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:: inform6
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 learn
95 Inform more quickly by trying it for yourself, rather than just taking
96 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 **Strict mode**, which makes the game less likely to misbehave when being
171 played, and **Debug mode**, which provides some extra commands which can
172 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 **comment**; it's often a
188 remark that we write to remind ourselves of how something works or why we
189 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 string) of characters to be displayed:
203 .. code-block:: inform6
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:: inform6
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 **constant** definitions for ``Story`` (the
226 game's name) and ``Headline`` (typically, information on the game's
227 theme, copyright, authorship and so on). These two **string** values,
228 along with a release number and date, and details of the compiler,
229 compose the **banner** which is displayed at the start of each game.
231 * Every game needs the three lines which ``Include`` the standard library
232 files -- that is, they merge those files' contents into your source file:
234 .. code-block:: inform6
241 They always have to be in this order, with ``Parser`` and ``VerbLib``
242 near the start of the file, and ``Grammar`` near the end.
244 * Every game needs to define an ``Initialise`` routine (note the British
247 .. code-block:: inform6
251 The **routine** that we've defined here doesn't do anything useful, but
252 it still needs to be present. Later, we'll come back to ``Initialise``
253 and explain what a routine is and why we need this one.
255 * You'll notice that each of the items mentioned in the previous three
256 rules ends with a semicolon. Inform is very fussy about its punctuation,
257 and gets really upset if you forget a terminating semicolon. In fact,
258 the compiler just keeps reading your source file until it finds one;
259 that's why we were able to take three lines to define the ``Headline``
262 .. code-block:: inform6
265 "^A simple Inform example
266 ^by Roger Firth and Sonja Kesserich.^";
268 Just to repeat what we said earlier: every game that you design will start
269 out from a basic source file like this (in fact, it might be sensible to
270 keep a copy of this template file in a safe place, as a starting point for
271 future games). Think of this stuff as the basic preparation which you'll
272 quickly come to take for granted, much as a landscape artist always begins
273 by sizing the canvas before starting to paint. So, now that we've taken a
274 quick tour of Inform's general needs, we can start thinking about what this
275 particular game requires.
277 Defining the game's locations
278 =============================
280 A good starting point in any game is to think about the locations which are
281 involved: this sketch map shows the four that we'll use:
283 .. image:: /images/heidi1.*
286 In IF, we talk about each of these locations as a **room**, even though in
287 this example none of them has four walls. So let's use Inform to define
288 those rooms. Here's a first attempt:
290 .. code-block:: inform6
292 Object "In front of a cottage"
294 "You stand outside a cottage. The forest stretches east.",
297 Object "Deep in the forest"
299 "Through the dense foliage, you glimpse a building to the west.
300 A track heads to the northeast.",
303 Object "A forest clearing"
305 "A tall sycamore stands in the middle of this clearing.
306 The path winds southwest through the trees.",
309 Object "At the top of the tree"
310 with description "You cling precariously to the trunk.",
313 Again, we can infer some general principles from these four examples:
315 * A room definition starts with the word ``Object`` and ends, about four
316 lines later, with a semicolon. Each of the components that appears in
317 your game -- not only the rooms, but also the people, the things that you
318 see and touch, intangibles like a sound, a smell, a gust of wind -- is
319 defined in this way; think of an "object" simply as the general term for
320 the myriad thingies which together comprise the model world which your
323 * The phrase in double quotes following the word ``Object`` is the name
324 that the interpreter uses to provide the player character with a list of
325 the objects around her: where she is, what she can see, what she's
330 We're using the word "player" to mean both the person who is playing
331 the game, and the principal protagonist (often known as the player
332 character) within the game itself. Since the latter -- Heidi -- is
333 female, we'll refer to the player as "she" while discussing this game.
335 * A keyword ``with`` follows, which simply tells the compiler what to
338 * The word ``description``, introducing another piece of text which gives
339 more detail about the object: in the case of a room, it's the appearance
340 of the surrounding environment when the player character is in that room.
341 The textual description is given in double quotes, and is followed by a
344 * Near the end, the keyword ``has`` appears, which again tells the compiler
345 to expect a certain kind of information.
347 * The word ``light`` says that this object is a source of illumination, and
348 that therefore the player character can see what's happening here. There
349 has to be at least one light source in every room (unless you want the
350 player to be told that "It's pitch dark and you can't see a thing"); most
351 commonly, that light source is the room itself.
353 A smidgeon of background may help set this into context (there's more in
354 the next chapter). An object can have both **properties** (introduced by
355 the keyword ``with``) and **attributes** (written after the word ``has``).
356 A property has both a name (like ``description``) and a value (like the
357 character string "``You stand outside a cottage. The forest stretches
358 east.``"); an attribute has merely a name.
360 In a little while, when you play this game, you'll observe that it starts
363 .. code-block:: transcript
365 In front of a cottage
366 You stand outside a cottage. The forest stretches east.
368 And here you can see how the room's name (``In front of a cottage``) and
369 description (``You stand outside a cottage. The forest stretches east.``)
375 We said that this was a first attempt at defining the rooms; it's fine as
376 far as it goes, but a few bits of information are missing. If you look at
377 the game's sketch map, you can see how the rooms are intended to be
378 connected; from "Deep in the forest", for example, the player character
379 should be able to move west towards the cottage, or northeast to the
380 clearing. Now, although our descriptions mention or imply these available
381 routes, we also need to explicitly add them to the room definitions in a
382 form that the game itself can make sense of. Like this:
384 .. code-block:: inform6
386 Object before_cottage "In front of a cottage"
388 "You stand outside a cottage. The forest stretches east.",
392 Object forest "Deep in the forest"
394 "Through the dense foliage, you glimpse a building to the west.
395 A track heads to the northeast.",
400 Object clearing "A forest clearing"
402 "A tall sycamore stands in the middle of this clearing.
403 The path winds southwest through the trees.",
408 Object top_of_tree "At the top of the tree"
409 with description "You cling precariously to the trunk.",
413 We've made two changes to the room objects.
415 * First, between the word ``Object`` and the object's name in double
416 quotes, we've inserted a different type of name: a private, internal
417 identification, never seen by the player; one that we can use *within*
418 the source file when one object needs to refer to another object. For
419 example, the first room is identified as ``before_cottage``, and the
420 second as ``forest``.
422 Unlike the external name contained in double quotes, the internal
423 identifier has to be a single word -- that is, without spaces. To aid
424 readability, we often use an underscore character to act as sort of
425 pseudo-space: ``before_cottage`` is a bit clearer than ``beforecottage``.
427 * Second, we've added lines after the object descriptions which use those
428 internal identifiers to show how the rooms are connected; one line for
429 each connection. The ``before_cottage`` object has this additional
434 This means that a player standing in front of the cottage can type GO
435 EAST (or EAST, or just E), and the game will transport her to the room
436 whose internal identification is ``forest``. If she tries to move in any
437 other direction from this room, she'll be told "You can't go that way".
439 What we've just defined is a *one-way* easterly connection:
440 ``before_cottage`` → ``forest``. The forest object has two additional
446 The first line defines a westerly connection ``forest`` →
447 ``before_cottage`` (thus enabling the player character to return to the
448 cottage), and the second defines a connection ``forest`` → ``clearing``
449 which heads off to the northeast.
451 Inform provides for eight "horizontal" connections (``n_to``, ``ne_to``,
452 ``e_to``, ``se_to``, ``s_to``, ``sw_to``, ``w_to``, ``nw_to``) two
453 "vertical" ones (``u_to``, ``d_to``) and two specials ``in_to``, and
454 ``out_to``. You'll see some of these used for the remaining inter-room
457 There's one last detail to attend to before we can test what we've done.
458 You'll recollect that our story begins with Heidi standing in front of her
459 cottage. We need to tell the interpreter that ``before_cottage`` is the room
460 where the game starts, and we do this in the ``Initialise`` routine::
462 [ Initialise; location = before_cottage; ];
464 ``location`` is a **variable**, part of the library, which tells the
465 interpreter in which room the player character currently is. Here, we're
466 saying that, at the start of the game, the player character is in the
467 ``before_cottage`` room.
469 Now we can add what we've done to the ``Heidi.inf`` source file template.
470 At this stage, you should study the four room definitions, comparing them
471 with the sketch map until you're comfortable that you understand how to
472 create simple rooms and define the connections between them.
474 .. code-block:: inform6
476 !============================================================================
477 Constant Story "Heidi";
479 "^A simple Inform example
480 ^by Roger Firth and Sonja Kesserich.^";
485 !============================================================================
488 Object before_cottage "In front of a cottage"
490 "You stand outside a cottage. The forest stretches east.",
494 Object forest "Deep in the forest"
496 "Through the dense foliage, you glimpse a building to the west.
497 A track heads to the northeast.",
502 Object clearing "A forest clearing"
504 "A tall sycamore stands in the middle of this clearing.
505 The path winds southwest through the trees.",
510 Object top_of_tree "At the top of the tree"
511 with description "You cling precariously to the trunk.",
515 !============================================================================
516 ! Entry point routines
518 [ Initialise; location = before_cottage; ];
520 !============================================================================
521 ! Standard and extended grammar
525 !============================================================================
527 Type this in, as always taking great care with the punctuation -- watch
528 those commas and semicolons. Compile it, and fix any mistakes which the
529 compiler reports. You can then play the game in its current state.
530 Admittedly, you can't do very much, but you should be able to move freely
531 among the four rooms that you've defined.
535 In order to minimise the amount of typing that you have to do, the
536 descriptive text in this game has been kept as short as possible. In a
537 real game, you would typically provide more interesting descriptions
540 Adding the bird and the nest
541 ============================
543 Given what we said earlier, you won't be surprised to hear that both the
544 bird and its nest are Inform objects. We'll start their definitions like
547 .. code-block:: inform6
549 Object bird "baby bird"
550 with description "Too young to fly, the nestling tweets helplessly.",
553 Object nest "bird's nest"
554 with description "The nest is carefully woven of twigs and moss.",
557 You can see that these definitions have exactly the same format as the
558 rooms we defined previously: a one-word internal identifier (``bird``,
559 ``nest``), and a word or phrase naming the object for the player's benefit
560 (``baby bird``, ``bird's nest``). They both have some descriptive detail:
561 for a room this is printed when the player first enters, or when she types
562 LOOK; for other objects it's printed when she EXAMINEs that object. What
563 they *don't* have are connections (``e_to``, ``w_to``, etc. apply only to
564 rooms) or ``light`` (it's not necessary -- the rooms ensure that light is
567 When the game is running, the player will want to refer to these two
568 objects, saying for instance EXAMINE THE BABY BIRD or PICK UP THE NEST.
569 For this to work reliably, we need to specify the word (or words) which
570 relate to each object. Our aim here is flexibility: providing a choice of
571 relevant vocabulary so that the player can use whatever term seems
572 appropriate to her, with a good chance of it being understood. We add a
573 line to each definition:
575 .. code-block:: inform6
577 Object bird "baby bird"
578 with description "Too young to fly, the nestling tweets helplessly.",
579 name 'baby' 'bird' 'nestling',
582 Object nest "bird's nest"
583 with description "The nest is carefully woven of twigs and moss.",
584 name 'bird^s' 'nest' 'twigs' 'moss',
587 The ``name`` introduces a list in single quotes '...'. We call each of
588 those quoted things a **dictionary word**, and we do mean "word", not
589 "phrase" (``'baby'``\ ``'bird'`` rather than ``'baby bird'``); you can't
590 uses spaces, commas or periods in dictionary words, though there's a space
591 *between* each one, and the whole list ends with a comma. The idea is that
592 the interpreter decides which object a player is talking about by matching
593 what she types against the full set of all dictionary words. If the player
594 mentions BIRD, or BABY BIRD, or NESTLING, it's the ``baby bird`` that she
595 means; if she mentions NEST, BIRD'S NEST or MOSS, it's the ``bird's nest``.
596 And if she types NEST BABY or BIRD TWIGS, the interpreter will politely say
597 that it doesn't understand what on earth she's talking about.
601 You'll notice the use of ``'bird^s'`` to define the dictionary word
602 BIRD'S; this oddity is necessary because the compiler expects the single
603 quotes in the list always to come in pairs -- one at the start of the
604 dictionary word, and one at the end. If we had typed ``'bird's'`` then
605 the compiler would find the opening quote, the four letters ``b``,
606 ``i``, ``r`` and ``d``, and what looks like the closing quote. So far
607 so good; it's read the word BIRD and now expects a space before the next
608 opening quote... but instead finds ``s'`` which makes no sense. In
609 cases like this we must use the circumflex ``^`` to *represent* the
610 apostrophe, and the compiler then treats ``bird's`` as a dictionary
613 You may be wondering why we need a list of ``name`` words for the bird and
614 its nest, yet we didn't when we defined the rooms? It's because the player
615 can't interact with a room in the same way as with other objects; for
616 example, she doesn't need to say EXAMINE THE FOREST -- just being there and
617 typing LOOK is sufficient.
619 The bird's definition is complete, but there's an additional complexity
620 with the nest: we need to be able to put the bird into it. We do this by
621 labelling the nest as a ``container`` -- able to hold other objects -- so
622 that the player can type PUT (or INSERT) BIRD IN (or INTO) NEST.
623 Furthermore, we label it as ``open``; this prevents the interpreter from
624 asking us to open it before putting in the bird.
626 .. code-block:: inform6
628 Object nest "bird's nest"
629 with description "The nest is carefully woven of twigs and moss.",
630 name 'bird^s' 'nest' 'twigs' 'moss',
633 Both objects are now defined, and we can incorporate them into the game.
634 To do this, we need to choose the locations where the player will find
635 them. Let's say that the bird is found in the forest, while the nest is in
636 the clearing. This is how we set this up:
638 .. code-block:: inform6
640 Object bird "baby bird" forest
641 with description "Too young to fly, the nestling tweets helplessly.",
642 name 'baby' 'bird' 'nestling',
645 Object nest "bird's nest" clearing
646 with description "The nest is carefully woven of twigs and moss.",
647 name 'bird^s' 'nest' 'twigs' 'moss',
650 Read that first line as: "Here's the definition of an object which is
651 identified within this file as ``bird``, which is known to the player as
652 ``baby bird``, and which is initially located inside the object identified
653 within this file as ``forest``."
655 Where in the source file do these new objects fit? Well, anywhere really,
656 but you'll find it convenient to insert them following the rooms where
657 they're found. This means adding the bird just after the forest, and the
658 nest just after the clearing. Here's the middle piece of the source file:
660 .. code-block:: inform6
662 !============================================================================
665 Object before_cottage "In front of a cottage"
667 "You stand outside a cottage. The forest stretches east.",
671 Object forest "Deep in the forest"
673 "Through the dense foliage, you glimpse a building to the west.
674 A track heads to the northeast.",
679 Object bird "baby bird" forest
680 with description "Too young to fly, the nestling tweets helplessly.",
681 name 'baby' 'bird' 'nestling',
684 Object clearing "A forest clearing"
686 "A tall sycamore stands in the middle of this clearing.
687 The path winds southwest through the trees.",
692 Object nest "bird's nest" clearing
693 with description "The nest is carefully woven of twigs and moss.",
694 name 'bird^s' 'nest' 'twigs' 'moss',
697 Object top_of_tree "At the top of the tree"
698 with description "You cling precariously to the trunk.",
702 !============================================================================
704 Make those changes, recompile the game, play it and you'll see this:
706 .. code-block:: transcript
709 Through the dense foliage, you glimpse a building to the west. A track heads
712 You can see a baby bird here.
716 Adding the tree and the branch
717 ==============================
719 The description of the clearing mentions a tall sycamore tree, up which the
720 player character supposedly "climbs". We'd better define it:
722 .. code-block:: inform6
724 Object tree "tall sycamore tree" clearing
726 "Standing proud in the middle of the clearing,
727 the stout tree looks easy to climb.",
728 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
731 Everything there should be familiar, apart from that ``scenery`` at the
732 end. We've already mentioned the tree in the description of the forest
733 clearing, so we don't want the interpreter adding "You can see a tall
734 sycamore tree here" afterwards, as it does for the bird and the nest. By
735 labelling the tree as ``scenery`` we suppress that, and also prevent it
736 from being picked up by the player character. One final object: the branch
737 at the top of the tree. Again, not many surprises in this definition:
739 .. code-block:: inform6
741 Object branch "wide firm bough" top_of_tree
742 with description "It's flat enough to support a small object.",
743 name 'wide' 'firm' 'flat' 'bough' 'branch',
744 has static supporter;
746 The only new things are those two labels. ``static`` is similar to
747 ``scenery``: it prevents the branch from being picked up by the player
748 character, but *doesn't* suppress mention of it when describing the
749 setting. And ``supporter`` is rather like the ``container`` that we used
750 for the nest, except that this time the player character can put other
751 objects *onto* the branch. (In passing, we'll mention that an object can't
752 normally be both a ``container`` and a ``supporter``.) And so here are our
755 .. code-block:: inform6
757 !============================================================================
760 Object before_cottage "In front of a cottage"
762 "You stand outside a cottage. The forest stretches east.",
766 Object forest "Deep in the forest"
768 "Through the dense foliage, you glimpse a building to the west.
769 A track heads to the northeast.",
774 Object bird "baby bird" forest
775 with description "Too young to fly, the nestling tweets helplessly.",
776 name 'baby' 'bird' 'nestling',
779 Object clearing "A forest clearing"
781 "A tall sycamore stands in the middle of this clearing.
782 The path winds southwest through the trees.",
787 Object nest "bird's nest" clearing
788 with description "The nest is carefully woven of twigs and moss.",
789 name 'bird^s' 'nest' 'twigs' 'moss',
792 Object tree "tall sycamore tree" clearing
794 "Standing proud in the middle of the clearing,
795 the stout tree looks easy to climb.",
796 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
799 Object top_of_tree "At the top of the tree"
800 with description "You cling precariously to the trunk.",
804 Object branch "wide firm bough" top_of_tree
805 with description "It's flat enough to support a small object.",
806 name 'wide' 'firm' 'flat' 'bough' 'branch',
807 has static supporter;
809 !============================================================================
811 Once again, make the changes, recompile, and investigate what you can do in
817 Our first pass at the game is nearly done; just two more changes to
818 describe. The first is easy: Heidi wouldn't be able to climb the tree
819 carrying the bird and the nest separately: we want the player character to
820 put the bird into the nest first. One easy way to enforce this is by
821 adding a line near the top of the file:
823 .. code-block:: inform6
825 !============================================================================
826 Constant Story "Heidi";
828 "^A simple Inform example
829 ^by Roger Firth and Sonja Kesserich.^";
831 Constant MAX_CARRIED 1;
833 The value of ``MAX_CARRIED`` limits the number of objects that the player
834 character can be holding at any one time; by setting it to 1, we're saying
835 that she can carry the bird or the nest, but not both. However, the limit
836 ignores the contents of ``container`` or ``supporter`` objects, so the nest
837 with the bird inside it is still counted as one object.
839 The other change is slightly more complex and more important: there's
840 currently no way to "win" the game! The goal is for the player character
841 to put the bird in the nest, take the nest to the top of the tree, and
842 place it on the branch; when that happens, the game should be over. This
843 is one way of making it happen:
845 .. code-block:: inform6
847 Object branch "wide firm bough" top_of_tree
848 with description "It's flat enough to support a small object.",
849 name 'wide' 'firm' 'flat' 'bough' 'branch',
850 each_turn [; if (nest in branch) deadflag = 2; ],
851 has static supporter;
855 Here's an explanation of what's going on. If you find this difficult to
856 grasp, don't worry. It's the hardest bit so far, and it introduces
857 several new concepts all at once. Later in the guide, we'll explain
858 those concepts more clearly, so you can just skip this bit if you want.
860 The variable ``deadflag``, part of the library, is normally 0. If you set
861 its value to 2, the interpreter notices and ends the game with "You have
862 won". The statement::
864 if (nest in branch) deadflag = 2;
866 should be read as: "Test whether the ``nest`` is currently in the
867 ``branch`` (if the branch is a ``container``) or on it (if the
868 ``branch`` is a supporter); if it is, set the value of ``deadflag`` to
869 2; if it isn't, do nothing." The surrounding part::
873 should be read as: "At the end of each turn (when the player is in the
874 same room as the branch), do whatever is written inside the square
875 brackets". So, putting that all together:
877 * At the end of each turn (after the player has typed something and
878 pressed the Enter key, and the interpreter has done whatever was
879 requested) the interpreter checks whether the player and the
880 ``branch`` are in the same room. If not, nothing happens. If they're
881 together, it looks to see where the nest is. Initially it's in the
882 ``clearing``, so nothing happens.
884 * Also at the end of each turn, the interpreter checks the value of
885 ``deadflag``. Usually it's 0, so nothing happens.
887 * Finally the player character puts the ``nest`` on the ``branch``.
888 "Aha!" says the interpreter (to itself, of course), and sets the
889 value of ``deadflag`` to 2.
891 * Immediately afterwards, (another part of) the interpreter checks and
892 finds that the value of ``deadflag`` has changed to 2, which means
893 that the game is successfully completed; so, it says to the player,
896 That's as far as we'll take this example for now. Make those final
897 changes, recompile, and test what you've achieved. You'll probably find a
898 few things that could be done better -- even on a simple game like this
899 there's considerable scope for improvement -- so we'll revisit Heidi in her
900 forest shortly. First, though, we'll recap what we've learnt so far.