[ibg.git] / chapters / 03.rst

==============================
 Heidi: our first Inform game
==============================

.. epigraph::

   | |CENTER| *E was an esquire, with pride on his brow;*
   | |CENTER| *F was a farmer, and followed the plough.*

.. only:: html

  .. image:: /images/picE.png
     :align: left

|E|\ach of the three games in this guide is created step by step; you'll
get most benefit (especially to begin with) if you take an active part,
typing in the source code on your computer.  Our first game, described in
this chapter and the two which follow, tells this sentimental little story:

    "Heidi lives in a tiny cottage deep in the forest.  One sunny day,
    standing before the cottage, she hears the frenzied tweeting of baby
    bird; its nest has fallen from the tall tree in the clearing!  Heidi
    puts the bird into the nest, and then climbs the tree to place the nest
    back on its branch."

It's a very simple tale, but even so we'll cover quite a lot of ground
before we have a finished Inform game.  We'll get there in stages, first
making a very rough approximation of the story, and then successively
refining the details until it's good enough for an initial attempt (there's
time later for more advanced stuff).

Creating a basic source file
============================

The first task is to create an Inform source file template.  Every game
that we design will start out like this.  Follow these steps:

#. Create an ``Inform\Games\Heidi`` folder (maybe by copying
   ``Inform\Games\MyGame1``).

   .. note::

      In this guide, we use the PC convention of placing a backslash
      between folder names.  On a Macintosh, use a regular slash:
      ``Inform/Games/Heidi``.

#. In that folder, use your text editor to create this source file
   ``Heidi.inf``:

   .. code-block:: inform

      !% -SD
      !============================================================================
      Constant Story "Heidi";
      Constant Headline
                  "^A simple Inform example
                   ^by Roger Firth and Sonja Kesserich.^";

      Include "Parser";
      Include "VerbLib";

      !============================================================================
      ! The game objects

      !============================================================================
      ! Entry point routines

      [ Initialise; ];

      !============================================================================
      ! Standard and extended grammar

      Include "Grammar";

      !============================================================================

   Soon, we'll explain what this means.  For now, just type it all in,
   paying particular attention to those seven semicolons, and ensuring that
   the double quotes "..." always come in pairs.  The first line beginning
   with "``!%``" is special, and we'll talk about it in a moment; the
   remaining exclamation mark lines, on the other hand, are purely
   decorative; they just make the file's structure a little easier to
   understand.

   Ensure the file is named ``Heidi.inf``, rather than ``Heidi.txt`` or
   ``Heidi.inf.txt``.

   Remember that, throughout this guide, we place the "``TYPE``" symbol
   alongside pieces of code that we recommend you to type into your own
   game files as you read through the examples (which, conversely, means
   that you *don't* need to type the unmarked pieces of code).  You'll 
   learn Inform more quickly by trying it for yourself, rather than just 
   taking our word for how things work.

   .. todo::

      Again, revisit the TYPE symbol.  Maybe a standard indicator above
      each snippet?

#. In the same folder, use your text editor to create the compilation
   support file ``Heidi.bat`` (on a PC)::

       ..\..\Lib\Base\Inform Heidi
                   +include_path=.\,..\..\Lib\Base,..\..\Lib\Contrib | more

       pause "at end of compilation"

   or ``Heidi.command`` (on a Macintosh)::

       cd ~/Inform/Games/Heidi/

       ../../Lib/Base/inform30_macosx Heidi
                       +include_path=./,../../Lib/Base,../../Lib/Contrib

   Remember that there's just one space between "``Heidi``" and
   "``+include_path``".

   Type in the file from scratch, or copy and edit ``MyGame1.bat`` (or
   ``MyGame1.command``).  At this point, you should have a ``Heidi`` folder
   containing two files: ``Heidi.inf`` and either ``Heidi.bat`` or
   ``Heidi.command``.

#. Compile the source file ``Heidi.inf``; refer back to
   :ref:`inform-windows` or :ref:`inform-apple` for guidance.  If the
   compilation works, a story file ``Heidi.z5`` appears in the folder.  If
   the compilation *doesn't* work, you've probably made a typing mistake;
   check everything until you find it.

#. You can run the story file in your Inform interpreter; you should see
   this (except that the Serial number will be different -- it's based on
   the date):

   .. code-block:: transcript

      Heidi
      A simple Inform example
      by Roger Firth and Sonja Kesserich.
      Release 1 / Serial number 040804 / Inform v6.30 Library 6/11 SD

      Darkness
      It is pitch dark, and you can't see a thing.

      >

When you get that far, your template source file is correct.  Let's explain
what it contains.

Understanding the source file
=============================

Although we've got a certain amount of freedom of expression, source files
tend to conform to a standard overall structure: these lines at the start,
that material next, those pieces coming at the end, and so on.  What we're
doing here is mapping out a structure that suits us, giving ourselves a
clear framework onto which the elements of the game can be fitted.  Having
a clear (albeit sparse) map at the start will help us to keep things
organised as the game evolves.  We can infer several Inform rules just by
looking at the source file.

.. Generated by autoindex
.. index::
   single: Strict mode

* If the *very first line* (or lines) of the source file begin with the
  characters "``!%``", then the compiler treats what follows on those lines
  as control instructions to itself rather than as part of the game's
  source.  The instructions most commonly placed here are compiler
  switches, a way of controlling detailed aspects of how the compiler
  operates.  These particular switches, two of many, are turning on
  :term:`Strict mode`, which makes the game less likely to misbehave when
  being played, and :term:`Debug mode`, which provides some extra commands
  which can be helpful when tracking down problems.

  .. note::

     Actually, the :option:`-S` is redundant, since Strict mode is already
     on by default.  We include it here as a reminder that (a) to turn
     Strict mode *off*, you change this setting to :option:`-~S`, and (b)
     alphabetic case matters here: :option:`-s` causes a display of
     compiler statistics (and :option:`-~s` does nothing at all).

* Otherwise, when the compiler comes across an exclamation mark, it ignores
  the rest of the line.  If the ``!`` is at the start of a line, the whole
  line is ignored; if the ``!`` is halfway along a line, the compiler takes
  note of the first half, and then ignores the exclamation mark and
  everything after it on that line.  We call material following an
  exclamation mark, not seen by anybody else, a :term:`comment`; it's often
  a remark that we write to remind ourselves of how something works or why
  we tackled a problem in a particular way.  There's nothing special about
  those equals signs: they just produce clear lines across the page.

  It's always a good idea to comment code as you write it, for later it
  will help you to understand what was going on at a particular spot.
  Although it all seems clear in your head when you first write it, in a
  few months you may suspect that a totally alien mind must have produced
  that senseless gibberish.

  By the way, the compiler *doesn't* give special treatment to exclamation
  marks in quoted text: ``!`` within quotes "..." is treated as a normal
  character.  On this line, the first ``!`` is part of the sequence (or
  :term:`string`) of characters to be displayed:

  .. code-block:: inform

     print "Hello world!";         ! <- is the start of this comment

* The compiler ignores blank lines, and treats lots of space like a single
  space (except when the spaces are part of a character string).  So, these
  two rules tell us that we *could* have typed the source file like this:

  .. code-block:: inform

     Constant Story "Heidi";
     Constant Headline
     "^A simple Inform example^by Roger Firth and Sonja Kesserich.^";
     Include "Parser";Include "VerbLib";
     [ Initialise; ];
     Include "Grammar";

  We didn't type it that way because, though shorter, it's much harder to
  read.  When designing a game, you'll spend a lot of time studying what
  you've typed, so it's worthwhile taking a bit of care to make it as
  readable as possible.

* Every game should have the :term:`constant` definitions for ``Story``
  (the game's name) and ``Headline`` (typically, information on the game's
  theme, copyright, authorship and so on).  These two :term:`string`
  values, along with a release number and date, and details of the
  compiler, compose the :term:`banner` which is displayed at the start of
  each game.

* Every game needs the three lines which ``Include`` the standard library
  files -- that is, they merge those files' contents into your source file:

  .. code-block:: inform

     Include "Parser";
     Include "VerbLib";
     ...
     Include "Grammar";

  They always have to be in this order, with ``Parser`` and ``VerbLib``
  near the start of the file, and ``Grammar`` near the end.

* Every game needs to define an ``Initialise`` routine (note the British
  spelling):

  .. code-block:: inform

     [ Initialise; ];

  The :term:`routine` that we've defined here doesn't do anything useful,
  but it still needs to be present.  Later, we'll come back to
  ``Initialise`` and explain what a routine is and why we need this one.

* You'll notice that each of the items mentioned in the previous three
  rules ends with a semicolon.  Inform is very fussy about its punctuation,
  and gets really upset if you forget a terminating semicolon.  In fact,
  the compiler just keeps reading your source file until it finds one;
  that's why we were able to take three lines to define the ``Headline``
  constant

  .. code-block:: inform

     Constant Headline
           "^A simple Inform example
            ^by Roger Firth and Sonja Kesserich.^";

Just to repeat what we said earlier: every game that you design will start
out from a basic source file like this (in fact, it might be sensible to
keep a copy of this template file in a safe place, as a starting point for
future games).  Think of this stuff as the basic preparation which you'll
quickly come to take for granted, much as a landscape artist always begins
by sizing the canvas before starting to paint.  So, now that we've taken a
quick tour of Inform's general needs, we can start thinking about what this
particular game requires.

Defining the game's locations
=============================

A good starting point in any game is to think about the locations which are
involved: this sketch map shows the four that we'll use:

.. image:: /images/heidi1.*
   :align: center

In IF, we talk about each of these locations as a :term:`room`, even though
in this example none of them has four walls.  So let's use Inform to define
those rooms.  Here's a first attempt:

.. code-block:: inform

   Object   "In front of a cottage"
     with   description
                "You stand outside a cottage. The forest stretches east.",
      has   light;

   Object   "Deep in the forest"
     with   description
               "Through the dense foliage, you glimpse a building to the west.
                A track heads to the northeast.",
      has   light;

   Object   "A forest clearing"
     with   description
               "A tall sycamore stands in the middle of this clearing.
                The path winds southwest through the trees.",
      has   light;

   Object   "At the top of the tree"
     with   description "You cling precariously to the trunk.",
      has   light;

Again, we can infer some general principles from these four examples:

* A room definition starts with the word ``Object`` and ends, about four
  lines later, with a semicolon.  Each of the components that appears in
  your game -- not only the rooms, but also the people, the things that you
  see and touch, intangibles like a sound, a smell, a gust of wind -- is
  defined in this way; think of an "object" simply as the general term for
  the myriad thingies which together comprise the model world which your
  game inhabits.

* The phrase in double quotes following the word ``Object`` is the name
  that the interpreter uses to provide the player character with a list of
  the objects around her: where she is, what she can see, what she's
  holding, and so on.

  .. note::

     We're using the word "player" to mean both the person who is playing
     the game, and the principal protagonist (often known as the player
     character) within the game itself.  Since the latter -- Heidi -- is
     female, we'll refer to the player as "she" while discussing this game.

* A keyword ``with`` follows, which simply tells the compiler what to
  expect next.

.. Generated by autoindex
.. index::
   pair: description; library property

* The word :prop:`description`, introducing another piece of text which
  gives more detail about the object: in the case of a room, it's the
  appearance of the surrounding environment when the player character is in
  that room.  The textual description is given in double quotes, and is
  followed by a comma.

* Near the end, the keyword ``has`` appears, which again tells the compiler
  to expect a certain kind of information.

.. Generated by autoindex
.. index::
   pair: light; library attribute

* The word :attr:`light` says that this object is a source of illumination,
  and that therefore the player character can see what's happening here.
  There has to be at least one light source in every room (unless you want
  the player to be told that "It's pitch dark and you can't see a thing");
  most commonly, that light source is the room itself.

A smidgeon of background may help set this into context (there's more in
the next chapter).  An object can have both :term:`properties` (introduced
by the keyword ``with``) and :term:`attributes` (written after the word
``has``).  A property has both a name (like :prop:`description`) and a
value (like the character string "``You stand outside a cottage.  The
forest stretches east.``"); an attribute has merely a name.

In a little while, when you play this game, you'll observe that it starts
like this:

.. code-block:: transcript

   In front of a cottage
   You stand outside a cottage. The forest stretches east.

And here you can see how the room's name (``In front of a cottage``) and
description (``You stand outside a cottage.  The forest stretches east.``)
are used.

Joining up the rooms
====================

We said that this was a first attempt at defining the rooms; it's fine as
far as it goes, but a few bits of information are missing.  If you look at
the game's sketch map, you can see how the rooms are intended to be
connected; from "Deep in the forest", for example, the player character
should be able to move west towards the cottage, or northeast to the
clearing.  Now, although our descriptions mention or imply these available
routes, we also need to explicitly add them to the room definitions in a
form that the game itself can make sense of.  Like this:

.. code-block:: inform

   Object   before_cottage "In front of a cottage"
     with   description
                "You stand outside a cottage. The forest stretches east.",
            e_to forest,
     has    light;

   Object   forest "Deep in the forest"
     with   description
                "Through the dense foliage, you glimpse a building to the west.
                 A track heads to the northeast.",
            w_to before_cottage,
            ne_to clearing,
     has    light;

   Object   clearing "A forest clearing"
     with   description
                "A tall sycamore stands in the middle of this clearing.
                 The path winds southwest through the trees.",
            sw_to forest,
            u_to top_of_tree,
     has    light;

   Object   top_of_tree "At the top of the tree"
     with   description "You cling precariously to the trunk.",
            d_to clearing,
     has    light;

We've made two changes to the room objects.

* First, between the word ``Object`` and the object's name in double
  quotes, we've inserted a different type of name: a private, internal
  identification, never seen by the player; one that we can use *within*
  the source file when one object needs to refer to another object.  For
  example, the first room is identified as ``before_cottage``, and the
  second as ``forest``.

  Unlike the external name contained in double quotes, the internal
  identifier has to be a single word -- that is, without spaces.  To aid
  readability, we often use an underscore character to act as sort of
  pseudo-space: ``before_cottage`` is a bit clearer than ``beforecottage``.

* Second, we've added lines after the object descriptions which use those
  internal identifiers to show how the rooms are connected; one line for
  each connection.  The ``before_cottage`` object has this additional
  line::

     e_to forest,

  This means that a player standing in front of the cottage can type GO
  EAST (or EAST, or just E), and the game will transport her to the room
  whose internal identification is ``forest``.  If she tries to move in any
  other direction from this room, she'll be told "You can't go that way".

  What we've just defined is a *one-way* easterly connection:
  ``before_cottage`` → ``forest``.  The forest object has two additional
  lines::

     w_to before_cottage,
     ne_to clearing,

  The first line defines a westerly connection ``forest`` →
  ``before_cottage`` (thus enabling the player character to return to the
  cottage), and the second defines a connection ``forest`` → ``clearing``
  which heads off to the northeast.

  .. Generated by autoindex
  .. index::
     pair: d_to; library property
     pair: e_to; library property
     pair: in_to; library property
     pair: n_to; library property
     pair: ne_to; library property
     pair: nw_to; library property
     pair: out_to; library property
     pair: s_to; library property
     pair: se_to; library property
     pair: sw_to; library property
     pair: u_to; library property
     pair: w_to; library property

  Inform provides for eight "horizontal" connections (:prop:`n_to`,
  :prop:`ne_to`, :prop:`e_to`, :prop:`se_to`, :prop:`s_to`, :prop:`sw_to`,
  :prop:`w_to`, :prop:`nw_to`) two "vertical" ones (:prop:`u_to`,
  :prop:`d_to`) and two specials :prop:`in_to`, and :prop:`out_to`.  You'll
  see some of these used for the remaining inter-room connections.

There's one last detail to attend to before we can test what we've done.
You'll recollect that our story begins with Heidi standing in front of her
cottage.  We need to tell the interpreter that ``before_cottage`` is the
room where the game starts, and we do this in the ``Initialise`` routine::

    [ Initialise; location = before_cottage; ];

:var:`location` is a :term:`variable`, part of the library, which tells the
interpreter in which room the player character currently is.  Here, we're
saying that, at the start of the game, the player character is in the
``before_cottage`` room.

Now we can add what we've done to the ``Heidi.inf`` source file template.
At this stage, you should study the four room definitions, comparing them
with the sketch map until you're comfortable that you understand how to
create simple rooms and define the connections between them.

.. code-block:: inform

   !============================================================================
   Constant Story "Heidi";
   Constant Headline
               "^A simple Inform example
                ^by Roger Firth and Sonja Kesserich.^";

   Include "Parser";
   Include "VerbLib";

   !============================================================================
   ! The game objects

   Object   before_cottage "In front of a cottage"
     with   description
                "You stand outside a cottage. The forest stretches east.",
            e_to forest,
     has    light;

   Object   forest "Deep in the forest"
     with   description
                "Through the dense foliage, you glimpse a building to the west.
                 A track heads to the northeast.",
            w_to before_cottage,
            ne_to clearing,
     has    light;

   Object   clearing "A forest clearing"
     with   description
                "A tall sycamore stands in the middle of this clearing.
                 The path winds southwest through the trees.",
            sw_to forest,
            u_to top_of_tree,
     has    light;

   Object   top_of_tree "At the top of the tree"
     with   description "You cling precariously to the trunk.",
            d_to clearing,
     has    light;

   !============================================================================
   ! Entry point routines

   [ Initialise; location = before_cottage; ];

   !============================================================================
   ! Standard and extended grammar

   Include "Grammar";

   !============================================================================

Type this in, as always taking great care with the punctuation -- watch
those commas and semicolons.  Compile it, and fix any mistakes which the
compiler reports.  You can then play the game in its current state.
Admittedly, you can't do very much, but you should be able to move freely
among the four rooms that you've defined.

.. note::

   In order to minimise the amount of typing that you have to do, the
   descriptive text in this game has been kept as short as possible.  In a
   real game, you would typically provide more interesting descriptions
   than these.

Adding the bird and the nest
============================

Given what we said earlier, you won't be surprised to hear that both the
bird and its nest are Inform objects.  We'll start their definitions like
this:

.. code-block:: inform

   Object  bird "baby bird"
     with  description "Too young to fly, the nestling tweets helplessly.",
      has  ;

   Object  nest "bird's nest"
     with  description "The nest is carefully woven of twigs and moss.",
      has  ;

You can see that these definitions have exactly the same format as the
rooms we defined previously: a one-word internal identifier (``bird``,
``nest``), and a word or phrase naming the object for the player's benefit
(``baby bird``, ``bird's nest``).  They both have some descriptive detail:
for a room this is printed when the player first enters, or when she types
LOOK; for other objects it's printed when she EXAMINEs that object.  What
they *don't* have are connections (:prop:`e_to`, :prop:`w_to`, etc.  apply
only to rooms) or :attr:`light` (it's not necessary -- the rooms ensure
that light is available).

When the game is running, the player will want to refer to these two
objects, saying for instance EXAMINE THE BABY BIRD or PICK UP THE NEST.
For this to work reliably, we need to specify the word (or words) which
relate to each object.  Our aim here is flexibility: providing a choice of
relevant vocabulary so that the player can use whatever term seems
appropriate to her, with a good chance of it being understood.  We add a
line to each definition:

.. code-block:: inform

   Object  bird "baby bird"
     with  description "Too young to fly, the nestling tweets helplessly.",
           name 'baby' 'bird' 'nestling',
      has  ;

   Object  nest "bird's nest"
     with  description "The nest is carefully woven of twigs and moss.",
           name 'bird^s' 'nest' 'twigs' 'moss',
      has  ;

The :prop:`name` introduces a list in single quotes '...'.  We call each of
those quoted things a :term:`dictionary word`, and we do mean "word", not
"phrase" (``'baby'``\ ``'bird'`` rather than ``'baby bird'``); you can't
uses spaces, commas or periods *in* dictionary words, though there's a
space *between* each one, and the whole list ends with a comma.  The idea
is that the interpreter decides which object a player is talking about by
matching what she types against the full set of all dictionary words.  If
the player mentions BIRD, or BABY BIRD, or NESTLING, it's the ``baby bird``
that she means; if she mentions NEST, BIRD'S NEST or MOSS, it's the
``bird's nest``. And if she types NEST BABY or BIRD TWIGS, the interpreter
will politely say that it doesn't understand what on earth she's talking
about.

.. index::
   single: apostrophes

.. note::

   You'll notice the use of ``'bird^s'`` to define the dictionary word
   BIRD'S; this oddity is necessary because the compiler expects the single
   quotes in the list always to come in pairs -- one at the start of the
   dictionary word, and one at the end.  If we had typed ``'bird's'`` then
   the compiler would find the opening quote, the four letters ``b``,
   ``i``, ``r`` and ``d``, and what looks like the closing quote.  So far
   so good; it's read the word BIRD and now expects a space before the next
   opening quote... but instead finds ``s'`` which makes no sense.  In
   cases like this we must use the circumflex ``^`` to *represent* the
   apostrophe, and the compiler then treats ``bird's`` as a dictionary
   word.

You may be wondering why we need a list of :prop:`name` words for the bird
and its nest, yet we didn't when we defined the rooms?  It's because the
player can't interact with a room in the same way as with other objects;
for example, she doesn't need to say EXAMINE THE FOREST -- just being there
and typing LOOK is sufficient.

.. Generated by autoindex
.. index::
   pair: container; library attribute
   pair: open; library attribute

The bird's definition is complete, but there's an additional complexity
with the nest: we need to be able to put the bird into it.  We do this by
labelling the nest as a :attr:`container` -- able to hold other objects --
so that the player can type PUT (or INSERT) BIRD IN (or INTO) NEST.
Furthermore, we label it as :attr:`open`; this prevents the interpreter
from asking us to open it before putting in the bird.

.. code-block:: inform

   Object   nest "bird's nest"
     with   description "The nest is carefully woven of twigs and moss.",
            name 'bird^s' 'nest' 'twigs' 'moss',
     has    container open;

Both objects are now defined, and we can incorporate them into the game.
To do this, we need to choose the locations where the player will find
them.  Let's say that the bird is found in the forest, while the nest is in
the clearing.  This is how we set this up:

.. code-block:: inform

   Object   bird "baby bird" forest
     with   description "Too young to fly, the nestling tweets helplessly.",
            name 'baby' 'bird' 'nestling',
     has    ;

   Object   nest "bird's nest" clearing
     with   description "The nest is carefully woven of twigs and moss.",
            name 'bird^s' 'nest' 'twigs' 'moss',
     has    container open;

Read that first line as: "Here's the definition of an object which is
identified within this file as ``bird``, which is known to the player as
``baby bird``, and which is initially located inside the object identified
within this file as ``forest``."

Where in the source file do these new objects fit?  Well, anywhere really,
but you'll find it convenient to insert them following the rooms where
they're found.  This means adding the bird just after the forest, and the
nest just after the clearing.  Here's the middle piece of the source file:

.. code-block:: inform

   !============================================================================
   ! The game objects

   Object  before_cottage "In front of a cottage"
     with  description
                "You stand outside a cottage. The forest stretches east.",
           e_to forest,
      has  light;

   Object  forest "Deep in the forest"
     with  description
                "Through the dense foliage, you glimpse a building to the west.
                 A track heads to the northeast.",
           w_to before_cottage,
           ne_to clearing,
      has  light;

   Object  bird "baby bird" forest
     with  description "Too young to fly, the nestling tweets helplessly.",
           name 'baby' 'bird' 'nestling',
      has  ;

   Object  clearing "A forest clearing"
     with  description
                "A tall sycamore stands in the middle of this clearing.
                 The path winds southwest through the trees.",
           sw_to forest,
           u_to top_of_tree,
      has  light;

   Object  nest "bird's nest" clearing
     with  description "The nest is carefully woven of twigs and moss.",
           name 'bird^s' 'nest' 'twigs' 'moss',
      has  container open;

   Object  top_of_tree "At the top of the tree"
     with  description "You cling precariously to the trunk.",
           d_to clearing,
      has  light;

   !============================================================================

Make those changes, recompile the game, play it and you'll see this:

.. code-block:: transcript

   Deep in the forest
   Through the dense foliage, you glimpse a building to the west. A track heads
   to the northeast.

   You can see a baby bird here.

   >

Adding the tree and the branch
==============================

The description of the clearing mentions a tall sycamore tree, up which the
player character supposedly "climbs".  We'd better define it:

.. code-block:: inform

   Object   tree "tall sycamore tree" clearing
     with   description
                "Standing proud in the middle of the clearing,
                 the stout tree looks easy to climb.",
            name 'tall' 'sycamore' 'tree' 'stout' 'proud',
     has    scenery;

Everything there should be familiar, apart from that :attr:`scenery` at the
end. We've already mentioned the tree in the description of the forest
clearing, so we don't want the interpreter adding "You can see a tall
sycamore tree here" afterwards, as it does for the bird and the nest.  By
labelling the tree as :attr:`scenery` we suppress that, and also prevent it
from being picked up by the player character.  One final object: the branch
at the top of the tree.  Again, not many surprises in this definition:

.. code-block:: inform

   Object   branch "wide firm bough" top_of_tree
     with   description "It's flat enough to support a small object.",
            name 'wide' 'firm' 'flat' 'bough' 'branch',
     has    static supporter;

The only new things are those two labels.  :attr:`static` is similar to
:attr:`scenery`: it prevents the branch from being picked up by the player
character, but *doesn't* suppress mention of it when describing the
setting.  And :attr:`supporter` is rather like the :attr:`container` that
we used for the nest, except that this time the player character can put
other objects *onto* the branch.  (In passing, we'll mention that an object
can't normally be both a :attr:`container` *and* a :attr:`supporter`.)  And
so here are our objects again:

.. code-block:: inform

   !============================================================================
   ! The game objects

   Object   before_cottage "In front of a cottage"
     with   description
                "You stand outside a cottage. The forest stretches east.",
            e_to forest,
     has    light;

   Object   forest "Deep in the forest"
     with   description
                "Through the dense foliage, you glimpse a building to the west.
                 A track heads to the northeast.",
            w_to before_cottage,
            ne_to clearing,
     has    light;

   Object   bird "baby bird" forest
     with   description "Too young to fly, the nestling tweets helplessly.",
            name 'baby' 'bird' 'nestling',
     has    ;

   Object   clearing "A forest clearing"
     with   description
                "A tall sycamore stands in the middle of this clearing.
                 The path winds southwest through the trees.",
            sw_to forest,
            u_to top_of_tree,
     has    light;

   Object   nest "bird's nest" clearing
     with   description "The nest is carefully woven of twigs and moss.",
            name 'bird^s' 'nest' 'twigs' 'moss',
      has   container open;

   Object   tree "tall sycamore tree" clearing
     with   description
                "Standing proud in the middle of the clearing,
                 the stout tree looks easy to climb.",
            name 'tall' 'sycamore' 'tree' 'stout' 'proud',
      has   scenery;

   Object   top_of_tree "At the top of the tree"
     with   description "You cling precariously to the trunk.",
            d_to clearing,
      has   light;

   Object   branch "wide firm bough" top_of_tree
     with   description "It's flat enough to support a small object.",
            name 'wide' 'firm' 'flat' 'bough' 'branch',
      has   static supporter;

   !============================================================================

Once again, make the changes, recompile, and investigate what you can do in
your model world.

Finishing touches
=================

Our first pass at the game is nearly done; just two more changes to
describe.  The first is easy: Heidi wouldn't be able to climb the tree
carrying the bird and the nest separately: we want the player character to
put the bird into the nest first.  One easy way to enforce this is by
adding a line near the top of the file:

.. code-block:: inform

   !============================================================================
   Constant Story "Heidi";
   Constant Headline
               "^A simple Inform example
                ^by Roger Firth and Sonja Kesserich.^";

   Constant MAX_CARRIED 1;

The value of ``MAX_CARRIED`` limits the number of objects that the player
character can be holding at any one time; by setting it to 1, we're saying
that she can carry the bird or the nest, but not both.  However, the limit
ignores the contents of :attr:`container` or :attr:`supporter` objects, so
the nest with the bird inside it is still counted as one object.

The other change is slightly more complex and more important: there's
currently no way to "win" the game!  The goal is for the player character
to put the bird in the nest, take the nest to the top of the tree, and
place it on the branch; when that happens, the game should be over.  This
is one way of making it happen:

.. code-block:: inform

   Object  branch "wide firm bough" top_of_tree
     with  description "It's flat enough to support a small object.",
           name 'wide' 'firm' 'flat' 'bough' 'branch',
           each_turn [; if (nest in branch) deadflag = 2; ],
      has  static supporter;

.. note::

   Here's an explanation of what's going on.  If you find this difficult to
   grasp, don't worry.  It's the hardest bit so far, and it introduces
   several new concepts all at once.  Later in the guide, we'll explain
   those concepts more clearly, so you can just skip this bit if you want.

   .. Generated by autoindex
   .. index::
      pair: deadflag; library variable

   The variable :var:`deadflag`, part of the library, is normally 0.  If
   you set its value to 2, the interpreter notices and ends the game with
   "You have won".  The statement::

         if (nest in branch) deadflag = 2;

   should be read as: "Test whether the ``nest`` is currently in the
   ``branch`` (if the branch is a :attr:`container`) or on it (if the
   ``branch`` is a supporter); if it is, set the value of :var:`deadflag`
   to 2; if it isn't, do nothing."  The surrounding part::

         each_turn [; ... ],

   should be read as: "At the end of each turn (when the player is in the
   same room as the branch), do whatever is written inside the square
   brackets".  So, putting that all together:

   * At the end of each turn (after the player has typed something and
     pressed the Enter key, and the interpreter has done whatever was
     requested) the interpreter checks whether the player and the
     ``branch`` are in the same room.  If not, nothing happens.  If they're
     together, it looks to see where the nest is.  Initially it's in the
     ``clearing``, so nothing happens.

   * Also at the end of each turn, the interpreter checks the value of
     :var:`deadflag`.  Usually it's 0, so nothing happens.

   * Finally the player character puts the ``nest`` on the ``branch``.
     "Aha!"  says the interpreter (to itself, of course), and sets the
     value of :var:`deadflag` to 2.

   * Immediately afterwards, (another part of) the interpreter checks and
     finds that the value of :var:`deadflag` has changed to 2, which means
     that the game is successfully completed; so, it says to the player,
     "you've won!"

That's as far as we'll take this example for now.  Make those final
changes, recompile, and test what you've achieved.  You'll probably find a
few things that could be done better -- even on a simple game like this
there's considerable scope for improvement -- so we'll revisit Heidi in her
forest shortly.  First, though, we'll recap what we've learnt so far.