.. epigraph::
- | *I was an innkeeper, who loved to carouse;*
- | *J was a joiner, and built up a house.*
+ | |CENTER| *I was an innkeeper, who loved to carouse;*
+ | |CENTER| *J was a joiner, and built up a house.*
.. only:: html
.. image:: /images/picI.png
:align: left
-.. raw:: latex
-
- \dropcap{i}
-
-n even the simplest story, there's bound to be scope for the player to
+|I|\n even the simplest story, there's bound to be scope for the player to
attempt activities that you hadn't anticipated. Sometimes there may be
alternative ways of approaching a problem: if you can't be sure which
approach the player will take, you really ought to allow for all
need to substitute a more relevant response after LISTEN TO BIRD. Here's
how we do it:
-.. code-block:: inform6
+.. code-block:: inform
Object bird "baby bird" forest
with description "Too young to fly, the nestling tweets helplessly.",
We'll go through this a step at a time:
-#. We've added a new ``before`` property to our bird object. The
+#. We've added a new :prop:`before` property to our bird object. The
interpreter looks at the property *before* attempting to perform any
action which is directed specifically at this object::
>
The use of the ``return true`` statement probably needs a bit more
-explanation. An object's ``before`` property traps an action aimed at that
-object right at the start, before the interpreter has started to do
+explanation. An object's :prop:`before` property traps an action aimed at
+that object right at the start, before the interpreter has started to do
anything. That's the point at which the statements in the embedded routine
are executed. If the last of those statements is ``return true`` then the
interpreter assumes that the action has been dealt with by those
though it hadn't been intercepted. Sometimes that's what you want it to
do, but not here: if instead we'd written this:
-.. code-block:: inform6
+.. code-block:: inform
Object bird "baby bird" forest
with description "Too young to fly, the nestling tweets helplessly.",
Entering the cottage
====================
-At the start of the game the player character stands "outside a cottage", which
-might lead her to believe that she can go inside:
+At the start of the game the player character stands "outside a cottage",
+which might lead her to believe that she can go inside:
.. code-block:: transcript
Again, that isn't perhaps the most appropriate response, but it's easy to
change:
-.. code-block:: inform6
+.. code-block:: inform
Object before_cottage "In front of a cottage"
with description
cant_go "The only path lies to the east.",
has light;
-The ``in_to`` property would normally link to another room, in the same way
-as the ``e_to`` property contain the internal ID of the ``forest`` object.
-However, if instead you set its value to be a string, the interpreter
-displays that string when the player tries the IN direction. Other --
-unspecified -- directions like NORTH and UP still elicit the standard "You
-can't go that way" response, but we can change that too, by supplying a
-``cant_go`` property whose value is a suitable string. We then get this
-friendlier behaviour:
+The :prop:`in_to` property would normally link to another room, in the same
+way as the :prop:`e_to` property contain the internal ID of the ``forest``
+object. However, if instead you set its value to be a string, the
+interpreter displays that string when the player tries the IN direction.
+Other -- unspecified -- directions like NORTH and UP still elicit the
+standard "You can't go that way" response, but we can change that too, by
+supplying a :prop:`cant_go` property whose value is a suitable string. We
+then get this friendlier behaviour:
.. code-block:: transcript
to represent the cottage, a perfectly reasonable EXAMINE COTTAGE command
receives the obviously nonsensical reply "You can't see any such thing".
That's easy to fix; we can add a new ``cottage`` object, making it a piece
-of ``scenery`` just like the ``tree``:
+of :attr:`scenery` just like the ``tree``:
-.. code-block:: inform6
+.. code-block:: inform
Object cottage "tiny cottage" before_cottage
with description "It's small and simple, but you're very happy here.",
The situation here is similar to our LISTEN TO BIRD problem, and the
solution we adopt is similar as well:
-.. code-block:: inform6
+.. code-block:: inform
Object cottage "tiny cottage" before_cottage
with description "It's small and simple, but you're very happy here.",
],
has scenery;
-We use a ``before`` property to intercept the ``Enter`` action applied to
+We use a :prop:`before` property to intercept the ``Enter`` action applied to
the cottage object, so that we can display a more appropriate message.
This time, however, we've done it using one statement rather than two. It
turns out that the sequence "``print`` a string which ends with a newline
character, and then ``return true``" is so frequently needed that there's a
special statement which does it all. That is, this single statement (where
-you'll note that the string doesn't need to end in ``^``)::
+you'll note that the string *doesn't* need to end in ``^``)::
print_ret "It's such a lovely day -- much too nice to go inside.";
In the clearing, holding the nest and looking at the tree, the player is
meant to type UP. Just as likely, though, she'll try CLIMB TREE (which
currently gives the completely misleading response "I don't think much is
-to be achieved by that"). Yet another opportunity to use a ``before``
+to be achieved by that"). Yet another opportunity to use a :prop:`before`
property, but now with a difference.
-.. code-block:: inform6
+.. code-block:: inform
Object tree "tall sycamore tree" clearing
with description
object, it's not in order to display a better message; it's because we want
to move the player character to another room, just as if she'd typed UP.
Relocating the player character is actually quite a complex business, but
-fortunately all of that complexity is hidden: there's a standard **library
-routine** to do the job, not one that we've written, but one that's
-provided as part of the Inform system.
-
-You'll remember that, when we first mentioned routines (see "Standalone
-routines" on page 57), we used the example of ``Initialise()`` and said
-that "the routine's name followed by opening and closing parentheses is all
-that it takes to call a routine". That was true for ``Initialise()``, but
-not quite the whole story. To move the player character, we've got to
-specify where we want her to go, and we do that by supplying the internal
-ID of the destination room within the opening and closing parentheses.
-That is, instead of just ``PlayerTo()`` we call ``PlayerTo(top_of_tree)``,
-and we describe ``top_of_tree`` as the routine's **argument**.
+fortunately all of that complexity is hidden: there's a standard
+:term:`library routine` to do the job, not one that we've written, but one
+that's provided as part of the Inform system.
+
+.. index::
+ single: arguments (of a routine)
+
+You'll remember that, when we first mentioned routines (see
+:ref:`standalone-routines`), we used the example of ``Initialise()`` and
+said that "the routine's name followed by opening and closing parentheses
+is all that it takes to call a routine". That was true for
+``Initialise()``, but not quite the whole story. To move the player
+character, we've got to specify where we want her to go, and we do that by
+supplying the internal ID of the destination room within the opening and
+closing parentheses. That is, instead of just ``PlayerTo()`` we call
+``PlayerTo(top_of_tree)``, and we describe ``top_of_tree`` as the routine's
+:term:`argument`.
Although we've moved the player character to another room, we're still in
the middle of the intercepted ``Climb`` action. As previously, we need to
far, but instead for every ``Drop`` which takes place in our troublesome
``top_of_tree`` room. This is what we have to write:
-.. code-block:: inform6
+.. code-block:: inform
Object top_of_tree "At the top of the tree"
with description "You cling precariously to the trunk.",
Let's again take it a step at a time:
-#. We've added a new ``after`` property to our ``top_of_tree`` object. The
- interpreter looks at the property *subsequent to* performing any action in
- this room::
+#. We've added a new :prop:`after` property to our ``top_of_tree`` object.
+ The interpreter looks at the property *subsequent to* performing any
+ action in this room::
after [; ... ],
move noun to clearing;
- which takes the object which has just been moved from the ``player``
+ which takes the object which has just been moved from the :var:`player`
object to the ``top_of_tree`` object (by the successful ``Drop`` action)
and moves it again so that its parent becomes the ``clearing`` object.
- That ``noun`` is a library variable that always contains the internal ID
- of the object which is the target of the current action. If the player
- types DROP NEST, ``noun`` contains the internal ID of the ``nest``
- object; if she types DROP NESTLING then ``noun`` contains the internal
- ID of the ``bird`` object. Second, we execute::
+ That :var:`noun` is a library variable that always contains the internal
+ ID of the object which is the target of the current action. If the
+ player types DROP NEST, :var:`noun` contains the internal ID of the
+ ``nest`` object; if she types DROP NESTLING then :var:`noun` contains
+ the internal ID of the ``bird`` object. Second, we execute::
return false;
unhelpful in these non-standard circumstances. If you prefer to hint at
what's just happened, you could use this alternative solution:
-.. code-block:: inform6
+.. code-block:: inform
Object top_of_tree "At the top of the tree"
with description "You cling precariously to the trunk.",
has light;
The ``print_ret`` statement does two things for us: displays a more
-informative message, and returns ``true`` to tell the interpreter that
+informative message, and returns :const:`true` to tell the interpreter that
there's no need to let the player know what's happened -- we've handled
that ourselves.
checked for the bird actually being in the nest; fortunately, that's easy
to do:
-.. code-block:: inform6
+.. code-block:: inform
Object branch "wide firm bough" top_of_tree
with description "It's flat enough to support a small object.",
if (bird in nest && nest in branch) deadflag = 2;
should now be read as: "Test whether the ``bird`` is currently in (or on)
-the ``nest``, and whether the ``nest`` is currently on (or in) the
-``branch``; if both parts are ``true``, set the value of ``deadflag`` to 2;
-otherwise, do nothing".
+the ``nest``, *and* whether the ``nest`` is currently on (or in) the
+``branch``; if both parts are :const:`true`, set the value of
+:var:`deadflag` to 2; otherwise, do nothing".
Summing up
==========
The new topics that we've encountered here include these:
-.. rubric:: Object properties
-
-Objects can have a ``before`` property -- if there is one, the interpreter
-looks at it *before* performing an action which in some way involves that
-object. Similarly, you can provide an ``after`` property, which the
-interpreter looks at *after* performing an action but before telling the
-player what's happened. Both ``before`` and ``after`` properties can be
-used not only with tangible objects like the ``bird``, ``cottage`` and
-``tree`` (when they intercept actions aimed at that particular object) but
-also with rooms (when they intercept actions aimed at any object in that
-room).
-
-The value of each ``before`` and ``after`` property is an embedded routine.
-If such a routine ends with ``return false``, the interpreter then carries
-on with the next stage of the action which has been intercepted; if it ends
-with ``return true``, the interpreter does nothing further for that action.
-By combining these possibilities, you can supplement the work done by a
-standard action with statements of your own, or you can replace a standard
-action completely.
+Object properties
+-----------------
+
+Objects can have a :prop:`before` property -- if there is one, the
+interpreter looks at it *before* performing an action which in some way
+involves that object. Similarly, you can provide an :prop:`after`
+property, which the interpreter looks at *after* performing an action but
+before telling the player what's happened. Both :prop:`before` and
+:prop:`after` properties can be used not only with tangible objects like
+the ``bird``, ``cottage`` and ``tree`` (when they intercept actions aimed
+at that particular object) but also with rooms (when they intercept actions
+aimed at any object in that room).
+
+The value of each :prop:`before` and :prop:`after` property is an embedded
+routine. If such a routine ends with ``return false``, the interpreter
+then carries on with the next stage of the action which has been
+intercepted; if it ends with ``return true``, the interpreter does nothing
+further for that action. By combining these possibilities, you can
+supplement the work done by a standard action with statements of your own,
+or you can replace a standard action completely.
Previously, we've seen connection properties used with the internal ID of
the room to which they lead. In this chapter, we showed that the value
could also be a string (explaining why movement in that direction isn't
-possible). Here are examples of both, and also of the ``cant_go`` property
-which provides just such an explanation for *all* connections that aren't
-explicitly listed::
+possible). Here are examples of both, and also of the :prop:`cant_go`
+property which provides just such an explanation for *all* connections that
+aren't explicitly listed::
e_to forest,
in_to "It's such a lovely day -- much too nice to go inside.",
cant_go "The only path lies to the east.",
-.. rubric:: Routines and arguments
+.. _routines-args:
+
+Routines and arguments
+----------------------
The library includes a number of useful routines, available to perform
-certain common tasks if you require them; there's a list in "Library
-routines" on page 264. We used the ``PlayerTo`` routine, which moves the
+certain common tasks if you require them; there's a list in
+:ref:`library-routines`. We used the ``PlayerTo`` routine, which moves the
player character from her current room to another one -- not necessarily
adjacent to the first room.
+.. index::
+ single: arguments (of a routine)
+
When calling ``PlayerTo``, we had to tell the library which room is the
destination. We did this by supplying that room's internal ID within
parentheses, thus::
PlayerTo(clearing);
-A value given in parentheses like that is called an **argument** of the
+A value given in parentheses like that is called an :term:`argument` of the
routine. In fact, a routine can have more than one argument; if so,
they're separated by commas. For example, to move the player character to
a room *without* displaying that room's description, we could have supplied
In this example, the effect of the ``1`` is to prevent the description
being displayed.
-.. rubric:: Statements
+Statements
+----------
We encountered several new statements:
Inform defines quite a lot of actions, there are many fewer than there are
ways of expressing those same actions using English verbs.
+.. index::
+ pair: action; library variable
+
Each action is represented internally by a number, and the value of the
-current action is stored in a library variable called, erm, ``action``.
-Two more variables are also useful here: ``noun`` holds the internal ID of
-the object which is the focus of the action, and ``second`` holds the
+current action is stored in a library variable called, erm, :var:`action`.
+Two more variables are also useful here: :var:`noun` holds the internal ID
+of the object which is the focus of the action, and :var:`second` holds the
internal ID of the secondary object (if there is one). Here are some
examples of these:
PUT NEST ON BRANCH PutOn nest branch
=============================== ====== ======= =======
-The value ``nothing`` is a built-in constant (like ``true`` and ``false``)
-which means, well, there isn't any object to refer to. There's a list of
-standard library actions in "Group 1 actions" on page 270, "Group 2
-actions" on page 271 and "Group 3 actions" on page 271.
+The value ``nothing`` is a built-in constant (like :const:`true` and
+:const:`false`) which means, well, there isn't any object to refer to.
+There's a list of standard library actions in :ref:`group-1-actions`,
+:ref:`group-2-actions` and :ref:`group-3-actions`.
We've now reached the end of our first game. In these three chapters we've
shown you the basic principles on which almost all games are based, and
introduced you to many of the components that you'll need when creating
more interesting IF. We suggest that you take one last look at the source
-code (see "Heidi" story on page 213), and then move on to the next stage.
+code (see :doc:`/appendices/b`), and then move on to the next stage.
projects / ibg.git / blobdiff