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.*
10 .. image:: /images/picE.png
13 ach of the three games in this guide is created step by step; you'll get
14 most benefit (especially to begin with) if you take an active part, typing
15 in the source code on your computer. Our first game, described in this
16 chapter and the two which follow, tells this sentimental little story:
18 "Heidi lives in a tiny cottage deep in the forest. One sunny day,
19 standing before the cottage, she hears the frenzied tweeting of baby
20 bird; its nest has fallen from the tall tree in the clearing! Heidi
21 puts the bird into the nest, and then climbs the tree to place the nest
24 It's a very simple tale, but even so we'll cover quite a lot of ground
25 before we have a finished Inform game. We'll get there in stages, first
26 making a very rough approximation of the story, and then successively
27 refining the details until it's good enough for an initial attempt (there's
28 time later for more advanced stuff).
30 Creating a basic source file
31 ============================
33 The first task is to create an Inform source file template. Every game
34 that we design will start out like this. Follow these steps:
36 #. Create an ``Inform\Games\Heidi`` folder (maybe by copying ``Inform\Games\MyGame1``).
40 In this guide, we use the PC convention of placing a backslash
41 between folder names. On a Macintosh, use a regular slash:
42 ``Inform/Games/Heidi``.
44 #. In that folder, use your text editor to create this source file
47 .. code-block:: inform6
50 !============================================================================
51 Constant Story "Heidi";
53 "^A simple Inform example
54 ^by Roger Firth and Sonja Kesserich.^";
59 !============================================================================
62 !============================================================================
63 ! Entry point routines
67 !============================================================================
68 ! Standard and extended grammar
72 !============================================================================
74 Soon, we'll explain what this means. For now, just type it all in,
75 paying particular attention to those seven semicolons, and ensuring that
76 the double quotes "..." always come in pairs. The first line beginning
77 with "``!%``" is special, and we'll talk about it in a moment; the
78 remaining exclamation mark lines, on the other hand, are purely
79 decorative; they just make the file's structure a little easier to
82 Ensure the file is named ``Heidi.inf``, rather than ``Heidi.txt`` or
85 Remember that, throughout this guide, we place the "TYPE" symbol
86 alongside pieces of code that we recommend you to type into your own
87 game files as you read through the examples (which, conversely, means
88 that you don't need to type the unmarked pieces of code). You'll learn
89 Inform more quickly by trying it for yourself, rather than just taking
90 our word for how things work.
94 Again, revisit the TYPE symbol. Maybe a standard indicator above
97 #. In the same folder, use your text editor to create the compilation
98 support file ``Heidi.bat`` (on a PC)::
100 ..\..\Lib\Base\Inform Heidi
101 +include_path=.\,..\..\Lib\Base,..\..\Lib\Contrib | more
103 pause "at end of compilation"
105 or ``Heidi.command`` (on a Macintosh)::
107 cd ~/Inform/Games/Heidi/
109 ../../Lib/Base/inform30_macosx Heidi
110 +include_path=./,../../Lib/Base,../../Lib/Contrib
112 Remember that there's just one space between "``Heidi``" and
115 Type in the file from scratch, or copy and edit ``MyGame1.bat`` (or
116 ``MyGame1.command``). At this point, you should have a ``Heidi`` folder
117 containing two files: ``Heidi.inf`` and either ``Heidi.bat`` or
120 #. Compile the source file ``Heidi.inf``; refer back to "Inform on an IBM
121 PC (running Microsoft Windows)" on page 19 or "Inform on an Apple
122 Macintosh (running OS X)" on page 24 for guidance. If the compilation
123 works, a story file ``Heidi.z5`` appears in the folder. If the
124 compilation *doesn't* work, you've probably made a typing mistake; check
125 everything until you find it.
127 #. You can run the story file in your Inform interpreter; you should see
128 this (except that the Serial number will be different -- it's based on
131 .. code-block:: transcript
134 A simple Inform example
135 by Roger Firth and Sonja Kesserich.
136 Release 1 / Serial number 040804 / Inform v6.30 Library 6/11 SD
139 It is pitch dark, and you can't see a thing.
143 When you get that far, your template source file is correct. Let's explain
146 Understanding the source file
147 =============================
149 Although we've got a certain amount of freedom of expression, source files
150 tend to conform to a standard overall structure: these lines at the start,
151 that material next, those pieces coming at the end, and so on. What we're
152 doing here is mapping out a structure that suits us, giving ourselves a
153 clear framework onto which the elements of the game can be fitted. Having
154 a clear (albeit sparse) map at the start will help us to keep things
155 organised as the game evolves. We can infer several Inform rules just by
156 looking at the source file.
158 * If the *very first line* (or lines) of the source file begin with the
159 characters "``!%``", then the compiler treats what follows on those lines
160 as control instructions to itself rather than as part of the game's
161 source. The instructions most commonly placed here are compiler
162 switches, a way of controlling detailed aspects of how the compiler
163 operates. These particular switches, two of many, are turning on
164 **Strict mode**, which makes the game less likely to misbehave when being
165 played, and **Debug mode**, which provides some extra commands which can
166 be helpful when tracking down problems.
170 Actually, the ``-S`` is redundant, since Strict mode is already on by
171 default. We include it here as a reminder that (a) to turn Strict
172 mode *off*, you change this setting to ``-~S``, and (b) alphabetic
173 case matters here: ``-s`` causes a display of compiler statistics (and
174 ``-~s`` does nothing at all).
176 * Otherwise, when the compiler comes across an exclamation mark, it ignores
177 the rest of the line. If the ``!`` is at the start of a line, the whole
178 line is ignored; if the ``!`` is halfway along a line, the compiler takes
179 note of the first half, and then ignores the exclamation mark and
180 everything after it on that line. We call material following an
181 exclamation mark, not seen by anybody else, a **comment**; it's often a
182 remark that we write to remind ourselves of how something works or why we
183 tackled a problem in a particular way. There's nothing special about
184 those equals signs: they just produce clear lines across the page.
186 It's always a good idea to comment code as you write it, for later it
187 will help you to understand what was going on at a particular spot.
188 Although it all seems clear in your head when you first write it, in a
189 few months you may suspect that a totally alien mind must have produced
190 that senseless gibberish.
192 By the way, the compiler *doesn't* give special treatment to exclamation
193 marks in quoted text: ``!`` within quotes "..." is treated as a normal
194 character. On this line, the first ``!`` is part of the sequence (or
195 string) of characters to be displayed:
197 .. code-block:: inform6
199 print "Hello world!"; ! <- is the start of this comment
201 * The compiler ignores blank lines, and treats lots of space like a single
202 space (except when the spaces are part of a character string). So, these
203 two rules tell us that we *could* have typed the source file like this:
205 .. code-block:: inform6
207 Constant Story "Heidi";
209 "^A simple Inform example^by Roger Firth and Sonja Kesserich.^";
210 Include "Parser";Include "VerbLib";
214 We didn't type it that way because, though shorter, it's much harder to
215 read. When designing a game, you'll spend a lot of time studying what
216 you've typed, so it's worthwhile taking a bit of care to make it as
217 readable as possible.
219 * Every game should have the **constant** definitions for ``Story`` (the
220 game's name) and ``Headline`` (typically, information on the game's
221 theme, copyright, authorship and so on). These two **string** values,
222 along with a release number and date, and details of the compiler,
223 compose the **banner** which is displayed at the start of each game.
225 * Every game needs the three lines which ``Include`` the standard library
226 files -- that is, they merge those files' contents into your source file:
228 .. code-block:: inform6
235 They always have to be in this order, with ``Parser`` and ``VerbLib``
236 near the start of the file, and ``Grammar`` near the end.
238 * Every game needs to define an ``Initialise`` routine (note the British
241 .. code-block:: inform6
245 The **routine** that we've defined here doesn't do anything useful, but
246 it still needs to be present. Later, we'll come back to ``Initialise``
247 and explain what a routine is and why we need this one.
249 * You'll notice that each of the items mentioned in the previous three
250 rules ends with a semicolon. Inform is very fussy about its punctuation,
251 and gets really upset if you forget a terminating semicolon. In fact,
252 the compiler just keeps reading your source file until it finds one;
253 that's why we were able to take three lines to define the ``Headline``
256 .. code-block:: inform6
259 "^A simple Inform example
260 ^by Roger Firth and Sonja Kesserich.^";
262 Just to repeat what we said earlier: every game that you design will start
263 out from a basic source file like this (in fact, it might be sensible to
264 keep a copy of this template file in a safe place, as a starting point for
265 future games). Think of this stuff as the basic preparation which you'll
266 quickly come to take for granted, much as a landscape artist always begins
267 by sizing the canvas before starting to paint. So, now that we've taken a
268 quick tour of Inform's general needs, we can start thinking about what this
269 particular game requires.
271 Defining the game's locations
272 =============================
274 A good starting point in any game is to think about the locations which are
275 involved: this sketch map shows the four that we'll use:
277 .. image:: /images/heidi1.*
280 In IF, we talk about each of these locations as a **room**, even though in
281 this example none of them has four walls. So let's use Inform to define
282 those rooms. Here's a first attempt:
284 .. code-block:: inform6
286 Object "In front of a cottage"
288 "You stand outside a cottage. The forest stretches east.",
291 Object "Deep in the forest"
293 "Through the dense foliage, you glimpse a building to the west.
294 A track heads to the northeast.",
297 Object "A forest clearing"
299 "A tall sycamore stands in the middle of this clearing.
300 The path winds southwest through the trees.",
303 Object "At the top of the tree"
304 with description "You cling precariously to the trunk.",
307 Again, we can infer some general principles from these four examples:
309 * A room definition starts with the word ``Object`` and ends, about four
310 lines later, with a semicolon. Each of the components that appears in
311 your game -- not only the rooms, but also the people, the things that you
312 see and touch, intangibles like a sound, a smell, a gust of wind -- is
313 defined in this way; think of an "object" simply as the general term for
314 the myriad thingies which together comprise the model world which your
317 * The phrase in double quotes following the word ``Object`` is the name
318 that the interpreter uses to provide the player character with a list of
319 the objects around her: where she is, what she can see, what she's
324 We're using the word "player" to mean both the person who is playing
325 the game, and the principal protagonist (often known as the player
326 character) within the game itself. Since the latter -- Heidi -- is
327 female, we'll refer to the player as "she" while discussing this game.
329 * A keyword ``with`` follows, which simply tells the compiler what to
332 * The word ``description``, introducing another piece of text which gives
333 more detail about the object: in the case of a room, it's the appearance
334 of the surrounding environment when the player character is in that room.
335 The textual description is given in double quotes, and is followed by a
338 * Near the end, the keyword ``has`` appears, which again tells the compiler
339 to expect a certain kind of information.
341 * The word ``light`` says that this object is a source of illumination, and
342 that therefore the player character can see what's happening here. There
343 has to be at least one light source in every room (unless you want the
344 player to be told that "It's pitch dark and you can't see a thing"); most
345 commonly, that light source is the room itself.
347 A smidgeon of background may help set this into context (there's more in
348 the next chapter). An object can have both **properties** (introduced by
349 the keyword ``with``) and **attributes** (written after the word ``has``).
350 A property has both a name (like ``description``) and a value (like the
351 character string "``You stand outside a cottage. The forest stretches
352 east.``"); an attribute has merely a name.
354 In a little while, when you play this game, you'll observe that it starts
357 .. code-block:: transcript
359 In front of a cottage
360 You stand outside a cottage. The forest stretches east.
362 And here you can see how the room's name (``In front of a cottage``) and
363 description (``You stand outside a cottage. The forest stretches east.``)
369 We said that this was a first attempt at defining the rooms; it's fine as
370 far as it goes, but a few bits of information are missing. If you look at
371 the game's sketch map, you can see how the rooms are intended to be
372 connected; from "Deep in the forest", for example, the player character
373 should be able to move west towards the cottage, or northeast to the
374 clearing. Now, although our descriptions mention or imply these available
375 routes, we also need to explicitly add them to the room definitions in a
376 form that the game itself can make sense of. Like this:
378 .. code-block:: inform6
380 Object before_cottage "In front of a cottage"
382 "You stand outside a cottage. The forest stretches east.",
386 Object forest "Deep in the forest"
388 "Through the dense foliage, you glimpse a building to the west.
389 A track heads to the northeast.",
394 Object clearing "A forest clearing"
396 "A tall sycamore stands in the middle of this clearing.
397 The path winds southwest through the trees.",
402 Object top_of_tree "At the top of the tree"
403 with description "You cling precariously to the trunk.",
407 We've made two changes to the room objects.
409 * First, between the word ``Object`` and the object's name in double
410 quotes, we've inserted a different type of name: a private, internal
411 identification, never seen by the player; one that we can use *within*
412 the source file when one object needs to refer to another object. For
413 example, the first room is identified as ``before_cottage``, and the
414 second as ``forest``.
416 Unlike the external name contained in double quotes, the internal
417 identifier has to be a single word -- that is, without spaces. To aid
418 readability, we often use an underscore character to act as sort of
419 pseudo-space: ``before_cottage`` is a bit clearer than ``beforecottage``.
421 * Second, we've added lines after the object descriptions which use those
422 internal identifiers to show how the rooms are connected; one line for
423 each connection. The ``before_cottage`` object has this additional
428 This means that a player standing in front of the cottage can type GO
429 EAST (or EAST, or just E), and the game will transport her to the room
430 whose internal identification is ``forest``. If she tries to move in any
431 other direction from this room, she'll be told "You can't go that way".
433 What we've just defined is a *one-way* easterly connection:
434 ``before_cottage`` → ``forest``. The forest object has two additional
440 The first line defines a westerly connection ``forest`` →
441 ``before_cottage`` (thus enabling the player character to return to the
442 cottage), and the second defines a connection ``forest`` → ``clearing``
443 which heads off to the northeast.
445 Inform provides for eight "horizontal" connections (``n_to``, ``ne_to``,
446 ``e_to``, ``se_to``, ``s_to``, ``sw_to``, ``w_to``, ``nw_to``) two
447 "vertical" ones (``u_to``, ``d_to``) and two specials ``in_to``, and
448 ``out_to``. You'll see some of these used for the remaining inter-room
451 There's one last detail to attend to before we can test what we've done.
452 You'll recollect that our story begins with Heidi standing in front of her
453 cottage. We need to tell the interpreter that ``before_cottage`` is the room
454 where the game starts, and we do this in the ``Initialise`` routine::
456 [ Initialise; location = before_cottage; ];
458 ``location`` is a **variable**, part of the library, which tells the
459 interpreter in which room the player character currently is. Here, we're
460 saying that, at the start of the game, the player character is in the
461 ``before_cottage`` room.
463 Now we can add what we've done to the ``Heidi.inf`` source file template.
464 At this stage, you should study the four room definitions, comparing them
465 with the sketch map until you're comfortable that you understand how to
466 create simple rooms and define the connections between them.
468 .. code-block:: inform6
470 !============================================================================
471 Constant Story "Heidi";
473 "^A simple Inform example
474 ^by Roger Firth and Sonja Kesserich.^";
479 !============================================================================
482 Object before_cottage "In front of a cottage"
484 "You stand outside a cottage. The forest stretches east.",
488 Object forest "Deep in the forest"
490 "Through the dense foliage, you glimpse a building to the west.
491 A track heads to the northeast.",
496 Object clearing "A forest clearing"
498 "A tall sycamore stands in the middle of this clearing.
499 The path winds southwest through the trees.",
504 Object top_of_tree "At the top of the tree"
505 with description "You cling precariously to the trunk.",
509 !============================================================================
510 ! Entry point routines
512 [ Initialise; location = before_cottage; ];
514 !============================================================================
515 ! Standard and extended grammar
519 !============================================================================
521 Type this in, as always taking great care with the punctuation -- watch
522 those commas and semicolons. Compile it, and fix any mistakes which the
523 compiler reports. You can then play the game in its current state.
524 Admittedly, you can't do very much, but you should be able to move freely
525 among the four rooms that you've defined.
529 In order to minimise the amount of typing that you have to do, the
530 descriptive text in this game has been kept as short as possible. In a
531 real game, you would typically provide more interesting descriptions
534 Adding the bird and the nest
535 ============================
537 Given what we said earlier, you won't be surprised to hear that both the
538 bird and its nest are Inform objects. We'll start their definitions like
541 .. code-block:: inform6
543 Object bird "baby bird"
544 with description "Too young to fly, the nestling tweets helplessly.",
547 Object nest "bird's nest"
548 with description "The nest is carefully woven of twigs and moss.",
551 You can see that these definitions have exactly the same format as the
552 rooms we defined previously: a one-word internal identifier (``bird``,
553 ``nest``), and a word or phrase naming the object for the player's benefit
554 (``baby bird``, ``bird's nest``). They both have some descriptive detail:
555 for a room this is printed when the player first enters, or when she types
556 LOOK; for other objects it's printed when she EXAMINEs that object. What
557 they *don't* have are connections (``e_to``, ``w_to``, etc. apply only to
558 rooms) or ``light`` (it's not necessary -- the rooms ensure that light is
561 When the game is running, the player will want to refer to these two
562 objects, saying for instance EXAMINE THE BABY BIRD or PICK UP THE NEST.
563 For this to work reliably, we need to specify the word (or words) which
564 relate to each object. Our aim here is flexibility: providing a choice of
565 relevant vocabulary so that the player can use whatever term seems
566 appropriate to her, with a good chance of it being understood. We add a
567 line to each definition:
569 .. code-block:: inform6
571 Object bird "baby bird"
572 with description "Too young to fly, the nestling tweets helplessly.",
573 name 'baby' 'bird' 'nestling',
576 Object nest "bird's nest"
577 with description "The nest is carefully woven of twigs and moss.",
578 name 'bird^s' 'nest' 'twigs' 'moss',
581 The ``name`` introduces a list in single quotes '...'. We call each of
582 those quoted things a **dictionary word**, and we do mean "word", not
583 "phrase" (``'baby'``\ ``'bird'`` rather than ``'baby bird'``); you can't
584 uses spaces, commas or periods in dictionary words, though there's a space
585 *between* each one, and the whole list ends with a comma. The idea is that
586 the interpreter decides which object a player is talking about by matching
587 what she types against the full set of all dictionary words. If the player
588 mentions BIRD, or BABY BIRD, or NESTLING, it's the ``baby bird`` that she
589 means; if she mentions NEST, BIRD'S NEST or MOSS, it's the ``bird's nest``.
590 And if she types NEST BABY or BIRD TWIGS, the interpreter will politely say
591 that it doesn't understand what on earth she's talking about.
595 You'll notice the use of ``'bird^s'`` to define the dictionary word
596 BIRD'S; this oddity is necessary because the compiler expects the single
597 quotes in the list always to come in pairs -- one at the start of the
598 dictionary word, and one at the end. If we had typed ``'bird's'`` then
599 the compiler would find the opening quote, the four letters ``b``,
600 ``i``, ``r`` and ``d``, and what looks like the closing quote. So far
601 so good; it's read the word BIRD and now expects a space before the next
602 opening quote... but instead finds ``s'`` which makes no sense. In
603 cases like this we must use the circumflex ``^`` to *represent* the
604 apostrophe, and the compiler then treats ``bird's`` as a dictionary
607 You may be wondering why we need a list of ``name`` words for the bird and
608 its nest, yet we didn't when we defined the rooms? It's because the player
609 can't interact with a room in the same way as with other objects; for
610 example, she doesn't need to say EXAMINE THE FOREST -- just being there and
611 typing LOOK is sufficient.
613 The bird's definition is complete, but there's an additional complexity
614 with the nest: we need to be able to put the bird into it. We do this by
615 labelling the nest as a ``container`` -- able to hold other objects -- so
616 that the player can type PUT (or INSERT) BIRD IN (or INTO) NEST.
617 Furthermore, we label it as ``open``; this prevents the interpreter from
618 asking us to open it before putting in the bird.
620 .. code-block:: inform6
622 Object nest "bird's nest"
623 with description "The nest is carefully woven of twigs and moss.",
624 name 'bird^s' 'nest' 'twigs' 'moss',
627 Both objects are now defined, and we can incorporate them into the game.
628 To do this, we need to choose the locations where the player will find
629 them. Let's say that the bird is found in the forest, while the nest is in
630 the clearing. This is how we set this up:
632 .. code-block:: inform6
634 Object bird "baby bird" forest
635 with description "Too young to fly, the nestling tweets helplessly.",
636 name 'baby' 'bird' 'nestling',
639 Object nest "bird's nest" clearing
640 with description "The nest is carefully woven of twigs and moss.",
641 name 'bird^s' 'nest' 'twigs' 'moss',
644 Read that first line as: "Here's the definition of an object which is
645 identified within this file as ``bird``, which is known to the player as
646 ``baby bird``, and which is initially located inside the object identified
647 within this file as ``forest``."
649 Where in the source file do these new objects fit? Well, anywhere really,
650 but you'll find it convenient to insert them following the rooms where
651 they're found. This means adding the bird just after the forest, and the
652 nest just after the clearing. Here's the middle piece of the source file:
654 .. code-block:: inform6
656 !============================================================================
659 Object before_cottage "In front of a cottage"
661 "You stand outside a cottage. The forest stretches east.",
665 Object forest "Deep in the forest"
667 "Through the dense foliage, you glimpse a building to the west.
668 A track heads to the northeast.",
673 Object bird "baby bird" forest
674 with description "Too young to fly, the nestling tweets helplessly.",
675 name 'baby' 'bird' 'nestling',
678 Object clearing "A forest clearing"
680 "A tall sycamore stands in the middle of this clearing.
681 The path winds southwest through the trees.",
686 Object nest "bird's nest" clearing
687 with description "The nest is carefully woven of twigs and moss.",
688 name 'bird^s' 'nest' 'twigs' 'moss',
691 Object top_of_tree "At the top of the tree"
692 with description "You cling precariously to the trunk.",
696 !============================================================================
698 Make those changes, recompile the game, play it and you'll see this:
700 .. code-block:: transcript
703 Through the dense foliage, you glimpse a building to the west. A track heads
706 You can see a baby bird here.
710 Adding the tree and the branch
711 ==============================
713 The description of the clearing mentions a tall sycamore tree, up which the
714 player character supposedly "climbs". We'd better define it:
716 .. code-block:: inform6
718 Object tree "tall sycamore tree" clearing
720 "Standing proud in the middle of the clearing,
721 the stout tree looks easy to climb.",
722 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
725 Everything there should be familiar, apart from that ``scenery`` at the
726 end. We've already mentioned the tree in the description of the forest
727 clearing, so we don't want the interpreter adding "You can see a tall
728 sycamore tree here" afterwards, as it does for the bird and the nest. By
729 labelling the tree as ``scenery`` we suppress that, and also prevent it
730 from being picked up by the player character. One final object: the branch
731 at the top of the tree. Again, not many surprises in this definition:
733 .. code-block:: inform6
735 Object branch "wide firm bough" top_of_tree
736 with description "It's flat enough to support a small object.",
737 name 'wide' 'firm' 'flat' 'bough' 'branch',
738 has static supporter;
740 The only new things are those two labels. ``static`` is similar to
741 ``scenery``: it prevents the branch from being picked up by the player
742 character, but *doesn't* suppress mention of it when describing the
743 setting. And ``supporter`` is rather like the ``container`` that we used
744 for the nest, except that this time the player character can put other
745 objects *onto* the branch. (In passing, we'll mention that an object can't
746 normally be both a ``container`` and a ``supporter``.) And so here are our
749 .. code-block:: inform6
751 !============================================================================
754 Object before_cottage "In front of a cottage"
756 "You stand outside a cottage. The forest stretches east.",
760 Object forest "Deep in the forest"
762 "Through the dense foliage, you glimpse a building to the west.
763 A track heads to the northeast.",
768 Object bird "baby bird" forest
769 with description "Too young to fly, the nestling tweets helplessly.",
770 name 'baby' 'bird' 'nestling',
773 Object clearing "A forest clearing"
775 "A tall sycamore stands in the middle of this clearing.
776 The path winds southwest through the trees.",
781 Object nest "bird's nest" clearing
782 with description "The nest is carefully woven of twigs and moss.",
783 name 'bird^s' 'nest' 'twigs' 'moss',
786 Object tree "tall sycamore tree" clearing
788 "Standing proud in the middle of the clearing,
789 the stout tree looks easy to climb.",
790 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
793 Object top_of_tree "At the top of the tree"
794 with description "You cling precariously to the trunk.",
798 Object branch "wide firm bough" top_of_tree
799 with description "It's flat enough to support a small object.",
800 name 'wide' 'firm' 'flat' 'bough' 'branch',
801 has static supporter;
803 !============================================================================
805 Once again, make the changes, recompile, and investigate what you can do in
811 Our first pass at the game is nearly done; just two more changes to
812 describe. The first is easy: Heidi wouldn't be able to climb the tree
813 carrying the bird and the nest separately: we want the player character to
814 put the bird into the nest first. One easy way to enforce this is by
815 adding a line near the top of the file:
817 .. code-block:: inform6
819 !============================================================================
820 Constant Story "Heidi";
822 "^A simple Inform example
823 ^by Roger Firth and Sonja Kesserich.^";
825 Constant MAX_CARRIED 1;
827 The value of ``MAX_CARRIED`` limits the number of objects that the player
828 character can be holding at any one time; by setting it to 1, we're saying
829 that she can carry the bird or the nest, but not both. However, the limit
830 ignores the contents of ``container`` or ``supporter`` objects, so the nest
831 with the bird inside it is still counted as one object.
833 The other change is slightly more complex and more important: there's
834 currently no way to "win" the game! The goal is for the player character
835 to put the bird in the nest, take the nest to the top of the tree, and
836 place it on the branch; when that happens, the game should be over. This
837 is one way of making it happen:
839 .. code-block:: inform6
841 Object branch "wide firm bough" top_of_tree
842 with description "It's flat enough to support a small object.",
843 name 'wide' 'firm' 'flat' 'bough' 'branch',
844 each_turn [; if (nest in branch) deadflag = 2; ],
845 has static supporter;
849 Here's an explanation of what's going on. If you find this difficult to
850 grasp, don't worry. It's the hardest bit so far, and it introduces
851 several new concepts all at once. Later in the guide, we'll explain
852 those concepts more clearly, so you can just skip this bit if you want.
854 The variable ``deadflag``, part of the library, is normally 0. If you set
855 its value to 2, the interpreter notices and ends the game with "You have
856 won". The statement::
858 if (nest in branch) deadflag = 2;
860 should be read as: "Test whether the ``nest`` is currently in the
861 ``branch`` (if the branch is a ``container``) or on it (if the
862 ``branch`` is a supporter); if it is, set the value of ``deadflag`` to
863 2; if it isn't, do nothing." The surrounding part::
867 should be read as: "At the end of each turn (when the player is in the
868 same room as the branch), do whatever is written inside the square
869 brackets". So, putting that all together:
871 * At the end of each turn (after the player has typed something and
872 pressed the Enter key, and the interpreter has done whatever was
873 requested) the interpreter checks whether the player and the
874 ``branch`` are in the same room. If not, nothing happens. If they're
875 together, it looks to see where the nest is. Initially it's in the
876 ``clearing``, so nothing happens.
878 * Also at the end of each turn, the interpreter checks the value of
879 ``deadflag``. Usually it's 0, so nothing happens.
881 * Finally the player character puts the ``nest`` on the ``branch``.
882 "Aha!" says the interpreter (to itself, of course), and sets the
883 value of ``deadflag`` to 2.
885 * Immediately afterwards, (another part of) the interpreter checks and
886 finds that the value of ``deadflag`` has changed to 2, which means
887 that the game is successfully completed; so, it says to the player,
890 That's as far as we'll take this example for now. Make those final
891 changes, recompile, and test what you've achieved. You'll probably find a
892 few things that could be done better -- even on a simple game like this
893 there's considerable scope for improvement -- so we'll revisit Heidi in her
894 forest shortly. First, though, we'll recap what we've learnt so far.