not get an argument, it will provide its
own `UVECTOR`. The information returned is as follows: the first 8
elements indicate the number of garbage collections that are
attributable to certain causes, and the other 19 give information
about certain areas of storage. In detail:

1.  number of garbage collections caused by exhaustion of movable
    garbage-collected storage
2.  ditto by overflow of control stack(s)
3.  ditto by overflow of top-level-`LVAL` section of control stack(s)
4.  ditto by overflow of global vector
5.  ditto by overflow of `TYPE` vector
6.  ditto by exhaustion of immovable garbage-collected storage
7.  ditto by overflow of internal stack
8.  ditto by overflow of both stacks at the same time (rare)

9.  number of words of movable storage
10. number of words of movable storage used since last `BLOAT-STAT`
11. maximum number of words of movable storage ever existing
12. number of words of movable storage used since Muddle began running
13. maximum size of control stack
14. number of words on control stack in use
15. maximum size of control stack(s) ever reached
16. number of slots for top-level `LVAL`s
17. number of top-level `LVAL`s existing
18. number of slots for `GVAL`s in global vector
19. number of `GVAL`s existing
20. number of slots for `TYPE`s in `TYPE` vector
21. number of `TYPE`s existing
22. number of words of immovable garbage-collected storage
23. number of words of immovable storage unused
24. size of largest unused contiguous immovable-storage block
25. number of words on internal stack
26. number of words on internal stack in use
27. maximum size of internal stack ever reached

22.8. GC-MON
------------

    <GC-MOND pred>

("garbage-collector monitor") determines whether or not the
interpreter will hereafter print information on the terminal when a
garbage collection starts and finishes, according to whether or not
its argument is true. It returns the previous state. Calling it with
no argument returns the current state. The initial state is false.

When typing is enabled, the "copying" garbage collector prints, when
it starts:

    GIN reason subr-that-caused:atom

and, when it finishes:

    GOUT seconds-needed

The "mark-sweep" garbage collector prints `MSGIN` and `MSGOUT` instead
of `GIN` and `GOUT`.

22.9. Related Subroutines
-------------------------

Two `SUBR`s, described next, use only part of the garbage-collector
algorithm, in order to find all pointers to an object. `GC-DUMP` and
`GC-READ`, as their names imply, also use part in order to translate
between Muddle objects and binary representation thereof.

### 22.9.1. SUBSTITUTE

    <SUBSTITUTE new:any old:any>

returns *old*, after causing a miniature garbage collection to occur,
during which **all** references to *old* are changed so as to refer to
*new*. Neither argument can be of `PRIMTYPE` `STRING` or `BYTES` or
`LOCD` or live on the control stack, unless both are of the same
`PRIMTYPE`. One `TYPE` name cannot be substituted for another. One of
the few legitimate uses for it is to substitute the "right" `ATOM` for
the "wrong" one, after `OBLIST`s have been in the wrong state. This is
more or less the way `ATOM`s are impurified. It is also useful for
unlinking `RSUBR`s. `SUBSTITUTE` returns *old* as a favor: unless you
hang onto *old* at that point, it will be garbage-collected.

22.9.2 PURIFY
-------------

    <PURIFY any-1 ... any-N>

returns its last argument, after causing a miniature garbage
collection that results in all the arguments becoming pure and
sharable, and ignored afterward by the garbage collector. No argument
can live on the control stack or be of `PRIMTYPE` `PROCESS` or `LOCD`
or `ASOC`. Sharing between operating-system processes actually occurs
after a `SAVE`, if and when the `SAVE` file is `RESTORE`d.

Chapter 23. Muddle as a System Process
======================================

This chapter treats Muddle considered as executing in an
operating-system process, and interactions between Muddle and other
operating-system processes. See also section 21.8.13.

23.1. TIME
----------

`TIME` takes any number of arguments, which are evaluated but ignored,
and returns a `FLOAT` giving the number of seconds of CPU time the
Muddle process has used so far. `TIME` is often used in machine-level
debugging to examine the values of its arguments, by having Muddle's
superior process (say, DDT) plant a breakpoint in the code for `TIME`.

23.2. Names
-----------

    <UNAME>

returns a `STRING` which is the "user name" of Muddle's process. This
is the "uname" process-control variable in the ITS version and the
logged-in directory in the Tenex and Tops-20 versions.

    <XUNAME>

returns a `STRING` which is the "intended user name" of Muddle's
process. This is the "xuname" process-control variable in the ITS
version and identical to `<UNAME>` in the Tenex and Tops-20 versions.

    <JNAME>

returns a `STRING` which is the "job name" of Muddle's process. This
is the "jname" process-control variable in the ITS version and the
`SETNM` name in the Tenex and Tops-20 versions. The characters belong
to the "sixbit" or "printing" subset of ASCII, namely those between
`<ASCII *40*>` and `<ASCII *137*>` inclusive.

    <XJNAME>

returns a `STRING` which is the "intended job name" of Muddle's
process. This is the "xjname" process-control variable in the ITS
version and identical to `<JNAME>` in the Tenex and Tops-20 versions.

23.3. Exits
-----------

    <LOGOUT>

attempts to log out the process in which it is executed. It will
succeed only if the Muddle is the top-level process, that is, it is
running disowned or as a daemon. If it succeeds, it of course never
returns. If it does not, it returns `#FALSE ()`.

    <QUIT>

causes Muddle to stop running, in an orderly manner. In the ITS
version, it is equivalent to a `.LOGOUT 1` instruction. In the Tenex
and Tops-20 versions, it is equivalent to a control-C signal, and
control passes to the superior process.

    <VALRET string-or-fix>

("value return") seldom returns. It passes control back up the process
tree to the superior of Muddle, passing its argument as a message to
that superior. If it does return, the value is `#FALSE ()`. If the
argument is a `STRING`, it is passed to the superior as a command to
be executed, via `.VALUE` in the ITS version and `RSCAN` in the
Tops-20 version. If the argument is a `FIX`, it is passed to the
superior as the "effective address" of a `.BREAK 16`, instruction in
the ITS version and ignored in other versions.

23.4. Inter-process Communication
---------------------------------

All of the `SUBR`s in this section are available only in the ITS
version.

The IPC ("inter-process communication") device is treated as an I/O
device by ITS but not explicitly so by Muddle: that is, it is never
`OPEN`ed. It allows Muddle to communicate with other ITS processes by
means of sending and receiving messages. A process identifies itself
as sender or recipient of a message with an ordered pair of "sixbit"
`STRING`s, which are often but not always `<UNAME>` and `<JNAME>`. A
message has a "body" and a "type".

### 23.4.1. SEND and SEND-WAIT

    <SEND othern1 othern2 body type mynamel myname2>

    <SEND-WAIT othern1 othern2 body type mynamel myname2>

both send an IPC message to any job that is listening for it as
*othern1* *othern2*. *body* must be either a `STRING`, or a `UVECTOR`
of objects of `PRIMTYPE` `WORD`. *type* is an optional `FIX`, `0` by
default, which is part of the information the other guy receives. The
last two arguments are from whom the message is to be sent. These are
optional, and `<UNAME>` and `<JNAME>` respectively are used by
default. `SEND` returns a `FALSE` if no one is listening, while
`SEND-WAIT` hangs until someone wants it. Both return `T` if someone
accepts the message.

### 23.4.2. The "IPC" Interrupt

When your Muddle process receives an IPC message, `"IPC"` occurs
(chapter 21). A handler is called with either four or six arguments
gleaned from the message. *body*, *type*, *othern1*, and *othern2* are
supplied only if they are not this process's `<UNAME>` and `<JNAME>`.

There is a built-in `HANDLER` for the `"IPC"` interrupt, with a
handler named `IPC-HANDLER` and `0` in the `PROCESS` slot. The handler
prints out on the terminal the *body*, whom it is from, the *type* if
not `0`, and whom it is to if not `<UNAME>` `<JNAME>`. If the *type*
is `1` and the *body* is a `STRING`, then, after the message
information is printed out, the `STRING` is `PARSE`d and `EVAL`uated.

### 23.4.3. IPC-OFF

`<IPC-OFF>` stops all listening on the IPC device.

### 23.4.4. IPC-ON

    <IPC-ON myname1 myname2>

causes listening on the IPC device as *myname1* *myname2*. If no
arguments are provided, listening is on `<UNAME>` `<JNAME>`. When a
message arrives, `"IPC"` occurs.

Muddle is initially listening as `<UNAME>` `<JNAME>` with the built-in
`HANDLER` set up on the `"IPC"` interrupt with a priority of `1`.

### 23.4.5. DEMSIG

    <DEMSIG daemon:string>

signals to ITS (directly, not via the IPC device) that the daemon
named by its argument should run now. It returns `T` if the daemon
exists, `#FALSE ()` otherwise.

Chapter 24. Efficiency and Tastefulness
=======================================

24.1. Efficiency
----------------

Actually, you make Muddle programs efficient by thinking hard about
what they really make the interpreter **do**, and making them do less.
Some guidelines, in order of decreasing expense:

1.  Free storage is expensive.
2.  Calling functions is expensive.
3.  `PROG` and `REPEAT` are expensive, except when compiled.

Explanation:

1.  Unnecessary use of free storage (creating needless `LIST`s,
    `VECTOR`s, `UVECTOR`s, etc.) will cause the garbage collector to
    run more often. This is **expensive!** A fairly large Muddle (for
    example, 60,000 36-bit words) can take ten seconds of PDP-10 CPU
    time for a garbage collection. Be especially wary of constructions
    like `(0)`. Every time that is evaluated, it creates a new
    one-element `LIST`; it is too easy to write such things when they
    aren't really necessary. Unless you are doing `PUT`s or `PUTREST`s
    on it, use `'(0)` instead.
2.  Sad, but true. Also generally ignored. If you call a function only
    once, or if it is short (less than one line), you are much better
    off in speed if you substitute its body in by hand. On the other
    hand, you may be much worse off in modularity. There are
    techniques for combining several `FUNCTION`s into one `RSUBR`
    (with `RSUBR-ENTRY`s), either during or after compilation, and for
    changing `FUNCTION`s into `MACRO`s.
3.  `PROG` is almost never necessary, given (a) `"AUX"` in
    `FUNCTION`s; (b) the fact that `FUNCTION`s can contain any number
    of `FORM`s; (c) the fact that `COND` clauses can contain any
    number of `FORM`s; and (d) the fact that new variables can be
    generated and initialized by `REPEAT`. However, `PROG` may be
    useful when an error occurs, to establish bindings needed for
    cleaning things up or interacting with a human.

The use of `PROG` may be sensible when the normal flow of control can
be cut short by unusual conditions, so that the program wants to
`RETURN` before reaching the end of `PROG`. Of course, nested `COND`s
can accomplish the same end, but deep nesting may tend to make the
program unreadable. For example:

    <PROG (TEMP)
          <OR <SET TEMP <OK-FOR-STEP-1?>>
              <RETURN .TEMP>>
          <STEP-1>
          <OR <SET TEMP <OK-FOR-STEP-2?>>
              <RETURN .TEMP>>
          <STEP-2>>

could instead be written

    <COND (<OK-FOR-STEP-1?>
           <STEP-1>
           <COND (<OK-FOR-STEP-2?>
                  <STEP-2>)>)>

By the way, `REPEAT` is faster than `GO` in a `PROG`. The `<GO x>`
`FORM` has to be separately interpreted, right? In fact, if you
organize things properly you **very** seldom need a `GO`; using `GO`
is generally considered "bad style", but in some cases it's needed.
Very few.

In many cases, a `REPEAT` can be replaced with a `MAPF` or `MAPR`, or
an `ILIST`, `IVECTOR`, etc. of the form

    <ILIST .N '<SET X <+ .X 1>>

which generates an `N`-element `LIST` of successive numbers starting
at `X+1`.

Whether a program is interpreted or compiled, the first two
considerations mentioned above hold: garbage collection and function
calling remain expensive. Garbage collection is, clearly, exactly the
same. Function calling is relatively more expensive. However, the
compiler careth not whether you use `REPEAT`, `GO`, `PROG`, `ILIST`,
`MAPF`, or whatnot: it all gets compiled into practically the same
thing. However, the `REPEAT` or `PROG` will be slower if it has an
`ACTIVATION` that is `SPECIAL` or used other than by `RETURN` or
`AGAIN`.

### 24.1.1. Example

There follows an example of a `FUNCTION` that does many things wrong.
It is accompanied by commentary, and two better versions of the same
thing. (This function actually occurred in practice. Needless to say,
names are withheld to protect the guilty.)

Blunt comment: this is terrible. Its purpose is to output the
characters needed by a graphics terminal to draw lines connecting a
set of points. The points are specified by two input lists: `X` values
and `Y` values. The output channel is the third argument. The actual
characters for each line are returned in a `LIST` by the function
`TRANS`.

    <DEFINE PLOTVDSK (X Y CHN "AUX" L LIST)
       <COND (<NOT <==? <SET L <LENGTH .X>><LENGTH .Y> >>
              <ERROR "LENGTHS NOT EQUAL">)>
       <SET LIST (29)>
       <REPEAT ((N 1))
           <SET LIST (!.LIST !<TRANS <.N .X> <.N .Y>>)>
           <COND (<G? <SET N <+ .N 1>> .L><RETURN .N>)> >
       <REPEAT ((N 1) (L1 <LENGTH .LIST>))
           <PRINC <ASCII <.N .LIST>> .CHN>
           <COND (<G? <SET N <+ .N 1>> .L1>
                  <RETURN "DONE">)> >>

Comments:

1.  `LIST` is only temporarily necessary. It is just created and then
    thrown away.
2.  Worse, the construct `(!.LIST !<TRANS ...>)` **copies** the
    previous elements of `LIST` every time it is executed!
3.  Indexing down the elements of `LIST` as in `<.N .LIST>` takes a
    long time, if the `LIST` is long. `<3 ...>` or `<4 ...>` is not
    worth worrying about, but `<10 ...>` is, and `<100 ...>` takes
    quite a while. Even if the indexing were not phased out, the
    compiler would be happier with `<NTH .LIST .N>`.
4.  The variable `CHN` is unnecessary if `OUTCHAN` is bound to the
    argument `CHANNEL`.
5.  It is tasteful to call `ERROR` in the same way that F/SUBRs do.
    This includes using an `ATOM` from the `ERRORS` `OBLIST` (if one
    is appropriate) to tell what is wrong, and it includes identifying
    yourself.

So, do it this way:

    <DEFINE PLOTVDSK (X Y OUTCHAN)
    #DECL ((OUTCHAN <SPECIAL CHANNEL>)
    <COND (<NOT <==? <LENGTH .X> <LENGTH .Y>>>
            <ERROR VECTOR-LENGTHS-DIFFER!-ERRORS PLOTVDSK>)>
    <PRINC <ASCII 29>>
    <REPEAT ()
            <COND (<EMPTY? .X> <RETURN "DONE">)>
            <REPEAT ((OL <TRANS <1 .X> <1 .Y>>))
                    <PRINC <ASCII <1 .OL>>>
                    <COND (<EMPTY? <SET OL <REST .OL>>>
                           <RETURN>)>>
            <SET X <REST .X>>
            <SET Y <REST .Y>>>>

Of course, if you know how long is the `LIST` that `TRANS` returns,
you can avoid using the inner `REPEAT` loop and have explicit `PRINC`s
for each element. This can be done even better by using `MAPF`, as in
the next version, which does exactly the same thing as the previous
one, but uses `MAPF` to do the `REST`ing and the end conditional:

    <DEFINE PLOTVDSK (X Y OUTCHAN)
    #DECL ((OUTCHAN <SPECIAL CHANNEL>)
    <COND (<NOT <==? <LENGTH .X> <LENGTH .Y>>>
            <ERROR VECTOR-LENGTHS-DIFFER!-ERRORS PLOTVDSK>)>
    <PRINC <ASCII 29>> <MAPF <>
          #FUNCTION ((XE YE)
                    <MAPF <> #FUNCTION ((T) <PRINC <ASCII .T>>) <TRANS
    .XE .YE>>)
          .X
          .Y>
    "DONE">

24.2. Creating a LIST in Forward Order
--------------------------------------

If you must create the elements of a `LIST` in sequence from first to
last, you can avoid copying earlier ones when adding a later one to
the end. One way is to use `MAPF` or `MAPR` with a first argument of
`,LIST`: the elements are put on the control stack rather than in free
storage, until the final call to `LIST`. If you know how many elements
there will be, you can put them on the control stack yourself, in a
`TUPLE` built for that purpose. Another way is used when `REPEAT` is
necessary:

    <REPEAT ((FIRST (T)) (LAST .FIRST) ...)
            #DECL ((VALUE FIRST LAST) LIST ...)
            ...
            <SET LAST <REST <PUTREST .LAST (.NEW)>>>
            ...
            <RETURN <REST .FIRST>>>
            ...>

Here, `.LAST` always points to the current last element of the `LIST`.
Because of the order of evaluation, the `<SET LAST ...>` could also be
written `<PUTREST .LAST (SET LAST (.NEW)>>`.

24.3. Read-only Free Variables
------------------------------

If a Function uses the value of a free variable
(`<GVAL unmanifest:atom>` or `<LVAL special:atom>`) without changing
it, the compiled version may be more efficient if the value is
assigned to a dummy `UNSPECIAL` `ATOM` in the Function's `"AUX"` list.
This is true because an `UNSPECIAL` `ATOM` gets compiled into a slot
on the control stack, which is accessible very quickly. The tradeoff
is probably worthwhile if a *special* is referenced more than once, or
if an *unmanifest* is referenced more than twice. Example:

    <DEFINE MAP-LOOKUP (THINGS "AUX" (DB ,DATA-BASE))
            #DECL ((VALUE) VECTOR (THINGS DB) <UNSPECIAL <PRIMTYPE LIST>>)
            <MAPF ,VECTOR <FUNCTION (T) <MEMQ .T .DB>> .THINGS>>

24.4. Global and Local Values
-----------------------------

In the interpreter the sequence `,X .X ,X .X` is slower than
`,X ,X .X .X` because of interference between the `GVAL` and `LVAL`
mechanisms (appendix 1). Thus it is not good to use both the `GVAL`
and `LVAL` of the same `ATOM` frequently, unless references to the
`LVAL` will be compiled away (made into control stack references).

24.5. Making Offsets for Arrays
-------------------------------

It is often the case that you want to attach some meaning to each
element of an array and access it independently of other elements.
Firstly, it is a good idea to use names (`ATOM`s) rather than integers
(`FIX`es or even `OFFSET`s) for offsets into the array, to make future
changes easier. Secondly, it is a good idea to use the `GVAL`s of the
name `ATOM`s to remember the actual `FIX`es, so that the `ATOM`s can
be `MANIFEST` for the compiler's benefit. Thirdly, to establish the
`GVAL`s, both the interpreter and the compiler will be happier with
`<SETG name offset>` rather than
`<DEFINE name ("TUPLE" T) <offset !.T>>`.

24.6. Tables
------------

There are several ways in Muddle to store a table, that is, a
collection of (names and) values that will be searched.
Unsurprisingly, choosing the best way is often dictated by the size of
the table and/or the nature of the (names and) values.

For a small table, the names and values can be put in (separate)
structures -- the choice of `LIST` or array being determined by
volatility and limitability -- which are searched using `MEMQ` or
`MEMBER`. This method is very space-efficient. If the table gets
larger, and if the elements are completely orderable, a (uniform)
vector can be used, kept sorted, and searched with a binary search.

For a large table, where reasonably efficient searches are required, a
hashing scheme is probably best. Two methods are available in Muddle:
associations and `OBLIST`s.

In the first method, `PUTPROP` and `GETPROP` are used, which are very
fast. The number of hashing buckets is fixed. Duplicates are
eliminated by `==?` testing. If it is necessary to use `=?` testing,
or to find all the entries in the table, you can duplicate the table
in a `LIST` or array, to be used only for those purposes.

In the second method, `INSERT` and `LOOKUP` on a specially-built
`OBLIST` are used. (If the names are not `STRING`s, they can be
converted to `STRING`s using `UNPARSE`, which takes a little time.)
The number of hashing buckets can be chosen for best efficiency.
Duplicates are eliminated by `=?` testing. MAPF/R can be used to find
all the entries in the table.

24.7. Nesting
-------------

The beauty of deeply-nested control structures in a single `FUNCTION`
is definitely in the eye of the beholder. (`PPRINT`, a preloaded
`RSUBR`, finds them trying. However, the compiler often produces
better code from them.) **If** you don't like excessive nesting, then
you will agree that

    <SET X ...>
    <COND (<0? .X> ...) ...>

looks better than

    <COND (<0? <SET X ...>> ...) ...>

and that

    <REPEAT ...
            <COND ...
                  (... <RETURN ...>)>
            ...
            ...>

looks better than

    <REPEAT ...
            <COND ...
                  (... <RETURN ...>)
                  (ELSE ...)>
            ...>

You can see the nature of the choices. Nesting is still and all better
than `GO`.

Appendix 1. A Look Inside
=========================

This appendix tells about the mapping between Muddle objects and
PDP-10 storage -- in other words, the way things look "on the inside".
None of this information is essential to knowing how to program in
Muddle, but it does give some reasons for capabilities and
restrictions that otherwise you have to memorize. The notation and
terminology get a little awkward in this discussion, because we are in
a twilight zone between the worlds of Muddle objects and of bit
patterns. In general the words and phrases appearing in diagrams refer
to bit patterns not Muddle objects. A lower-case word (like "tuple")
refers to the storage occupied by an object of the corresponding
`PRIMTYPE` (like `TUPLE`).

First some terminology needs discussion. The sine qua non of any
Muddle object is a **pair** of 36-bit computer words. In general,
lists consist of pairs chained together by pointers (addresses), and
vectors consist of contiguous blocks of pairs. `==?` essentially tests
two pairs to see whether they contain the same bit patterns.

The first (lower-addressed) word of a pair is called the **`TYPE`
word**, because it contains a numeric **`TYPE` code** that represents
the object's `TYPE`. The second (higher-addressed) word of a pair is
called the **value word**, because it contains (part of or the
beginning of) the "data part" of the object. The `TYPE` word (and
sometimes the value word) is considered to be made of a left half and
a right half. We will picture a pair like this:

    ---------------------------------
    |      TYPE     |               |
    | - - - - - - - - - - - - - - - |
    |             value             |
    ---------------------------------

where a vertical bar in the middle of a word means the word's halves
are used independently. You can see that the `TYPE` code is confined
to the left half of the `TYPE` word. (Half-)words are sometimes
subdivided into **fields** appropriate for the context; fields are
also pictured as separated by vertical bars. The right half of the
`TYPE` word is used for different purposes depending on the `TYPE` of
the object and actual location of the value.

Actually the 18-bit `TYPE` field is further decoded. The high-order
(leftmost) bit is the mark bit, used exclusively by the garbage
collector when it runs. The next two bits are monitor bits, used to
cause `"READ"` and `"WRITE"` interrupts on read and write references
to the pair. The next bit is used to differentiate between list
elements and vector dope words. The next bit is unused but could be
used in the future for an "execute" monitor. The remaining 13 bits
specify the actual `TYPE` code. What `CHTYPE` does is to copy the pair
and put a new `TYPE` code into the new pair.

Each data `TYPE` (predefined and `NEWTYPE`s) must belong to one of
about 25 "storage allocation classes" (roughly corresponding to Muddle
`PRIMTYPE`s). These classes are characterized primarily by the manner
in which the garbage collector treats them. Some of these classes will
now be described.

"One Word"

This class includes all data that are not pointers to some kind of
structure. All external (program-available) `TYPE`s in this class are
of `PRIMTYPE` `WORD`. Example:

    ---------------------------------
    |       FIX     |       0       |
    | - - - - - - - - - - - - - - - |
    |              105              |
    ---------------------------------

"Two Word"

The members of this class are all 18-bit pointers to list elements.
All external `TYPE`s in this class are of `PRIMTYPE` `LIST`. Example:

    ---------------------------------
    |      LIST     |       0       |
    | - - - - - - - - - - - - - - - |
    |       0       |    pointer    |
    ---------------------------------

where `pointer` is a pointer to the first list element. If there are
no elements, `pointer` is zero; thus empty objects of `PRIMTYPE`
`LIST` are `==?` if their `TYPE`s are the same.

"Two N Word"

Members of this class are all "counting pointers" to blocks of
two-word pairs. The right half of a counting pointer is an address,
and the left half is the negative of the number of 36-bit words in the
block. (This format is tailored to the PDP-10 `AOBJN` instruction.)
The number of pairs in the block (`LENGTH`) is half that number, since
each pair is two words. All external `TYPE`s in this class are of
`PRIMTYPE` `VECTOR`. Example:

    ---------------------------------
    |     VECTOR    |       0       |
    | - - - - - - - - - - - - - - - |
    |   -2*length   |    pointer    |
    ---------------------------------

where `length` is the `LENGTH` of the `VECTOR` and `pointer` is the
location of the start (the element selected by an `NTH` argument of 1)
of the `VECTOR`.

"N word"

This class is the same as the previous one, except that the block
contains objects all of the same `TYPE` without individual `TYPE`
words. The `TYPE` code for all the elements is in vector dope words,
which are at addresses just larger than the block itself. Thus, any
object that carries information in its `TYPE` word cannot go into the
block: `PRIMTYPE`s `STRING`, `BYTES`, `TUPLE` (and the corresponding
locatives `LOCS`, `LOCB`, `LOCA`), `FRAME`, and `LOCD`. All external
`TYPE`s in this class are of `PRIMTYPE` `UVECTOR`. Example:

    ---------------------------------
    |    UVECTOR    |       0       |
    | - - - - - - - - - - - - - - - |
    |    -length    |    pointer    |
    ---------------------------------

where `length` is the `LENGTH` of the `UVECTOR` and `pointer` points
to the beginning of the `UVECTOR`.

"Byte String" and "Character String"

These two classes are almost identical. Byte strings are byte pointers
to strings of arbitrary-size bytes. `PRIMTYPE` `BYTES` is the only
member of this class. Character strings are byte pointers to strings
of ASCII characters. `PRIMTYPE` `STRING` is the only member of this
class. Both of these classes consist of a length and a PDP-10 byte
pointer. In the case of character strings, the byte-size field in the
byte pointer is always seven bits per byte (hence five bytes per
word). Example:

    ---------------------------------
    |     STRING    |    length     |
    | - - - - - - - - - - - - - - - |
    |         byte-pointer          |
    ---------------------------------

where `length` is the `LENGTH` of the `STRING` (in bytes) and
`byte-pointer` points to a byte just before the beginning of the
string (an `ILDB` instruction is needed to get the first byte). A
newly-created `STRING` always has `*010700*` in the left half of
`byte-pointer`. Unless the string was created by `SPNAME`,
`byte-pointer` points to a uvector, where the elements (characters) of
the `STRING` are stored, packed together five to a word.

"Frame"

This class gives the user program a handle on its control and
variable-reference structures. All external `TYPE`s in this class are
of `PRIMTYPE` `FRAME`. Three numbers are needed to designate a frame:
a unique 18-bit identifying number, a pointer to the frame's storage
on a control stack, and a pointer to the `PROCESS` associated with the
frame. Example:

    ---------------------------------
    |     FRAME     |PROCESS-pointer|
    | - - - - - - - - - - - - - - - |
    |   unique-id   | frame-pointer |
    ---------------------------------

where `PROCESS-pointer` points to the dope words of a `PROCESS`
vector, and `unique-id` is used for validating (testing `LEGAL?`) the
`frame-pointer`, which points to a frame for some Subroutine call on
the control stack.

"Tuple"

A tuple pointer is a counting pointer to a vector on the control
stack. It may be a pointer to the arguments to a Subroutine or a
pointer generated by the `"TUPLE"` declaration in a `FUNCTION`. Like
objects in the previous class, these objects contain a unique
identifying number used for validation. `PRIMTYPE` `TUPLE` is the only
member of this class. Example:

    ---------------------------------
    |     TUPLE     |   unique-id   |
    | - - - - - - - - - - - - - - - |
    |   -2*length   |    pointer    |
    ---------------------------------

Other Storage Classes

The rest of the storage classes include strictly internal `TYPE`s and
pointers to special kinds of lists and vectors like locatives, `ATOM`s
and `ASOC`s. A pair for any `LOCATIVE` except a `LOCD` looks like a
pair for the corresponding structure, except of course that the `TYPE`
is different. A `LOCD` pair looks like a tuple pair and needs a word
and a half for its value; the `unique-id` refers to a binding on the
control stack or to the "global stack" if zero. Thus `LOCD`s are in a
sense "stack objects" and are more restricted than other locatives.

An `OFFSET` is stored with the `INDEX` in the right half of the value
word and the Pattern in the left half. Since the Pattern can be either
an `ATOM` or a `FORM`, the left half actually points to a pair, which
points to the actual Pattern. The Patttern `ANY` is recognized as a
special case: the left-half pointer is zero, and no pair is used.
Thus, if you're making the production version of your program and want
to save some storage, can do something like
`<SETG FOO <PUT-DECL ,FOO ANY>>` for all `OFFSET`s.

Basic Data Structures
---------------------

Lists

List elements are pairs linked together by the right halves of their
first words. The list is terminated by a zero in the right half of the
last pair. For example the `LIST` `(1 2 3)` would look like this:

    -------------
    | LIST | 0  |
    | - - - - - |   -----------     -----------     -----------
    |  0   | ------>| FIX | ------->| FIX | ------->| FIX | 0 |
    -------------   | - - - - |     | - - - - |     | - - - - |
                    |    1    |     |    2    |     |    3    |
                    -----------     -----------     -----------

The use of pointers to tie together elements explains why new elements
can be added easily to a list, how sharing and circularity work, etc.
The links go in only one direction through the list, which is why a
list cannot be `BACK`ed or `TOP`ped: there's no way to find the
`REST`ed elements.

Since some Muddle values require a word and a half for the value in
the pair, they do not fit directly into list elements. This problem is
solved by having "deferred pointers". Instead of putting the datum
directly into the list element, a pointer to another pair is used as
the value with the special internal `TYPE` `DEFER`, and the real datum
is put in the deferred pair. For example the `LIST` `(1 "hello" 3)`
would look like this:

    -------------
    | LIST | 0  |
    | - - - - - |   -----------     -----------     -----------
    |  0   | ------>| FIX | ------->|DEFER| ------->| FIX | 0 |
    -------------   | - - - - |     | - - - - |     | - - - - |
                    |    1    |     |       -----   |    3    |
                    -----------     ----------- |   -----------
                                                |
                                    ----------- |
                                    |STRING| 5|<-
                                    | - - - - |
                                    |byte-pntr|
                                    -----------

Vectors

A vector is a block of contiguous words. More than one pair can point
to the block, possibly at different places in the block; this is how
sharing occurs among vectors. Pointers that are different arise from
`REST` or `GROW`/`BACK` operations. The block is followed by two "dope
words", at addresses just larger than the largest address in the
block. Dope words have the following format:

    /                               /
    |                               |
    |                               |
    ---------------------------------
    |      type     |      grow     |
    | - - - - - - - - - - - - - - - |
    |     length    |       gc      |
    ---------------------------------

The various fields have the following meanings:

`type` -- The fourth bit from the left (the "vector bit", `40000`
octal) is always one, to distinguish these vector dope words from a
`TYPE`/value pair.

If the high-order bit is zero, then the vector is a `UVECTOR`, and the
remaining bits specify the uniform `TYPE` of the elements. `CHUTYPE`
just puts a new `TYPE` code in this field. Each element is limited to
a one-word value: clearly `PRIMTYPE` `STRING`s and `BYTES`es and stack
objects can't go in uniform vectors.

If the high-order bit is one and the `TYPE` bits are zero, then this
is a regular `VECTOR`.

If the high-order bit is one and the `TYPE` bits are not all zero,
then this is either an `ATOM`, a `PROCESS`, an `ASOC`, or a
`TEMPLATE`. The special internal format of these objects will be
described a little later in this appendix.

`length` -- The high-order bit is the mark bit, used by the garbage
collector. The rest of this field specifies the number of words in the
block, including the dope words. This differs from the length given in
pairs pointing to this vector, since such pairs may be the result of
`REST` operations.

`grow` -- This is actually two nine-bit fields, specifying either
growth or shrinkage at both the high and low ends of the vector. The
fields are usually set only when a stack must be grown or shrunk.

`gc` -- This is used by the garbage collector to specify where this
vector is moving during compaction.

Examples (numbers in octal): the `VECTOR` `[1 "bye" 3]` looks like:

    ---------------
    | VECTOR |  0 |
    | - - - - - - |         -----------------
    |   -6   |  ----------->|  FIX  |       |
    ---------------         | - - - - - - - |
                            |       1       |
                            -----------------
                            | STRING |  3   |
                            | - - - - - - - |
                            |  byte pointer |
                            -----------------
                            |  FIX  |       |
                            | - - - - - - - |
                            |       3       |
                            -----------------
                            | 440000 |  0   |
                            | - - - - - - - |
                            |   10   |      |
                            -----------------

The `UVECTOR` `![-1 7 -4!]` looks like:

    ---------------
    | UVECTOR | 0 |
    | - - - - - - |         -----------------
    |   -3    | ----------->|       -1      |
    ---------------         -----------------
                            |        7      |
                            -----------------
                            |       -4      |
                            -----------------
                            | 40000+FIX | 0 |
                            | - - - - - - - |
                            |   5       |   |
                            -----------------

Atoms

Internally, atoms are special vector-like objects. An atom contains a
value cell (the first two words of the block, filled in whenever the
global or local value of the `ATOM` is referenced and is not already
there), an `OBLIST` pointer, and a print name (`PNAME`), in the
following format:

    ---------------------------------
    |      type     |     bindid    |
    ---------------------------------
    |       pointer-to-value        |
    ---------------------------------
    |       pointer-to-oblist       |
    ---------------------------------
    |           print-name          |
    /                               /
    /                               /
    |(ASCII with NUL padding on end)|
    ---------------------------------
    |      ATOM     |   valid-type  |
    | - - - - - - - - - - - - - - - |
    |     length    |       gc      |
    ---------------------------------

If the type field corresponds to `TYPE` `UNBOUND`, then the `ATOM` is
locally and globally unbound. (This is different from a pair, where
the same `TYPE` `UNBOUND` is used to mean unassigned.) If it
corresponds to `TYPE` `LOCI` (an internal `TYPE`), then the value cell
points either to the global stack, if `bindid` is zero, or to a local
control stack, if `bindid` is non-zero. The `bindid` field is used to
verify whether the local value pointed to by the value cell is valid
in the current environment. The `pointer-to-OBLIST` is either a
counting pointer to an oblist (uvector). a positive offset into the
"transfer vector" (for pure `ATOM`s), or zero, meaning that this
`ATOM` is not on an `OBLIST`. The `valid-type` field tells whether or
not the `ATOM` represents a `TYPE` and if so the code for that `TYPE`:
`grow` values are never needed for atoms.

Associations

Associations are also special vector-like objects. The first six words
of the block contain `TYPE`/value pairs for the `ITEM`, `INDICATOR`
and `AVALUE` of the `ASOC`. The next word contains forward and
backward pointers in the chain for that bucket of the association hash
table. The last word contains forward and backward pointers in the
chain of all the associations.

    ---------------------------------
    |             ITEM              |
    | - - - - - - - - - - - - - - - |
    |             pair              |
    ---------------------------------
    |          INDICATOR            |
    | - - - - - - - - - - - - - - - |
    |             pair              |
    ---------------------------------
    |            AVALUE             |
    | - - - - - - - - - - - - - - - |
    |             pair              |
    ---------------------------------
    |     bucket-chain-pointers     |
    ---------------------------------
    |  association-chain-pointers   |
    ---------------------------------
    |      ASOC     |       0       |
    | - - - - - - - - - - - - - - - |
    |    12 octal   |       gc      |
    ---------------------------------

`PROCESS`es

A `PROCESS` vector looks exactly like a vector of `TYPE`/value pairs.
It is different only in that the garbage collector treats it
differently from a normal vector, and it contains extremely volatile
information when the `PROCESS` is `RUNNING`.

Templates

In a template, the number in the type field (left half or first dope
word) identifies to which "storage allocation class" this `TEMPLATE`
belongs, and it is used to find PDP-10 instructions in internal tables
(frozen uvectors) for performing `LENGTH`, `NTH`, and `PUT` operations
on any object of this `TYPE`. The programs to build these tables are
not part of the interpreter, but the interpreter does know how to use
them properly. The compiler can put these instructions directly in
compiled programs if a `TEMPLATE` is never `REST`ed; otherwise it must
let the interpreter discover the appropriate instruction. The value
word of a template pair contains, not a counting pointer, but the
number of elements that have been `REST`ed off in the left half and a
pointer to the first dope word in the right half.

The Control Stack
-----------------

Accumulators with symbolic names `AB`, `TB`, and `TP` are all pointers
into the `RUNNING` `PROCESS`'s control stack. `AB` ("argument base")
is a pointer to the arguments to the Subroutine now being run. It is
set up by the Subroutine-call mediator, and its old value is always
restored after a mediated Subroutine call returns. `TB` ("temporaries
base") points to the frame for the running Subroutine and also serves
as a stack base pointer. The `TB` pointer is really all that is
necessary to return from a Subroutine -- given a value to return, for
example by `ERRET` -- since the frame specifies the entire state of
the calling routine. `TP` ("temporaries pointer") is the actual stack
pointer and always points to the current top of the control stack.

While we're on the subject of accumulators, we might as well be
complete. Each accumulator contains the value word of a pair, the
corresponding `TYPE` words residing in the `RUNNING` `PROCESS` vector.
When a `PROCESS` is not `RUNNING` (or when the garbage collector is
running), the accumulator contents are stored in the vector, so that
the Objects they point to look like elements of the `PROCESS` and thus
are not garbage-collectible.

Accumulators `A`, `B`, `C`, `D`, `E` and `O` are used almost entirely
as scratch accumulators, and they are not saved or restored across
Subroutine calls. Of course the interrupt machinery always saves these
and all other accumulators. `A` and `B` are used to return a pair as
the value of a Subroutine call. Other than that special feature, they
are just like the other scratch accumulators.

`M` and `R` are used in running `RSUBR`s. `M` is always set up to
point to the start of the `RSUBR`'s code, which is actually just a
uniform vector of instructions. All jumps and other references to the
code use `M` as an index register. This makes the code
location-insensitive, which is necessary because the code uvector will
move around. `R` is set up to point to the vector of objects needed by
the `RSUBR`. This accumulator is necessary because objects in
garbage-collected space can move around, but the pointers to them in
the reference vector are always at the same place relative to its
beginning.

`FRM` is the internal frame pointer, used in compiled code to keep
track of pending Subroutine calls when the control stack is heavily
used. `P` is the internal-stack pointer, used primarily for internal
calls in the interpreter.

One of the nicest features of the Muddle environment is the uniformity
of the calling and returning sequence. All Subroutines -- both
built-in F/SUBRs and compiled `RSUBR(-ENTRY)`s -- are called in
exactly the same way and return the same way. Arguments are always
passed on the control stack and results always end up in the same
accumulators. For efficiency reasons, a lot of internal calls within
the interpreter circumvent the calling sequence. However, all calls
made by the interpreter when running user programs go through the
standard calling sequence.

A Subroutine call is initiated by one of three UUOs (PDP-10
instructions executed by software rather than hardware). `MCALL`
("Muddle call") is used when the number of arguments is known at
assemble or compile time, and this number is less than 16. `QCALL`
("quick call") may be used if, in addition, an `RSUBR(-ENTRY)` is
being called that can be called "quickly" by virtue of its having
special information in its reference vector. `ACALL` ("accumulator
call") is used otherwise. The general method of calling a Subroutine
is to `PUSH` (a PDP-10 instruction) pairs representing the arguments
onto the control stack via `TP` and then either (1) `MCALL` or `QCALL`
or (2) put the number of arguments into an accumulator and `ACALL`.
Upon return the object returned by the Subroutine will be in
accumulators `A` and `B`, and the arguments will have been `POP`ped
off the control stack.

The call mediator stores the contents of `P` and `TP` and the address
of the calling instruction in the current frame (pointed to by `TB`).
It also stores Muddle's "binding pointer" to the topmost binding in
the control stack. (The bindings are linked together through the
control stack so that searching through them is more efficient than
looking at every object on the stack.) This frame now specifies the
entire state of the caller when the call occurred. The mediator then
builds a new frame on the control stack and stores a pointer back to
the caller's frame (the current contents of `TB`), a pointer to the
Subroutine being called, and the new contents of `AB`, which is a
counting pointer to the arguments and is computed from the information
in the `MCALL` or `QCALL` instruction or the `ACALL` accumulator. `TB`
is then set up to point to the new frame, and its left half is
incremented by one, making a new `unique-id`. The mediator then
transfers control to the Subroutine.

A control stack frame has seven words as shown:

    ---------------------------------
    |     ENTRY     |  called-addr  |
    ---------------------------------
    |   unique-id   |  prev frame   |
    ---------------------------------
    |       argument pointer        |
    ---------------------------------
    |    saved binding pointer      |
    ---------------------------------
    |           saved P             |
    ---------------------------------
    |           saved TP            |
    ---------------------------------
    |    saved calling address      |
    ---------------------------------

The first three words are set up during the call to the Subroutine.
The rest are filled in when this routine calls another Subroutine. The
left half of `TB` is incremented every time a Subroutine call occurs
and is used as the `unique-id` for the frame, stored in frame and
tuple pairs as mentioned before. Obviously this `id` is not strictly
unique, since each 256K calls it wraps around to zero. The right half
of `TB` is always left pointing one word past the
saved-calling-address word in the frame. `TP` is also left pointing at
that word, since that is the top of the control stack at Subroutine
entry. The arguments to the called Subroutine are below the frame on
the control stack (at lower storage addresses), and the temporaries
for the called Subroutine are above the frame (at higher storage
addresses). These arguments and temporaries are just pairs stored on
the control stack while needed: they are all that remain of
`UNSPECIAL` values in compiled programs.

The following figure shows what the control stack might look like
after several Subroutine calls.

    /               /
    |               |
    -----------------
    |               |
    |  args for S1  |
    |               |
    -----------------
    | frame for S1  |
    ----------------- <--
    |               |   |
    | temps for S1  |   |
    |               |   |
    -----------------   |
    |               |   |
    |  args for S2  |   |
    |               |   |
    -----------------   |
    | frame for S2  | ---
    ----------------- <------
    |               |       |
    | temps for S2  |       |
    |               |       |
    -----------------       |
    |  args for S3  |       |
    -----------------       |
    | frame for S3  | -------
    -----------------
    |               |
    | temps for S3  |
    |               |
    |               |
    -----------------
          (top)

The above figure shows the frames all linked together through the
control stack (the "execution path"), so that it is easy to return to
the caller of a given Subroutine (`ERRET` or `RETRY`).

Subroutine exit is accomplished simply by the call mediator, which
loads the right half of `TB` from the previous frame pointer, restores
the "binding pointer", `P`, and `TP`, and transfers control back to
the instruction following the saved calling address.

Variable Bindings
-----------------

All local `ATOM` values are kept on the control stack of the `PROCESS`
to which they are local. As described before, the atom contains a word
that points to the value on the control stack. The pointer is actually
to a six-word "binding block" on the control stack. Binding blocks
have the following format:

    ---------------------------------
    | BIND or UBIND |      prev     |
    ---------------------------------
    |        pointer to ATOM        |
    ---------------------------------
    |             value             |
    | - - - - - - - - - - - - - - - |
    |             pair              |
    ---------------------------------
    |     decl      |   unique-id   |
    ---------------------------------
    |       previous-binding        |
    ---------------------------------

where:

-   `BIND` means this is a binding for a `SPECIAL` `ATOM` (the only
    kind used by compiled programs), and `UBIND` means this is a
    binding for an `UNSPECIAL` `ATOM` -- for `SPECIAL` checking by the
    interpreter;
-   `prev` points to the closest previous binding block for any `ATOM`
    (the "access path" -- `UNWIND` objects are also linked in this
    chain);
-   `decl` points to a `DECL` associated with this value, for
    `SET(LOC)` to check;
-   `unique-id` is used for validation of this block; and
-   `previous-binding` points to the closest previous binding for this
    `ATOM` (used in unbinding).

Bindings are generated by an internal subroutine called `SPECBIND`
(name comes from `SPECIAL`). The caller to `SPECBIND` `PUSH`es
consecutive six-word blocks onto the control stack via `TP` before
calling `SPECBIND`. The first word of each block contains the `TYPE`
code for `ATOM` in its left half and all ones in its right half.
`SPECBIND` uses this bit pattern to identify the binding blocks.
`SPECBIND`'s caller also fills in the next three words and leaves the
last two words empty. `SPECBIND` fills in the rest and leaves the
"binding pointer" pointing at the topmost binding on the control
stack. `SPECBIND` also stores a pointer to the current binding in the
value cell of the atom.

Unbinding is accomplished during Subroutine return. When the previous
frame is being restored, the call mediator checks to see if the saved
"binding pointer" and the current one are different; if they are,
`SPECSTORE` is called. `SPECSTORE` runs through the binding blocks,
restoring old value pointers in atoms until the "binding pointer" is
equal to the one saved in the frame.

Obviously variable binding is more complicated than this, because
`ATOM`s can have both local and global values and even different local
values in different `PROCESS`es. The solution to all of these
additional problems lies in the `bindid` field of the atom. Each
`PROCESS` vector also contains a current `bindid`. Whenever an ATOM's
local value is desired, the `RUNNING` `PROCESS`'s `bindid` is checked
against that of the atom: if they are the same, the atom points to the
current value; if not, the current `PROCESS`'s control stack must be
searched to find a binding block for this `ATOM`. This binding scheme
might be called "shallow binding". The searching is facilitated by
having all binding blocks linked together. Accessing global variables
is accomplished in a similar way, using a `VECTOR` that is referred to
as the "global stack". The global stack has only an `ATOM` and a value
slot for each variable, since global values never get rebound.

`EVAL` with respect to a different environment causes some additional
problems. Whenever this kind of `EVAL` is done, a brand new `bindid`
is generated, forcing all current local value cells of atoms to appear
invalid. Local values must now be obtained by searching the control
stack, which is inefficient compared to just pulling them out of the
atoms. (The greatest inefficiency occurs when an `ATOM`'s `LVAL` is
never accessed twice in a row in the same environment.) A special
block is built on the control stack and linked into the binding-block
chain. This block is called a "skip block" or "environment splice",
and it diverts the "access path" to the new environment, causing
searches to become relative to this new environment.