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 Each of the three games in this guide is created step by step; you'll get
11 most benefit (especially to begin with) if you take an active part, typing
12 in the source code on your computer. Our first game, described in this
13 chapter and the two which follow, tells this sentimental little story:
15 "Heidi lives in a tiny cottage deep in the forest. One sunny day,
16 standing before the cottage, she hears the frenzied tweeting of baby
17 bird; its nest has fallen from the tall tree in the clearing! Heidi
18 puts the bird into the nest, and then climbs the tree to place the nest
21 It's a very simple tale, but even so we'll cover quite a lot of ground
22 before we have a finished Inform game. We'll get there in stages, first
23 making a very rough approximation of the story, and then successively
24 refining the details until it's good enough for an initial attempt (there's
25 time later for more advanced stuff).
27 Creating a basic source file
28 ============================
30 The first task is to create an Inform source file template. Every game
31 that we design will start out like this. Follow these steps:
33 #. Create an ``Inform\Games\Heidi`` folder (maybe by copying ``Inform\Games\MyGame1``).
37 In this guide, we use the PC convention of placing a backslash
38 between folder names. On a Macintosh, use a regular slash:
39 ``Inform/Games/Heidi``.
41 #. In that folder, use your text editor to create this source file
44 .. code-block:: inform6
47 !============================================================================
48 Constant Story "Heidi";
50 "^A simple Inform example
51 ^by Roger Firth and Sonja Kesserich.^";
56 !============================================================================
59 !============================================================================
60 ! Entry point routines
64 !============================================================================
65 ! Standard and extended grammar
69 !============================================================================
71 Soon, we'll explain what this means. For now, just type it all in,
72 paying particular attention to those seven semicolons, and ensuring that
73 the double quotes "..." always come in pairs. The first line beginning
74 with "``!%``" is special, and we'll talk about it in a moment; the
75 remaining exclamation mark lines, on the other hand, are purely
76 decorative; they just make the file's structure a little easier to
79 Ensure the file is named ``Heidi.inf``, rather than ``Heidi.txt`` or
82 Remember that, throughout this guide, we place the "TYPE" symbol
83 alongside pieces of code that we recommend you to type into your own
84 game files as you read through the examples (which, conversely, means
85 that you don't need to type the unmarked pieces of code). You'll learn
86 Inform more quickly by trying it for yourself, rather than just taking
87 our word for how things work.
91 Again, revisit the TYPE symbol. Maybe a standard indicator above
94 #. In the same folder, use your text editor to create the compilation
95 support file ``Heidi.bat`` (on a PC)::
97 ..\..\Lib\Base\Inform Heidi
98 +include_path=.\,..\..\Lib\Base,..\..\Lib\Contrib | more
100 pause "at end of compilation"
102 or ``Heidi.command`` (on a Macintosh)::
104 cd ~/Inform/Games/Heidi/
106 ../../Lib/Base/inform30_macosx Heidi
107 +include_path=./,../../Lib/Base,../../Lib/Contrib
109 Remember that there's just one space between "``Heidi``" and
112 Type in the file from scratch, or copy and edit ``MyGame1.bat`` (or
113 ``MyGame1.command``). At this point, you should have a ``Heidi`` folder
114 containing two files: ``Heidi.inf`` and either ``Heidi.bat`` or
117 #. Compile the source file ``Heidi.inf``; refer back to "Inform on an IBM
118 PC (running Microsoft Windows)" on page 19 or "Inform on an Apple
119 Macintosh (running OS X)" on page 24 for guidance. If the compilation
120 works, a story file ``Heidi.z5`` appears in the folder. If the
121 compilation *doesn't* work, you've probably made a typing mistake; check
122 everything until you find it.
124 #. You can run the story file in your Inform interpreter; you should see
125 this (except that the Serial number will be different -- it's based on
128 .. code-block:: transcript
131 A simple Inform example
132 by Roger Firth and Sonja Kesserich.
133 Release 1 / Serial number 040804 / Inform v6.30 Library 6/11 SD
136 It is pitch dark, and you can't see a thing.
140 When you get that far, your template source file is correct. Let's explain
143 Understanding the source file
144 =============================
146 Although we've got a certain amount of freedom of expression, source files
147 tend to conform to a standard overall structure: these lines at the start,
148 that material next, those pieces coming at the end, and so on. What we're
149 doing here is mapping out a structure that suits us, giving ourselves a
150 clear framework onto which the elements of the game can be fitted. Having
151 a clear (albeit sparse) map at the start will help us to keep things
152 organised as the game evolves. We can infer several Inform rules just by
153 looking at the source file.
155 * If the *very first line* (or lines) of the source file begin with the
156 characters "``!%``", then the compiler treats what follows on those lines
157 as control instructions to itself rather than as part of the game's
158 source. The instructions most commonly placed here are compiler
159 switches, a way of controlling detailed aspects of how the compiler
160 operates. These particular switches, two of many, are turning on
161 **Strict mode**, which makes the game less likely to misbehave when being
162 played, and **Debug mode**, which provides some extra commands which can
163 be helpful when tracking down problems.
167 Actually, the ``-S`` is redundant, since Strict mode is already on by
168 default. We include it here as a reminder that (a) to turn Strict
169 mode *off*, you change this setting to ``-~S``, and (b) alphabetic
170 case matters here: ``-s`` causes a display of compiler statistics (and
171 ``-~s`` does nothing at all).
173 * Otherwise, when the compiler comes across an exclamation mark, it ignores
174 the rest of the line. If the ``!`` is at the start of a line, the whole
175 line is ignored; if the ``!`` is halfway along a line, the compiler takes
176 note of the first half, and then ignores the exclamation mark and
177 everything after it on that line. We call material following an
178 exclamation mark, not seen by anybody else, a **comment**; it's often a
179 remark that we write to remind ourselves of how something works or why we
180 tackled a problem in a particular way. There's nothing special about
181 those equals signs: they just produce clear lines across the page.
183 It's always a good idea to comment code as you write it, for later it
184 will help you to understand what was going on at a particular spot.
185 Although it all seems clear in your head when you first write it, in a
186 few months you may suspect that a totally alien mind must have produced
187 that senseless gibberish.
189 By the way, the compiler *doesn't* give special treatment to exclamation
190 marks in quoted text: ``!`` within quotes "..." is treated as a normal
191 character. On this line, the first ``!`` is part of the sequence (or
192 string) of characters to be displayed:
194 .. code-block:: inform6
196 print "Hello world!"; ! <- is the start of this comment
198 * The compiler ignores blank lines, and treats lots of space like a single
199 space (except when the spaces are part of a character string). So, these
200 two rules tell us that we *could* have typed the source file like this:
202 .. code-block:: inform6
204 Constant Story "Heidi";
206 "^A simple Inform example^by Roger Firth and Sonja Kesserich.^";
207 Include "Parser";Include "VerbLib";
211 We didn't type it that way because, though shorter, it's much harder to
212 read. When designing a game, you'll spend a lot of time studying what
213 you've typed, so it's worthwhile taking a bit of care to make it as
214 readable as possible.
216 * Every game should have the **constant** definitions for ``Story`` (the
217 game's name) and ``Headline`` (typically, information on the game's
218 theme, copyright, authorship and so on). These two **string** values,
219 along with a release number and date, and details of the compiler,
220 compose the **banner** which is displayed at the start of each game.
222 * Every game needs the three lines which ``Include`` the standard library
223 files -- that is, they merge those files' contents into your source file:
225 .. code-block:: inform6
232 They always have to be in this order, with ``Parser`` and ``VerbLib``
233 near the start of the file, and ``Grammar`` near the end.
235 * Every game needs to define an ``Initialise`` routine (note the British
238 .. code-block:: inform6
242 The **routine** that we've defined here doesn't do anything useful, but
243 it still needs to be present. Later, we'll come back to ``Initialise``
244 and explain what a routine is and why we need this one.
246 * You'll notice that each of the items mentioned in the previous three
247 rules ends with a semicolon. Inform is very fussy about its punctuation,
248 and gets really upset if you forget a terminating semicolon. In fact,
249 the compiler just keeps reading your source file until it finds one;
250 that's why we were able to take three lines to define the ``Headline``
253 .. code-block:: inform6
256 "^A simple Inform example
257 ^by Roger Firth and Sonja Kesserich.^";
259 Just to repeat what we said earlier: every game that you design will start
260 out from a basic source file like this (in fact, it might be sensible to
261 keep a copy of this template file in a safe place, as a starting point for
262 future games). Think of this stuff as the basic preparation which you'll
263 quickly come to take for granted, much as a landscape artist always begins
264 by sizing the canvas before starting to paint. So, now that we've taken a
265 quick tour of Inform's general needs, we can start thinking about what this
266 particular game requires.
268 Defining the game's locations
269 =============================
271 A good starting point in any game is to think about the locations which are
272 involved: this sketch map shows the four that we'll use:
274 .. image:: /images/heidi1.*
277 In IF, we talk about each of these locations as a **room**, even though in
278 this example none of them has four walls. So let's use Inform to define
279 those rooms. Here's a first attempt:
281 .. code-block:: inform6
283 Object "In front of a cottage"
285 "You stand outside a cottage. The forest stretches east.",
288 Object "Deep in the forest"
290 "Through the dense foliage, you glimpse a building to the west.
291 A track heads to the northeast.",
294 Object "A forest clearing"
296 "A tall sycamore stands in the middle of this clearing.
297 The path winds southwest through the trees.",
300 Object "At the top of the tree"
301 with description "You cling precariously to the trunk.",
304 Again, we can infer some general principles from these four examples:
306 * A room definition starts with the word ``Object`` and ends, about four
307 lines later, with a semicolon. Each of the components that appears in
308 your game -- not only the rooms, but also the people, the things that you
309 see and touch, intangibles like a sound, a smell, a gust of wind -- is
310 defined in this way; think of an "object" simply as the general term for
311 the myriad thingies which together comprise the model world which your
314 * The phrase in double quotes following the word ``Object`` is the name
315 that the interpreter uses to provide the player character with a list of
316 the objects around her: where she is, what she can see, what she's
321 We're using the word "player" to mean both the person who is playing
322 the game, and the principal protagonist (often known as the player
323 character) within the game itself. Since the latter -- Heidi -- is
324 female, we'll refer to the player as "she" while discussing this game.
326 * A keyword ``with`` follows, which simply tells the compiler what to
329 * The word ``description``, introducing another piece of text which gives
330 more detail about the object: in the case of a room, it's the appearance
331 of the surrounding environment when the player character is in that room.
332 The textual description is given in double quotes, and is followed by a
335 * Near the end, the keyword ``has`` appears, which again tells the compiler
336 to expect a certain kind of information.
338 * The word ``light`` says that this object is a source of illumination, and
339 that therefore the player character can see what's happening here. There
340 has to be at least one light source in every room (unless you want the
341 player to be told that "It's pitch dark and you can't see a thing"); most
342 commonly, that light source is the room itself.
344 A smidgeon of background may help set this into context (there's more in
345 the next chapter). An object can have both **properties** (introduced by
346 the keyword ``with``) and **attributes** (written after the word ``has``).
347 A property has both a name (like ``description``) and a value (like the
348 character string "``You stand outside a cottage. The forest stretches
349 east.``"); an attribute has merely a name.
351 In a little while, when you play this game, you'll observe that it starts
354 .. code-block:: transcript
356 In front of a cottage
357 You stand outside a cottage. The forest stretches east.
359 And here you can see how the room's name (``In front of a cottage``) and
360 description (``You stand outside a cottage. The forest stretches east.``)
366 We said that this was a first attempt at defining the rooms; it's fine as
367 far as it goes, but a few bits of information are missing. If you look at
368 the game's sketch map, you can see how the rooms are intended to be
369 connected; from "Deep in the forest", for example, the player character
370 should be able to move west towards the cottage, or northeast to the
371 clearing. Now, although our descriptions mention or imply these available
372 routes, we also need to explicitly add them to the room definitions in a
373 form that the game itself can make sense of. Like this:
375 .. code-block:: inform6
377 Object before_cottage "In front of a cottage"
379 "You stand outside a cottage. The forest stretches east.",
383 Object forest "Deep in the forest"
385 "Through the dense foliage, you glimpse a building to the west.
386 A track heads to the northeast.",
391 Object clearing "A forest clearing"
393 "A tall sycamore stands in the middle of this clearing.
394 The path winds southwest through the trees.",
399 Object top_of_tree "At the top of the tree"
400 with description "You cling precariously to the trunk.",
404 We've made two changes to the room objects.
406 * First, between the word ``Object`` and the object's name in double
407 quotes, we've inserted a different type of name: a private, internal
408 identification, never seen by the player; one that we can use *within*
409 the source file when one object needs to refer to another object. For
410 example, the first room is identified as ``before_cottage``, and the
411 second as ``forest``.
413 Unlike the external name contained in double quotes, the internal
414 identifier has to be a single word -- that is, without spaces. To aid
415 readability, we often use an underscore character to act as sort of
416 pseudo-space: ``before_cottage`` is a bit clearer than ``beforecottage``.
418 * Second, we've added lines after the object descriptions which use those
419 internal identifiers to show how the rooms are connected; one line for
420 each connection. The ``before_cottage`` object has this additional
425 This means that a player standing in front of the cottage can type GO
426 EAST (or EAST, or just E), and the game will transport her to the room
427 whose internal identification is ``forest``. If she tries to move in any
428 other direction from this room, she'll be told "You can't go that way".
430 What we've just defined is a *one-way* easterly connection:
431 ``before_cottage`` → ``forest``. The forest object has two additional
437 The first line defines a westerly connection ``forest`` →
438 ``before_cottage`` (thus enabling the player character to return to the
439 cottage), and the second defines a connection ``forest`` → ``clearing``
440 which heads off to the northeast.
442 Inform provides for eight "horizontal" connections (``n_to``, ``ne_to``,
443 ``e_to``, ``se_to``, ``s_to``, ``sw_to``, ``w_to``, ``nw_to``) two
444 "vertical" ones (``u_to``, ``d_to``) and two specials ``in_to``, and
445 ``out_to``. You'll see some of these used for the remaining inter-room
448 There's one last detail to attend to before we can test what we've done.
449 You'll recollect that our story begins with Heidi standing in front of her
450 cottage. We need to tell the interpreter that ``before_cottage`` is the room
451 where the game starts, and we do this in the ``Initialise`` routine::
453 [ Initialise; location = before_cottage; ];
455 ``location`` is a **variable**, part of the library, which tells the
456 interpreter in which room the player character currently is. Here, we're
457 saying that, at the start of the game, the player character is in the
458 ``before_cottage`` room.
460 Now we can add what we've done to the ``Heidi.inf`` source file template.
461 At this stage, you should study the four room definitions, comparing them
462 with the sketch map until you're comfortable that you understand how to
463 create simple rooms and define the connections between them.
465 .. code-block:: inform6
467 !============================================================================
468 Constant Story "Heidi";
470 "^A simple Inform example
471 ^by Roger Firth and Sonja Kesserich.^";
476 !============================================================================
479 Object before_cottage "In front of a cottage"
481 "You stand outside a cottage. The forest stretches east.",
485 Object forest "Deep in the forest"
487 "Through the dense foliage, you glimpse a building to the west.
488 A track heads to the northeast.",
493 Object clearing "A forest clearing"
495 "A tall sycamore stands in the middle of this clearing.
496 The path winds southwest through the trees.",
501 Object top_of_tree "At the top of the tree"
502 with description "You cling precariously to the trunk.",
506 !============================================================================
507 ! Entry point routines
509 [ Initialise; location = before_cottage; ];
511 !============================================================================
512 ! Standard and extended grammar
516 !============================================================================
518 Type this in, as always taking great care with the punctuation -- watch
519 those commas and semicolons. Compile it, and fix any mistakes which the
520 compiler reports. You can then play the game in its current state.
521 Admittedly, you can't do very much, but you should be able to move freely
522 among the four rooms that you've defined.
526 In order to minimise the amount of typing that you have to do, the
527 descriptive text in this game has been kept as short as possible. In a
528 real game, you would typically provide more interesting descriptions
531 Adding the bird and the nest
532 ============================
534 Given what we said earlier, you won't be surprised to hear that both the
535 bird and its nest are Inform objects. We'll start their definitions like
538 .. code-block:: inform6
540 Object bird "baby bird"
541 with description "Too young to fly, the nestling tweets helplessly.",
544 Object nest "bird's nest"
545 with description "The nest is carefully woven of twigs and moss.",
548 You can see that these definitions have exactly the same format as the
549 rooms we defined previously: a one-word internal identifier (``bird``,
550 ``nest``), and a word or phrase naming the object for the player's benefit
551 (``baby bird``, ``bird's nest``). They both have some descriptive detail:
552 for a room this is printed when the player first enters, or when she types
553 LOOK; for other objects it's printed when she EXAMINEs that object. What
554 they *don't* have are connections (``e_to``, ``w_to``, etc. apply only to
555 rooms) or ``light`` (it's not necessary -- the rooms ensure that light is
558 When the game is running, the player will want to refer to these two
559 objects, saying for instance EXAMINE THE BABY BIRD or PICK UP THE NEST.
560 For this to work reliably, we need to specify the word (or words) which
561 relate to each object. Our aim here is flexibility: providing a choice of
562 relevant vocabulary so that the player can use whatever term seems
563 appropriate to her, with a good chance of it being understood. We add a
564 line to each definition:
566 .. code-block:: inform6
568 Object bird "baby bird"
569 with description "Too young to fly, the nestling tweets helplessly.",
570 name 'baby' 'bird' 'nestling',
573 Object nest "bird's nest"
574 with description "The nest is carefully woven of twigs and moss.",
575 name 'bird^s' 'nest' 'twigs' 'moss',
578 The ``name`` introduces a list in single quotes '...'. We call each of
579 those quoted things a **dictionary word**, and we do mean "word", not
580 "phrase" (``'baby'``\ ``'bird'`` rather than ``'baby bird'``); you can't
581 uses spaces, commas or periods in dictionary words, though there's a space
582 *between* each one, and the whole list ends with a comma. The idea is that
583 the interpreter decides which object a player is talking about by matching
584 what she types against the full set of all dictionary words. If the player
585 mentions BIRD, or BABY BIRD, or NESTLING, it's the ``baby bird`` that she
586 means; if she mentions NEST, BIRD'S NEST or MOSS, it's the ``bird's nest``.
587 And if she types NEST BABY or BIRD TWIGS, the interpreter will politely say
588 that it doesn't understand what on earth she's talking about.
592 You'll notice the use of ``'bird^s'`` to define the dictionary word
593 BIRD'S; this oddity is necessary because the compiler expects the single
594 quotes in the list always to come in pairs -- one at the start of the
595 dictionary word, and one at the end. If we had typed ``'bird's'`` then
596 the compiler would find the opening quote, the four letters ``b``,
597 ``i``, ``r`` and ``d``, and what looks like the closing quote. So far
598 so good; it's read the word BIRD and now expects a space before the next
599 opening quote... but instead finds ``s'`` which makes no sense. In
600 cases like this we must use the circumflex ``^`` to *represent* the
601 apostrophe, and the compiler then treats ``bird's`` as a dictionary
604 You may be wondering why we need a list of ``name`` words for the bird and
605 its nest, yet we didn't when we defined the rooms? It's because the player
606 can't interact with a room in the same way as with other objects; for
607 example, she doesn't need to say EXAMINE THE FOREST -- just being there and
608 typing LOOK is sufficient.
610 The bird's definition is complete, but there's an additional complexity
611 with the nest: we need to be able to put the bird into it. We do this by
612 labelling the nest as a ``container`` -- able to hold other objects -- so
613 that the player can type PUT (or INSERT) BIRD IN (or INTO) NEST.
614 Furthermore, we label it as ``open``; this prevents the interpreter from
615 asking us to open it before putting in the bird.
617 .. code-block:: inform6
619 Object nest "bird's nest"
620 with description "The nest is carefully woven of twigs and moss.",
621 name 'bird^s' 'nest' 'twigs' 'moss',
624 Both objects are now defined, and we can incorporate them into the game.
625 To do this, we need to choose the locations where the player will find
626 them. Let's say that the bird is found in the forest, while the nest is in
627 the clearing. This is how we set this up:
629 .. code-block:: inform6
631 Object bird "baby bird" forest
632 with description "Too young to fly, the nestling tweets helplessly.",
633 name 'baby' 'bird' 'nestling',
636 Object nest "bird's nest" clearing
637 with description "The nest is carefully woven of twigs and moss.",
638 name 'bird^s' 'nest' 'twigs' 'moss',
641 Read that first line as: "Here's the definition of an object which is
642 identified within this file as ``bird``, which is known to the player as
643 ``baby bird``, and which is initially located inside the object identified
644 within this file as ``forest``."
646 Where in the source file do these new objects fit? Well, anywhere really,
647 but you'll find it convenient to insert them following the rooms where
648 they're found. This means adding the bird just after the forest, and the
649 nest just after the clearing. Here's the middle piece of the source file:
651 .. code-block:: inform6
653 !============================================================================
656 Object before_cottage "In front of a cottage"
658 "You stand outside a cottage. The forest stretches east.",
662 Object forest "Deep in the forest"
664 "Through the dense foliage, you glimpse a building to the west.
665 A track heads to the northeast.",
670 Object bird "baby bird" forest
671 with description "Too young to fly, the nestling tweets helplessly.",
672 name 'baby' 'bird' 'nestling',
675 Object clearing "A forest clearing"
677 "A tall sycamore stands in the middle of this clearing.
678 The path winds southwest through the trees.",
683 Object nest "bird's nest" clearing
684 with description "The nest is carefully woven of twigs and moss.",
685 name 'bird^s' 'nest' 'twigs' 'moss',
688 Object top_of_tree "At the top of the tree"
689 with description "You cling precariously to the trunk.",
693 !============================================================================
695 Make those changes, recompile the game, play it and you'll see this:
697 .. code-block:: transcript
700 Through the dense foliage, you glimpse a building to the west. A track heads
703 You can see a baby bird here.
707 Adding the tree and the branch
708 ==============================
710 The description of the clearing mentions a tall sycamore tree, up which the
711 player character supposedly "climbs". We'd better define it:
713 .. code-block:: inform6
715 Object tree "tall sycamore tree" clearing
717 "Standing proud in the middle of the clearing,
718 the stout tree looks easy to climb.",
719 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
722 Everything there should be familiar, apart from that ``scenery`` at the
723 end. We've already mentioned the tree in the description of the forest
724 clearing, so we don't want the interpreter adding "You can see a tall
725 sycamore tree here" afterwards, as it does for the bird and the nest. By
726 labelling the tree as ``scenery`` we suppress that, and also prevent it
727 from being picked up by the player character. One final object: the branch
728 at the top of the tree. Again, not many surprises in this definition:
730 .. code-block:: inform6
732 Object branch "wide firm bough" top_of_tree
733 with description "It's flat enough to support a small object.",
734 name 'wide' 'firm' 'flat' 'bough' 'branch',
735 has static supporter;
737 The only new things are those two labels. ``static`` is similar to
738 ``scenery``: it prevents the branch from being picked up by the player
739 character, but *doesn't* suppress mention of it when describing the
740 setting. And ``supporter`` is rather like the ``container`` that we used
741 for the nest, except that this time the player character can put other
742 objects *onto* the branch. (In passing, we'll mention that an object can't
743 normally be both a ``container`` and a ``supporter``.) And so here are our
746 .. code-block:: inform6
748 !============================================================================
751 Object before_cottage "In front of a cottage"
753 "You stand outside a cottage. The forest stretches east.",
757 Object forest "Deep in the forest"
759 "Through the dense foliage, you glimpse a building to the west.
760 A track heads to the northeast.",
765 Object bird "baby bird" forest
766 with description "Too young to fly, the nestling tweets helplessly.",
767 name 'baby' 'bird' 'nestling',
770 Object clearing "A forest clearing"
772 "A tall sycamore stands in the middle of this clearing.
773 The path winds southwest through the trees.",
778 Object nest "bird's nest" clearing
779 with description "The nest is carefully woven of twigs and moss.",
780 name 'bird^s' 'nest' 'twigs' 'moss',
783 Object tree "tall sycamore tree" clearing
785 "Standing proud in the middle of the clearing,
786 the stout tree looks easy to climb.",
787 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
790 Object top_of_tree "At the top of the tree"
791 with description "You cling precariously to the trunk.",
795 Object branch "wide firm bough" top_of_tree
796 with description "It's flat enough to support a small object.",
797 name 'wide' 'firm' 'flat' 'bough' 'branch',
798 has static supporter;
800 !============================================================================
802 Once again, make the changes, recompile, and investigate what you can do in
808 Our first pass at the game is nearly done; just two more changes to
809 describe. The first is easy: Heidi wouldn't be able to climb the tree
810 carrying the bird and the nest separately: we want the player character to
811 put the bird into the nest first. One easy way to enforce this is by
812 adding a line near the top of the file:
814 .. code-block:: inform6
816 !============================================================================
817 Constant Story "Heidi";
819 "^A simple Inform example
820 ^by Roger Firth and Sonja Kesserich.^";
822 Constant MAX_CARRIED 1;
824 The value of ``MAX_CARRIED`` limits the number of objects that the player
825 character can be holding at any one time; by setting it to 1, we're saying
826 that she can carry the bird or the nest, but not both. However, the limit
827 ignores the contents of ``container`` or ``supporter`` objects, so the nest
828 with the bird inside it is still counted as one object.
830 The other change is slightly more complex and more important: there's
831 currently no way to "win" the game! The goal is for the player character
832 to put the bird in the nest, take the nest to the top of the tree, and
833 place it on the branch; when that happens, the game should be over. This
834 is one way of making it happen:
836 .. code-block:: inform6
838 Object branch "wide firm bough" top_of_tree
839 with description "It's flat enough to support a small object.",
840 name 'wide' 'firm' 'flat' 'bough' 'branch',
841 each_turn [; if (nest in branch) deadflag = 2; ],
842 has static supporter;
846 Here's an explanation of what's going on. If you find this difficult to
847 grasp, don't worry. It's the hardest bit so far, and it introduces
848 several new concepts all at once. Later in the guide, we'll explain
849 those concepts more clearly, so you can just skip this bit if you want.
851 The variable ``deadflag``, part of the library, is normally 0. If you set
852 its value to 2, the interpreter notices and ends the game with "You have
853 won". The statement::
855 if (nest in branch) deadflag = 2;
857 should be read as: "Test whether the ``nest`` is currently in the
858 ``branch`` (if the branch is a ``container``) or on it (if the
859 ``branch`` is a supporter); if it is, set the value of ``deadflag`` to
860 2; if it isn't, do nothing." The surrounding part::
864 should be read as: "At the end of each turn (when the player is in the
865 same room as the branch), do whatever is written inside the square
866 brackets". So, putting that all together:
868 * At the end of each turn (after the player has typed something and
869 pressed the Enter key, and the interpreter has done whatever was
870 requested) the interpreter checks whether the player and the
871 ``branch`` are in the same room. If not, nothing happens. If they're
872 together, it looks to see where the nest is. Initially it's in the
873 ``clearing``, so nothing happens.
875 * Also at the end of each turn, the interpreter checks the value of
876 ``deadflag``. Usually it's 0, so nothing happens.
878 * Finally the player character puts the ``nest`` on the ``branch``.
879 "Aha!" says the interpreter (to itself, of course), and sets the
880 value of ``deadflag`` to 2.
882 * Immediately afterwards, (another part of) the interpreter checks and
883 finds that the value of ``deadflag`` has changed to 2, which means
884 that the game is successfully completed; so, it says to the player,
887 That's as far as we'll take this example for now. Make those final
888 changes, recompile, and test what you've achieved. You'll probably find a
889 few things that could be done better -- even on a simple game like this
890 there's considerable scope for improvement -- so we'll revisit Heidi in her
891 forest shortly. First, though, we'll recap what we've learnt so far.