1 ==============================
2 Heidi: our first Inform game
3 ==============================
7 | *E was an esquire, with pride on his brow;*
8 | *F was a farmer, and followed the plough.*
12 .. image:: /images/picE.png
19 ach of the three games in this guide is created step by step; you'll get
20 most benefit (especially to begin with) if you take an active part, typing
21 in the source code on your computer. Our first game, described in this
22 chapter and the two which follow, tells this sentimental little story:
24 "Heidi lives in a tiny cottage deep in the forest. One sunny day,
25 standing before the cottage, she hears the frenzied tweeting of baby
26 bird; its nest has fallen from the tall tree in the clearing! Heidi
27 puts the bird into the nest, and then climbs the tree to place the nest
30 It's a very simple tale, but even so we'll cover quite a lot of ground
31 before we have a finished Inform game. We'll get there in stages, first
32 making a very rough approximation of the story, and then successively
33 refining the details until it's good enough for an initial attempt (there's
34 time later for more advanced stuff).
36 Creating a basic source file
37 ============================
39 The first task is to create an Inform source file template. Every game
40 that we design will start out like this. Follow these steps:
42 #. Create an ``Inform\Games\Heidi`` folder (maybe by copying ``Inform\Games\MyGame1``).
46 In this guide, we use the PC convention of placing a backslash
47 between folder names. On a Macintosh, use a regular slash:
48 ``Inform/Games/Heidi``.
50 #. In that folder, use your text editor to create this source file
53 .. code-block:: inform
56 !============================================================================
57 Constant Story "Heidi";
59 "^A simple Inform example
60 ^by Roger Firth and Sonja Kesserich.^";
65 !============================================================================
68 !============================================================================
69 ! Entry point routines
73 !============================================================================
74 ! Standard and extended grammar
78 !============================================================================
80 Soon, we'll explain what this means. For now, just type it all in,
81 paying particular attention to those seven semicolons, and ensuring that
82 the double quotes "..." always come in pairs. The first line beginning
83 with "``!%``" is special, and we'll talk about it in a moment; the
84 remaining exclamation mark lines, on the other hand, are purely
85 decorative; they just make the file's structure a little easier to
88 Ensure the file is named ``Heidi.inf``, rather than ``Heidi.txt`` or
91 Remember that, throughout this guide, we place the "``TYPE``" symbol
92 alongside pieces of code that we recommend you to type into your own
93 game files as you read through the examples (which, conversely, means
94 that you *don't* need to type the unmarked pieces of code). You'll
95 learn Inform more quickly by trying it for yourself, rather than just
96 taking our word for how things work.
100 Again, revisit the TYPE symbol. Maybe a standard indicator above
103 #. In the same folder, use your text editor to create the compilation
104 support file ``Heidi.bat`` (on a PC)::
106 ..\..\Lib\Base\Inform Heidi
107 +include_path=.\,..\..\Lib\Base,..\..\Lib\Contrib | more
109 pause "at end of compilation"
111 or ``Heidi.command`` (on a Macintosh)::
113 cd ~/Inform/Games/Heidi/
115 ../../Lib/Base/inform30_macosx Heidi
116 +include_path=./,../../Lib/Base,../../Lib/Contrib
118 Remember that there's just one space between "``Heidi``" and
121 Type in the file from scratch, or copy and edit ``MyGame1.bat`` (or
122 ``MyGame1.command``). At this point, you should have a ``Heidi`` folder
123 containing two files: ``Heidi.inf`` and either ``Heidi.bat`` or
126 #. Compile the source file ``Heidi.inf``; refer back to "Inform on an IBM
127 PC (running Microsoft Windows)" on page 19 or "Inform on an Apple
128 Macintosh (running OS X)" on page 24 for guidance. If the compilation
129 works, a story file ``Heidi.z5`` appears in the folder. If the
130 compilation *doesn't* work, you've probably made a typing mistake; check
131 everything until you find it.
133 #. You can run the story file in your Inform interpreter; you should see
134 this (except that the Serial number will be different -- it's based on
137 .. code-block:: transcript
140 A simple Inform example
141 by Roger Firth and Sonja Kesserich.
142 Release 1 / Serial number 040804 / Inform v6.30 Library 6/11 SD
145 It is pitch dark, and you can't see a thing.
149 When you get that far, your template source file is correct. Let's explain
152 Understanding the source file
153 =============================
155 Although we've got a certain amount of freedom of expression, source files
156 tend to conform to a standard overall structure: these lines at the start,
157 that material next, those pieces coming at the end, and so on. What we're
158 doing here is mapping out a structure that suits us, giving ourselves a
159 clear framework onto which the elements of the game can be fitted. Having
160 a clear (albeit sparse) map at the start will help us to keep things
161 organised as the game evolves. We can infer several Inform rules just by
162 looking at the source file.
164 * If the *very first line* (or lines) of the source file begin with the
165 characters "``!%``", then the compiler treats what follows on those lines
166 as control instructions to itself rather than as part of the game's
167 source. The instructions most commonly placed here are compiler
168 switches, a way of controlling detailed aspects of how the compiler
169 operates. These particular switches, two of many, are turning on
170 :term:`Strict mode`, which makes the game less likely to misbehave when
171 being played, and :term:`Debug mode`, which provides some extra commands
172 which can be helpful when tracking down problems.
176 Actually, the ``-S`` is redundant, since Strict mode is already on by
177 default. We include it here as a reminder that (a) to turn Strict
178 mode *off*, you change this setting to ``-~S``, and (b) alphabetic
179 case matters here: ``-s`` causes a display of compiler statistics (and
180 ``-~s`` does nothing at all).
182 * Otherwise, when the compiler comes across an exclamation mark, it ignores
183 the rest of the line. If the ``!`` is at the start of a line, the whole
184 line is ignored; if the ``!`` is halfway along a line, the compiler takes
185 note of the first half, and then ignores the exclamation mark and
186 everything after it on that line. We call material following an
187 exclamation mark, not seen by anybody else, a :term:`comment`; it's often
188 a remark that we write to remind ourselves of how something works or why
189 we tackled a problem in a particular way. There's nothing special about
190 those equals signs: they just produce clear lines across the page.
192 It's always a good idea to comment code as you write it, for later it
193 will help you to understand what was going on at a particular spot.
194 Although it all seems clear in your head when you first write it, in a
195 few months you may suspect that a totally alien mind must have produced
196 that senseless gibberish.
198 By the way, the compiler *doesn't* give special treatment to exclamation
199 marks in quoted text: ``!`` within quotes "..." is treated as a normal
200 character. On this line, the first ``!`` is part of the sequence (or
201 :term:`string`) of characters to be displayed:
203 .. code-block:: inform
205 print "Hello world!"; ! <- is the start of this comment
207 * The compiler ignores blank lines, and treats lots of space like a single
208 space (except when the spaces are part of a character string). So, these
209 two rules tell us that we *could* have typed the source file like this:
211 .. code-block:: inform
213 Constant Story "Heidi";
215 "^A simple Inform example^by Roger Firth and Sonja Kesserich.^";
216 Include "Parser";Include "VerbLib";
220 We didn't type it that way because, though shorter, it's much harder to
221 read. When designing a game, you'll spend a lot of time studying what
222 you've typed, so it's worthwhile taking a bit of care to make it as
223 readable as possible.
225 * Every game should have the :term:`constant` definitions for ``Story``
226 (the game's name) and ``Headline`` (typically, information on the game's
227 theme, copyright, authorship and so on). These two :term:`string`
228 values, along with a release number and date, and details of the
229 compiler, compose the :term:`banner` which is displayed at the start of
232 * Every game needs the three lines which ``Include`` the standard library
233 files -- that is, they merge those files' contents into your source file:
235 .. code-block:: inform
242 They always have to be in this order, with ``Parser`` and ``VerbLib``
243 near the start of the file, and ``Grammar`` near the end.
245 * Every game needs to define an ``Initialise`` routine (note the British
248 .. code-block:: inform
252 The :term:`routine` that we've defined here doesn't do anything useful,
253 but it still needs to be present. Later, we'll come back to
254 ``Initialise`` and explain what a routine is and why we need this one.
256 * You'll notice that each of the items mentioned in the previous three
257 rules ends with a semicolon. Inform is very fussy about its punctuation,
258 and gets really upset if you forget a terminating semicolon. In fact,
259 the compiler just keeps reading your source file until it finds one;
260 that's why we were able to take three lines to define the ``Headline``
263 .. code-block:: inform
266 "^A simple Inform example
267 ^by Roger Firth and Sonja Kesserich.^";
269 Just to repeat what we said earlier: every game that you design will start
270 out from a basic source file like this (in fact, it might be sensible to
271 keep a copy of this template file in a safe place, as a starting point for
272 future games). Think of this stuff as the basic preparation which you'll
273 quickly come to take for granted, much as a landscape artist always begins
274 by sizing the canvas before starting to paint. So, now that we've taken a
275 quick tour of Inform's general needs, we can start thinking about what this
276 particular game requires.
278 Defining the game's locations
279 =============================
281 A good starting point in any game is to think about the locations which are
282 involved: this sketch map shows the four that we'll use:
284 .. image:: /images/heidi1.*
287 In IF, we talk about each of these locations as a :term:`room`, even though
288 in this example none of them has four walls. So let's use Inform to define
289 those rooms. Here's a first attempt:
291 .. code-block:: inform
293 Object "In front of a cottage"
295 "You stand outside a cottage. The forest stretches east.",
298 Object "Deep in the forest"
300 "Through the dense foliage, you glimpse a building to the west.
301 A track heads to the northeast.",
304 Object "A forest clearing"
306 "A tall sycamore stands in the middle of this clearing.
307 The path winds southwest through the trees.",
310 Object "At the top of the tree"
311 with description "You cling precariously to the trunk.",
314 Again, we can infer some general principles from these four examples:
316 * A room definition starts with the word ``Object`` and ends, about four
317 lines later, with a semicolon. Each of the components that appears in
318 your game -- not only the rooms, but also the people, the things that you
319 see and touch, intangibles like a sound, a smell, a gust of wind -- is
320 defined in this way; think of an "object" simply as the general term for
321 the myriad thingies which together comprise the model world which your
324 * The phrase in double quotes following the word ``Object`` is the name
325 that the interpreter uses to provide the player character with a list of
326 the objects around her: where she is, what she can see, what she's
331 We're using the word "player" to mean both the person who is playing
332 the game, and the principal protagonist (often known as the player
333 character) within the game itself. Since the latter -- Heidi -- is
334 female, we'll refer to the player as "she" while discussing this game.
336 * A keyword ``with`` follows, which simply tells the compiler what to
339 * The word ``description``, introducing another piece of text which gives
340 more detail about the object: in the case of a room, it's the appearance
341 of the surrounding environment when the player character is in that room.
342 The textual description is given in double quotes, and is followed by a
345 * Near the end, the keyword ``has`` appears, which again tells the compiler
346 to expect a certain kind of information.
348 * The word ``light`` says that this object is a source of illumination, and
349 that therefore the player character can see what's happening here. There
350 has to be at least one light source in every room (unless you want the
351 player to be told that "It's pitch dark and you can't see a thing"); most
352 commonly, that light source is the room itself.
354 A smidgeon of background may help set this into context (there's more in
355 the next chapter). An object can have both :term:`properties` (introduced
356 by the keyword ``with``) and :term:`attributes` (written after the word
357 ``has``). A property has both a name (like ``description``) and a value
358 (like the character string "``You stand outside a cottage. The forest
359 stretches east.``"); an attribute has merely a name.
361 In a little while, when you play this game, you'll observe that it starts
364 .. code-block:: transcript
366 In front of a cottage
367 You stand outside a cottage. The forest stretches east.
369 And here you can see how the room's name (``In front of a cottage``) and
370 description (``You stand outside a cottage. The forest stretches east.``)
376 We said that this was a first attempt at defining the rooms; it's fine as
377 far as it goes, but a few bits of information are missing. If you look at
378 the game's sketch map, you can see how the rooms are intended to be
379 connected; from "Deep in the forest", for example, the player character
380 should be able to move west towards the cottage, or northeast to the
381 clearing. Now, although our descriptions mention or imply these available
382 routes, we also need to explicitly add them to the room definitions in a
383 form that the game itself can make sense of. Like this:
385 .. code-block:: inform
387 Object before_cottage "In front of a cottage"
389 "You stand outside a cottage. The forest stretches east.",
393 Object forest "Deep in the forest"
395 "Through the dense foliage, you glimpse a building to the west.
396 A track heads to the northeast.",
401 Object clearing "A forest clearing"
403 "A tall sycamore stands in the middle of this clearing.
404 The path winds southwest through the trees.",
409 Object top_of_tree "At the top of the tree"
410 with description "You cling precariously to the trunk.",
414 We've made two changes to the room objects.
416 * First, between the word ``Object`` and the object's name in double
417 quotes, we've inserted a different type of name: a private, internal
418 identification, never seen by the player; one that we can use *within*
419 the source file when one object needs to refer to another object. For
420 example, the first room is identified as ``before_cottage``, and the
421 second as ``forest``.
423 Unlike the external name contained in double quotes, the internal
424 identifier has to be a single word -- that is, without spaces. To aid
425 readability, we often use an underscore character to act as sort of
426 pseudo-space: ``before_cottage`` is a bit clearer than ``beforecottage``.
428 * Second, we've added lines after the object descriptions which use those
429 internal identifiers to show how the rooms are connected; one line for
430 each connection. The ``before_cottage`` object has this additional
435 This means that a player standing in front of the cottage can type GO
436 EAST (or EAST, or just E), and the game will transport her to the room
437 whose internal identification is ``forest``. If she tries to move in any
438 other direction from this room, she'll be told "You can't go that way".
440 What we've just defined is a *one-way* easterly connection:
441 ``before_cottage`` → ``forest``. The forest object has two additional
447 The first line defines a westerly connection ``forest`` →
448 ``before_cottage`` (thus enabling the player character to return to the
449 cottage), and the second defines a connection ``forest`` → ``clearing``
450 which heads off to the northeast.
452 Inform provides for eight "horizontal" connections (``n_to``, ``ne_to``,
453 ``e_to``, ``se_to``, ``s_to``, ``sw_to``, ``w_to``, ``nw_to``) two
454 "vertical" ones (``u_to``, ``d_to``) and two specials ``in_to``, and
455 ``out_to``. You'll see some of these used for the remaining inter-room
458 There's one last detail to attend to before we can test what we've done.
459 You'll recollect that our story begins with Heidi standing in front of her
460 cottage. We need to tell the interpreter that ``before_cottage`` is the room
461 where the game starts, and we do this in the ``Initialise`` routine::
463 [ Initialise; location = before_cottage; ];
465 ``location`` is a :term:`variable`, part of the library, which tells the
466 interpreter in which room the player character currently is. Here, we're
467 saying that, at the start of the game, the player character is in the
468 ``before_cottage`` room.
470 Now we can add what we've done to the ``Heidi.inf`` source file template.
471 At this stage, you should study the four room definitions, comparing them
472 with the sketch map until you're comfortable that you understand how to
473 create simple rooms and define the connections between them.
475 .. code-block:: inform
477 !============================================================================
478 Constant Story "Heidi";
480 "^A simple Inform example
481 ^by Roger Firth and Sonja Kesserich.^";
486 !============================================================================
489 Object before_cottage "In front of a cottage"
491 "You stand outside a cottage. The forest stretches east.",
495 Object forest "Deep in the forest"
497 "Through the dense foliage, you glimpse a building to the west.
498 A track heads to the northeast.",
503 Object clearing "A forest clearing"
505 "A tall sycamore stands in the middle of this clearing.
506 The path winds southwest through the trees.",
511 Object top_of_tree "At the top of the tree"
512 with description "You cling precariously to the trunk.",
516 !============================================================================
517 ! Entry point routines
519 [ Initialise; location = before_cottage; ];
521 !============================================================================
522 ! Standard and extended grammar
526 !============================================================================
528 Type this in, as always taking great care with the punctuation -- watch
529 those commas and semicolons. Compile it, and fix any mistakes which the
530 compiler reports. You can then play the game in its current state.
531 Admittedly, you can't do very much, but you should be able to move freely
532 among the four rooms that you've defined.
536 In order to minimise the amount of typing that you have to do, the
537 descriptive text in this game has been kept as short as possible. In a
538 real game, you would typically provide more interesting descriptions
541 Adding the bird and the nest
542 ============================
544 Given what we said earlier, you won't be surprised to hear that both the
545 bird and its nest are Inform objects. We'll start their definitions like
548 .. code-block:: inform
550 Object bird "baby bird"
551 with description "Too young to fly, the nestling tweets helplessly.",
554 Object nest "bird's nest"
555 with description "The nest is carefully woven of twigs and moss.",
558 You can see that these definitions have exactly the same format as the
559 rooms we defined previously: a one-word internal identifier (``bird``,
560 ``nest``), and a word or phrase naming the object for the player's benefit
561 (``baby bird``, ``bird's nest``). They both have some descriptive detail:
562 for a room this is printed when the player first enters, or when she types
563 LOOK; for other objects it's printed when she EXAMINEs that object. What
564 they *don't* have are connections (``e_to``, ``w_to``, etc. apply only to
565 rooms) or ``light`` (it's not necessary -- the rooms ensure that light is
568 When the game is running, the player will want to refer to these two
569 objects, saying for instance EXAMINE THE BABY BIRD or PICK UP THE NEST.
570 For this to work reliably, we need to specify the word (or words) which
571 relate to each object. Our aim here is flexibility: providing a choice of
572 relevant vocabulary so that the player can use whatever term seems
573 appropriate to her, with a good chance of it being understood. We add a
574 line to each definition:
576 .. code-block:: inform
578 Object bird "baby bird"
579 with description "Too young to fly, the nestling tweets helplessly.",
580 name 'baby' 'bird' 'nestling',
583 Object nest "bird's nest"
584 with description "The nest is carefully woven of twigs and moss.",
585 name 'bird^s' 'nest' 'twigs' 'moss',
588 The ``name`` introduces a list in single quotes '...'. We call each of
589 those quoted things a :term:`dictionary word`, and we do mean "word", not
590 "phrase" (``'baby'``\ ``'bird'`` rather than ``'baby bird'``); you can't
591 uses spaces, commas or periods *in* dictionary words, though there's a
592 space *between* each one, and the whole list ends with a comma. The idea
593 is that the interpreter decides which object a player is talking about by
594 matching what she types against the full set of all dictionary words. If
595 the player mentions BIRD, or BABY BIRD, or NESTLING, it's the ``baby bird``
596 that she means; if she mentions NEST, BIRD'S NEST or MOSS, it's the
597 ``bird's nest``. And if she types NEST BABY or BIRD TWIGS, the interpreter
598 will politely say that it doesn't understand what on earth she's talking
603 You'll notice the use of ``'bird^s'`` to define the dictionary word
604 BIRD'S; this oddity is necessary because the compiler expects the single
605 quotes in the list always to come in pairs -- one at the start of the
606 dictionary word, and one at the end. If we had typed ``'bird's'`` then
607 the compiler would find the opening quote, the four letters ``b``,
608 ``i``, ``r`` and ``d``, and what looks like the closing quote. So far
609 so good; it's read the word BIRD and now expects a space before the next
610 opening quote... but instead finds ``s'`` which makes no sense. In
611 cases like this we must use the circumflex ``^`` to *represent* the
612 apostrophe, and the compiler then treats ``bird's`` as a dictionary
615 You may be wondering why we need a list of ``name`` words for the bird and
616 its nest, yet we didn't when we defined the rooms? It's because the player
617 can't interact with a room in the same way as with other objects; for
618 example, she doesn't need to say EXAMINE THE FOREST -- just being there and
619 typing LOOK is sufficient.
621 The bird's definition is complete, but there's an additional complexity
622 with the nest: we need to be able to put the bird into it. We do this by
623 labelling the nest as a ``container`` -- able to hold other objects -- so
624 that the player can type PUT (or INSERT) BIRD IN (or INTO) NEST.
625 Furthermore, we label it as ``open``; this prevents the interpreter from
626 asking us to open it before putting in the bird.
628 .. code-block:: inform
630 Object nest "bird's nest"
631 with description "The nest is carefully woven of twigs and moss.",
632 name 'bird^s' 'nest' 'twigs' 'moss',
635 Both objects are now defined, and we can incorporate them into the game.
636 To do this, we need to choose the locations where the player will find
637 them. Let's say that the bird is found in the forest, while the nest is in
638 the clearing. This is how we set this up:
640 .. code-block:: inform
642 Object bird "baby bird" forest
643 with description "Too young to fly, the nestling tweets helplessly.",
644 name 'baby' 'bird' 'nestling',
647 Object nest "bird's nest" clearing
648 with description "The nest is carefully woven of twigs and moss.",
649 name 'bird^s' 'nest' 'twigs' 'moss',
652 Read that first line as: "Here's the definition of an object which is
653 identified within this file as ``bird``, which is known to the player as
654 ``baby bird``, and which is initially located inside the object identified
655 within this file as ``forest``."
657 Where in the source file do these new objects fit? Well, anywhere really,
658 but you'll find it convenient to insert them following the rooms where
659 they're found. This means adding the bird just after the forest, and the
660 nest just after the clearing. Here's the middle piece of the source file:
662 .. code-block:: inform
664 !============================================================================
667 Object before_cottage "In front of a cottage"
669 "You stand outside a cottage. The forest stretches east.",
673 Object forest "Deep in the forest"
675 "Through the dense foliage, you glimpse a building to the west.
676 A track heads to the northeast.",
681 Object bird "baby bird" forest
682 with description "Too young to fly, the nestling tweets helplessly.",
683 name 'baby' 'bird' 'nestling',
686 Object clearing "A forest clearing"
688 "A tall sycamore stands in the middle of this clearing.
689 The path winds southwest through the trees.",
694 Object nest "bird's nest" clearing
695 with description "The nest is carefully woven of twigs and moss.",
696 name 'bird^s' 'nest' 'twigs' 'moss',
699 Object top_of_tree "At the top of the tree"
700 with description "You cling precariously to the trunk.",
704 !============================================================================
706 Make those changes, recompile the game, play it and you'll see this:
708 .. code-block:: transcript
711 Through the dense foliage, you glimpse a building to the west. A track heads
714 You can see a baby bird here.
718 Adding the tree and the branch
719 ==============================
721 The description of the clearing mentions a tall sycamore tree, up which the
722 player character supposedly "climbs". We'd better define it:
724 .. code-block:: inform
726 Object tree "tall sycamore tree" clearing
728 "Standing proud in the middle of the clearing,
729 the stout tree looks easy to climb.",
730 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
733 Everything there should be familiar, apart from that ``scenery`` at the
734 end. We've already mentioned the tree in the description of the forest
735 clearing, so we don't want the interpreter adding "You can see a tall
736 sycamore tree here" afterwards, as it does for the bird and the nest. By
737 labelling the tree as ``scenery`` we suppress that, and also prevent it
738 from being picked up by the player character. One final object: the branch
739 at the top of the tree. Again, not many surprises in this definition:
741 .. code-block:: inform
743 Object branch "wide firm bough" top_of_tree
744 with description "It's flat enough to support a small object.",
745 name 'wide' 'firm' 'flat' 'bough' 'branch',
746 has static supporter;
748 The only new things are those two labels. ``static`` is similar to
749 ``scenery``: it prevents the branch from being picked up by the player
750 character, but *doesn't* suppress mention of it when describing the
751 setting. And ``supporter`` is rather like the ``container`` that we
752 used for the nest, except that this time the player character can put
753 other objects *onto* the branch. (In passing, we'll mention that an
754 object can't normally be both a ``container`` *and* a ``supporter``.)
755 And so here are our objects again:
757 .. code-block:: inform
759 !============================================================================
762 Object before_cottage "In front of a cottage"
764 "You stand outside a cottage. The forest stretches east.",
768 Object forest "Deep in the forest"
770 "Through the dense foliage, you glimpse a building to the west.
771 A track heads to the northeast.",
776 Object bird "baby bird" forest
777 with description "Too young to fly, the nestling tweets helplessly.",
778 name 'baby' 'bird' 'nestling',
781 Object clearing "A forest clearing"
783 "A tall sycamore stands in the middle of this clearing.
784 The path winds southwest through the trees.",
789 Object nest "bird's nest" clearing
790 with description "The nest is carefully woven of twigs and moss.",
791 name 'bird^s' 'nest' 'twigs' 'moss',
794 Object tree "tall sycamore tree" clearing
796 "Standing proud in the middle of the clearing,
797 the stout tree looks easy to climb.",
798 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
801 Object top_of_tree "At the top of the tree"
802 with description "You cling precariously to the trunk.",
806 Object branch "wide firm bough" top_of_tree
807 with description "It's flat enough to support a small object.",
808 name 'wide' 'firm' 'flat' 'bough' 'branch',
809 has static supporter;
811 !============================================================================
813 Once again, make the changes, recompile, and investigate what you can do in
819 Our first pass at the game is nearly done; just two more changes to
820 describe. The first is easy: Heidi wouldn't be able to climb the tree
821 carrying the bird and the nest separately: we want the player character to
822 put the bird into the nest first. One easy way to enforce this is by
823 adding a line near the top of the file:
825 .. code-block:: inform
827 !============================================================================
828 Constant Story "Heidi";
830 "^A simple Inform example
831 ^by Roger Firth and Sonja Kesserich.^";
833 Constant MAX_CARRIED 1;
835 The value of ``MAX_CARRIED`` limits the number of objects that the player
836 character can be holding at any one time; by setting it to 1, we're saying
837 that she can carry the bird or the nest, but not both. However, the limit
838 ignores the contents of ``container`` or ``supporter`` objects, so the nest
839 with the bird inside it is still counted as one object.
841 The other change is slightly more complex and more important: there's
842 currently no way to "win" the game! The goal is for the player character
843 to put the bird in the nest, take the nest to the top of the tree, and
844 place it on the branch; when that happens, the game should be over. This
845 is one way of making it happen:
847 .. code-block:: inform
849 Object branch "wide firm bough" top_of_tree
850 with description "It's flat enough to support a small object.",
851 name 'wide' 'firm' 'flat' 'bough' 'branch',
852 each_turn [; if (nest in branch) deadflag = 2; ],
853 has static supporter;
857 Here's an explanation of what's going on. If you find this difficult to
858 grasp, don't worry. It's the hardest bit so far, and it introduces
859 several new concepts all at once. Later in the guide, we'll explain
860 those concepts more clearly, so you can just skip this bit if you want.
862 The variable ``deadflag``, part of the library, is normally 0. If you set
863 its value to 2, the interpreter notices and ends the game with "You have
864 won". The statement::
866 if (nest in branch) deadflag = 2;
868 should be read as: "Test whether the ``nest`` is currently in the
869 ``branch`` (if the branch is a ``container``) or on it (if the
870 ``branch`` is a supporter); if it is, set the value of ``deadflag`` to
871 2; if it isn't, do nothing." The surrounding part::
875 should be read as: "At the end of each turn (when the player is in the
876 same room as the branch), do whatever is written inside the square
877 brackets". So, putting that all together:
879 * At the end of each turn (after the player has typed something and
880 pressed the Enter key, and the interpreter has done whatever was
881 requested) the interpreter checks whether the player and the
882 ``branch`` are in the same room. If not, nothing happens. If they're
883 together, it looks to see where the nest is. Initially it's in the
884 ``clearing``, so nothing happens.
886 * Also at the end of each turn, the interpreter checks the value of
887 ``deadflag``. Usually it's 0, so nothing happens.
889 * Finally the player character puts the ``nest`` on the ``branch``.
890 "Aha!" says the interpreter (to itself, of course), and sets the
891 value of ``deadflag`` to 2.
893 * Immediately afterwards, (another part of) the interpreter checks and
894 finds that the value of ``deadflag`` has changed to 2, which means
895 that the game is successfully completed; so, it says to the player,
898 That's as far as we'll take this example for now. Make those final
899 changes, recompile, and test what you've achieved. You'll probably find a
900 few things that could be done better -- even on a simple game like this
901 there's considerable scope for improvement -- so we'll revisit Heidi in her
902 forest shortly. First, though, we'll recap what we've learnt so far.