1 ==============================
2 Heidi: our first Inform game
3 ==============================
7 | |CENTER| *E was an esquire, with pride on his brow;*
8 | |CENTER| *F was a farmer, and followed the plough.*
12 .. image:: /images/picE.png
15 |E|\ach of the three games in this guide is created step by step; you'll
16 get most benefit (especially to begin with) if you take an active part,
17 typing in the source code on your computer. Our first game, described in
18 this chapter and the two which follow, tells this sentimental little story:
20 "Heidi lives in a tiny cottage deep in the forest. One sunny day,
21 standing before the cottage, she hears the frenzied tweeting of baby
22 bird; its nest has fallen from the tall tree in the clearing! Heidi
23 puts the bird into the nest, and then climbs the tree to place the nest
26 It's a very simple tale, but even so we'll cover quite a lot of ground
27 before we have a finished Inform game. We'll get there in stages, first
28 making a very rough approximation of the story, and then successively
29 refining the details until it's good enough for an initial attempt (there's
30 time later for more advanced stuff).
32 Creating a basic source file
33 ============================
35 The first task is to create an Inform source file template. Every game
36 that we design will start out like this. Follow these steps:
38 #. Create an ``Inform\Games\Heidi`` folder (maybe by copying
39 ``Inform\Games\MyGame1``).
43 In this guide, we use the PC convention of placing a backslash
44 between folder names. On a Macintosh, use a regular slash:
45 ``Inform/Games/Heidi``.
47 #. In that folder, use your text editor to create this source file
50 .. code-block:: inform
53 !============================================================================
54 Constant Story "Heidi";
56 "^A simple Inform example
57 ^by Roger Firth and Sonja Kesserich.^";
62 !============================================================================
65 !============================================================================
66 ! Entry point routines
70 !============================================================================
71 ! Standard and extended grammar
75 !============================================================================
77 Soon, we'll explain what this means. For now, just type it all in,
78 paying particular attention to those seven semicolons, and ensuring that
79 the double quotes "..." always come in pairs. The first line beginning
80 with "``!%``" is special, and we'll talk about it in a moment; the
81 remaining exclamation mark lines, on the other hand, are purely
82 decorative; they just make the file's structure a little easier to
85 Ensure the file is named ``Heidi.inf``, rather than ``Heidi.txt`` or
88 Remember that, throughout this guide, we place the "``TYPE``" symbol
89 alongside pieces of code that we recommend you to type into your own
90 game files as you read through the examples (which, conversely, means
91 that you *don't* need to type the unmarked pieces of code). You'll
92 learn Inform more quickly by trying it for yourself, rather than just
93 taking our word for how things work.
97 Again, revisit the TYPE symbol. Maybe a standard indicator above
100 #. In the same folder, use your text editor to create the compilation
101 support file ``Heidi.bat`` (on a PC)::
103 ..\..\Lib\Base\Inform Heidi
104 +include_path=.\,..\..\Lib\Base,..\..\Lib\Contrib | more
106 pause "at end of compilation"
108 or ``Heidi.command`` (on a Macintosh)::
110 cd ~/Inform/Games/Heidi/
112 ../../Lib/Base/inform30_macosx Heidi
113 +include_path=./,../../Lib/Base,../../Lib/Contrib
115 Remember that there's just one space between "``Heidi``" and
118 Type in the file from scratch, or copy and edit ``MyGame1.bat`` (or
119 ``MyGame1.command``). At this point, you should have a ``Heidi`` folder
120 containing two files: ``Heidi.inf`` and either ``Heidi.bat`` or
123 #. Compile the source file ``Heidi.inf``; refer back to
124 :ref:`inform-windows` or :ref:`inform-apple` for guidance. If the
125 compilation works, a story file ``Heidi.z5`` appears in the folder. If
126 the compilation *doesn't* work, you've probably made a typing mistake;
127 check everything until you find it.
129 #. You can run the story file in your Inform interpreter; you should see
130 this (except that the Serial number will be different -- it's based on
133 .. code-block:: transcript
136 A simple Inform example
137 by Roger Firth and Sonja Kesserich.
138 Release 1 / Serial number 040804 / Inform v6.30 Library 6/11 SD
141 It is pitch dark, and you can't see a thing.
145 When you get that far, your template source file is correct. Let's explain
148 Understanding the source file
149 =============================
151 Although we've got a certain amount of freedom of expression, source files
152 tend to conform to a standard overall structure: these lines at the start,
153 that material next, those pieces coming at the end, and so on. What we're
154 doing here is mapping out a structure that suits us, giving ourselves a
155 clear framework onto which the elements of the game can be fitted. Having
156 a clear (albeit sparse) map at the start will help us to keep things
157 organised as the game evolves. We can infer several Inform rules just by
158 looking at the source file.
160 .. Generated by autoindex
164 * If the *very first line* (or lines) of the source file begin with the
165 characters "``!%``", then the compiler treats what follows on those lines
166 as control instructions to itself rather than as part of the game's
167 source. The instructions most commonly placed here are compiler
168 switches, a way of controlling detailed aspects of how the compiler
169 operates. These particular switches, two of many, are turning on
170 :term:`Strict mode`, which makes the game less likely to misbehave when
171 being played, and :term:`Debug mode`, which provides some extra commands
172 which can be helpful when tracking down problems.
176 Actually, the :option:`-S` is redundant, since Strict mode is already
177 on by default. We include it here as a reminder that (a) to turn
178 Strict mode *off*, you change this setting to :option:`-~S`, and (b)
179 alphabetic case matters here: :option:`-s` causes a display of
180 compiler statistics (and :option:`-~s` does nothing at all).
182 * Otherwise, when the compiler comes across an exclamation mark, it ignores
183 the rest of the line. If the ``!`` is at the start of a line, the whole
184 line is ignored; if the ``!`` is halfway along a line, the compiler takes
185 note of the first half, and then ignores the exclamation mark and
186 everything after it on that line. We call material following an
187 exclamation mark, not seen by anybody else, a :term:`comment`; it's often
188 a remark that we write to remind ourselves of how something works or why
189 we tackled a problem in a particular way. There's nothing special about
190 those equals signs: they just produce clear lines across the page.
192 It's always a good idea to comment code as you write it, for later it
193 will help you to understand what was going on at a particular spot.
194 Although it all seems clear in your head when you first write it, in a
195 few months you may suspect that a totally alien mind must have produced
196 that senseless gibberish.
198 By the way, the compiler *doesn't* give special treatment to exclamation
199 marks in quoted text: ``!`` within quotes "..." is treated as a normal
200 character. On this line, the first ``!`` is part of the sequence (or
201 :term:`string`) of characters to be displayed:
203 .. code-block:: inform
205 print "Hello world!"; ! <- is the start of this comment
207 * The compiler ignores blank lines, and treats lots of space like a single
208 space (except when the spaces are part of a character string). So, these
209 two rules tell us that we *could* have typed the source file like this:
211 .. code-block:: inform
213 Constant Story "Heidi";
215 "^A simple Inform example^by Roger Firth and Sonja Kesserich.^";
216 Include "Parser";Include "VerbLib";
220 We didn't type it that way because, though shorter, it's much harder to
221 read. When designing a game, you'll spend a lot of time studying what
222 you've typed, so it's worthwhile taking a bit of care to make it as
223 readable as possible.
225 * Every game should have the :term:`constant` definitions for ``Story``
226 (the game's name) and ``Headline`` (typically, information on the game's
227 theme, copyright, authorship and so on). These two :term:`string`
228 values, along with a release number and date, and details of the
229 compiler, compose the :term:`banner` which is displayed at the start of
232 * Every game needs the three lines which ``Include`` the standard library
233 files -- that is, they merge those files' contents into your source file:
235 .. code-block:: inform
242 They always have to be in this order, with ``Parser`` and ``VerbLib``
243 near the start of the file, and ``Grammar`` near the end.
245 * Every game needs to define an ``Initialise`` routine (note the British
248 .. code-block:: inform
252 The :term:`routine` that we've defined here doesn't do anything useful,
253 but it still needs to be present. Later, we'll come back to
254 ``Initialise`` and explain what a routine is and why we need this one.
256 * You'll notice that each of the items mentioned in the previous three
257 rules ends with a semicolon. Inform is very fussy about its punctuation,
258 and gets really upset if you forget a terminating semicolon. In fact,
259 the compiler just keeps reading your source file until it finds one;
260 that's why we were able to take three lines to define the ``Headline``
263 .. code-block:: inform
266 "^A simple Inform example
267 ^by Roger Firth and Sonja Kesserich.^";
269 Just to repeat what we said earlier: every game that you design will start
270 out from a basic source file like this (in fact, it might be sensible to
271 keep a copy of this template file in a safe place, as a starting point for
272 future games). Think of this stuff as the basic preparation which you'll
273 quickly come to take for granted, much as a landscape artist always begins
274 by sizing the canvas before starting to paint. So, now that we've taken a
275 quick tour of Inform's general needs, we can start thinking about what this
276 particular game requires.
278 Defining the game's locations
279 =============================
281 A good starting point in any game is to think about the locations which are
282 involved: this sketch map shows the four that we'll use:
284 .. image:: /images/heidi1.*
287 In IF, we talk about each of these locations as a :term:`room`, even though
288 in this example none of them has four walls. So let's use Inform to define
289 those rooms. Here's a first attempt:
291 .. code-block:: inform
293 Object "In front of a cottage"
295 "You stand outside a cottage. The forest stretches east.",
298 Object "Deep in the forest"
300 "Through the dense foliage, you glimpse a building to the west.
301 A track heads to the northeast.",
304 Object "A forest clearing"
306 "A tall sycamore stands in the middle of this clearing.
307 The path winds southwest through the trees.",
310 Object "At the top of the tree"
311 with description "You cling precariously to the trunk.",
314 Again, we can infer some general principles from these four examples:
316 * A room definition starts with the word ``Object`` and ends, about four
317 lines later, with a semicolon. Each of the components that appears in
318 your game -- not only the rooms, but also the people, the things that you
319 see and touch, intangibles like a sound, a smell, a gust of wind -- is
320 defined in this way; think of an "object" simply as the general term for
321 the myriad thingies which together comprise the model world which your
324 * The phrase in double quotes following the word ``Object`` is the name
325 that the interpreter uses to provide the player character with a list of
326 the objects around her: where she is, what she can see, what she's
331 We're using the word "player" to mean both the person who is playing
332 the game, and the principal protagonist (often known as the player
333 character) within the game itself. Since the latter -- Heidi -- is
334 female, we'll refer to the player as "she" while discussing this game.
336 * A keyword ``with`` follows, which simply tells the compiler what to
339 .. Generated by autoindex
341 pair: description; library property
343 * The word :prop:`description`, introducing another piece of text which
344 gives more detail about the object: in the case of a room, it's the
345 appearance of the surrounding environment when the player character is in
346 that room. The textual description is given in double quotes, and is
349 * Near the end, the keyword ``has`` appears, which again tells the compiler
350 to expect a certain kind of information.
352 .. Generated by autoindex
354 pair: light; library attribute
356 * The word :attr:`light` says that this object is a source of illumination,
357 and that therefore the player character can see what's happening here.
358 There has to be at least one light source in every room (unless you want
359 the player to be told that "It's pitch dark and you can't see a thing");
360 most commonly, that light source is the room itself.
362 A smidgeon of background may help set this into context (there's more in
363 the next chapter). An object can have both :term:`properties` (introduced
364 by the keyword ``with``) and :term:`attributes` (written after the word
365 ``has``). A property has both a name (like :prop:`description`) and a
366 value (like the character string "``You stand outside a cottage. The
367 forest stretches east.``"); an attribute has merely a name.
369 In a little while, when you play this game, you'll observe that it starts
372 .. code-block:: transcript
374 In front of a cottage
375 You stand outside a cottage. The forest stretches east.
377 And here you can see how the room's name (``In front of a cottage``) and
378 description (``You stand outside a cottage. The forest stretches east.``)
384 We said that this was a first attempt at defining the rooms; it's fine as
385 far as it goes, but a few bits of information are missing. If you look at
386 the game's sketch map, you can see how the rooms are intended to be
387 connected; from "Deep in the forest", for example, the player character
388 should be able to move west towards the cottage, or northeast to the
389 clearing. Now, although our descriptions mention or imply these available
390 routes, we also need to explicitly add them to the room definitions in a
391 form that the game itself can make sense of. Like this:
393 .. code-block:: inform
395 Object before_cottage "In front of a cottage"
397 "You stand outside a cottage. The forest stretches east.",
401 Object forest "Deep in the forest"
403 "Through the dense foliage, you glimpse a building to the west.
404 A track heads to the northeast.",
409 Object clearing "A forest clearing"
411 "A tall sycamore stands in the middle of this clearing.
412 The path winds southwest through the trees.",
417 Object top_of_tree "At the top of the tree"
418 with description "You cling precariously to the trunk.",
422 We've made two changes to the room objects.
424 * First, between the word ``Object`` and the object's name in double
425 quotes, we've inserted a different type of name: a private, internal
426 identification, never seen by the player; one that we can use *within*
427 the source file when one object needs to refer to another object. For
428 example, the first room is identified as ``before_cottage``, and the
429 second as ``forest``.
431 Unlike the external name contained in double quotes, the internal
432 identifier has to be a single word -- that is, without spaces. To aid
433 readability, we often use an underscore character to act as sort of
434 pseudo-space: ``before_cottage`` is a bit clearer than ``beforecottage``.
436 * Second, we've added lines after the object descriptions which use those
437 internal identifiers to show how the rooms are connected; one line for
438 each connection. The ``before_cottage`` object has this additional
443 This means that a player standing in front of the cottage can type GO
444 EAST (or EAST, or just E), and the game will transport her to the room
445 whose internal identification is ``forest``. If she tries to move in any
446 other direction from this room, she'll be told "You can't go that way".
448 What we've just defined is a *one-way* easterly connection:
449 ``before_cottage`` → ``forest``. The forest object has two additional
455 The first line defines a westerly connection ``forest`` →
456 ``before_cottage`` (thus enabling the player character to return to the
457 cottage), and the second defines a connection ``forest`` → ``clearing``
458 which heads off to the northeast.
460 .. Generated by autoindex
462 pair: d_to; library property
463 pair: e_to; library property
464 pair: in_to; library property
465 pair: n_to; library property
466 pair: ne_to; library property
467 pair: nw_to; library property
468 pair: out_to; library property
469 pair: s_to; library property
470 pair: se_to; library property
471 pair: sw_to; library property
472 pair: u_to; library property
473 pair: w_to; library property
475 Inform provides for eight "horizontal" connections (:prop:`n_to`,
476 :prop:`ne_to`, :prop:`e_to`, :prop:`se_to`, :prop:`s_to`, :prop:`sw_to`,
477 :prop:`w_to`, :prop:`nw_to`) two "vertical" ones (:prop:`u_to`,
478 :prop:`d_to`) and two specials :prop:`in_to`, and :prop:`out_to`. You'll
479 see some of these used for the remaining inter-room connections.
481 There's one last detail to attend to before we can test what we've done.
482 You'll recollect that our story begins with Heidi standing in front of her
483 cottage. We need to tell the interpreter that ``before_cottage`` is the
484 room where the game starts, and we do this in the ``Initialise`` routine::
486 [ Initialise; location = before_cottage; ];
488 :var:`location` is a :term:`variable`, part of the library, which tells the
489 interpreter in which room the player character currently is. Here, we're
490 saying that, at the start of the game, the player character is in the
491 ``before_cottage`` room.
493 Now we can add what we've done to the ``Heidi.inf`` source file template.
494 At this stage, you should study the four room definitions, comparing them
495 with the sketch map until you're comfortable that you understand how to
496 create simple rooms and define the connections between them.
498 .. code-block:: inform
500 !============================================================================
501 Constant Story "Heidi";
503 "^A simple Inform example
504 ^by Roger Firth and Sonja Kesserich.^";
509 !============================================================================
512 Object before_cottage "In front of a cottage"
514 "You stand outside a cottage. The forest stretches east.",
518 Object forest "Deep in the forest"
520 "Through the dense foliage, you glimpse a building to the west.
521 A track heads to the northeast.",
526 Object clearing "A forest clearing"
528 "A tall sycamore stands in the middle of this clearing.
529 The path winds southwest through the trees.",
534 Object top_of_tree "At the top of the tree"
535 with description "You cling precariously to the trunk.",
539 !============================================================================
540 ! Entry point routines
542 [ Initialise; location = before_cottage; ];
544 !============================================================================
545 ! Standard and extended grammar
549 !============================================================================
551 Type this in, as always taking great care with the punctuation -- watch
552 those commas and semicolons. Compile it, and fix any mistakes which the
553 compiler reports. You can then play the game in its current state.
554 Admittedly, you can't do very much, but you should be able to move freely
555 among the four rooms that you've defined.
559 In order to minimise the amount of typing that you have to do, the
560 descriptive text in this game has been kept as short as possible. In a
561 real game, you would typically provide more interesting descriptions
564 Adding the bird and the nest
565 ============================
567 Given what we said earlier, you won't be surprised to hear that both the
568 bird and its nest are Inform objects. We'll start their definitions like
571 .. code-block:: inform
573 Object bird "baby bird"
574 with description "Too young to fly, the nestling tweets helplessly.",
577 Object nest "bird's nest"
578 with description "The nest is carefully woven of twigs and moss.",
581 You can see that these definitions have exactly the same format as the
582 rooms we defined previously: a one-word internal identifier (``bird``,
583 ``nest``), and a word or phrase naming the object for the player's benefit
584 (``baby bird``, ``bird's nest``). They both have some descriptive detail:
585 for a room this is printed when the player first enters, or when she types
586 LOOK; for other objects it's printed when she EXAMINEs that object. What
587 they *don't* have are connections (:prop:`e_to`, :prop:`w_to`, etc. apply
588 only to rooms) or :attr:`light` (it's not necessary -- the rooms ensure
589 that light is available).
591 When the game is running, the player will want to refer to these two
592 objects, saying for instance EXAMINE THE BABY BIRD or PICK UP THE NEST.
593 For this to work reliably, we need to specify the word (or words) which
594 relate to each object. Our aim here is flexibility: providing a choice of
595 relevant vocabulary so that the player can use whatever term seems
596 appropriate to her, with a good chance of it being understood. We add a
597 line to each definition:
599 .. code-block:: inform
601 Object bird "baby bird"
602 with description "Too young to fly, the nestling tweets helplessly.",
603 name 'baby' 'bird' 'nestling',
606 Object nest "bird's nest"
607 with description "The nest is carefully woven of twigs and moss.",
608 name 'bird^s' 'nest' 'twigs' 'moss',
611 The :prop:`name` introduces a list in single quotes '...'. We call each of
612 those quoted things a :term:`dictionary word`, and we do mean "word", not
613 "phrase" (``'baby'``\ ``'bird'`` rather than ``'baby bird'``); you can't
614 uses spaces, commas or periods *in* dictionary words, though there's a
615 space *between* each one, and the whole list ends with a comma. The idea
616 is that the interpreter decides which object a player is talking about by
617 matching what she types against the full set of all dictionary words. If
618 the player mentions BIRD, or BABY BIRD, or NESTLING, it's the ``baby bird``
619 that she means; if she mentions NEST, BIRD'S NEST or MOSS, it's the
620 ``bird's nest``. And if she types NEST BABY or BIRD TWIGS, the interpreter
621 will politely say that it doesn't understand what on earth she's talking
629 You'll notice the use of ``'bird^s'`` to define the dictionary word
630 BIRD'S; this oddity is necessary because the compiler expects the single
631 quotes in the list always to come in pairs -- one at the start of the
632 dictionary word, and one at the end. If we had typed ``'bird's'`` then
633 the compiler would find the opening quote, the four letters ``b``,
634 ``i``, ``r`` and ``d``, and what looks like the closing quote. So far
635 so good; it's read the word BIRD and now expects a space before the next
636 opening quote... but instead finds ``s'`` which makes no sense. In
637 cases like this we must use the circumflex ``^`` to *represent* the
638 apostrophe, and the compiler then treats ``bird's`` as a dictionary
641 You may be wondering why we need a list of :prop:`name` words for the bird
642 and its nest, yet we didn't when we defined the rooms? It's because the
643 player can't interact with a room in the same way as with other objects;
644 for example, she doesn't need to say EXAMINE THE FOREST -- just being there
645 and typing LOOK is sufficient.
647 .. Generated by autoindex
649 pair: container; library attribute
650 pair: open; library attribute
652 The bird's definition is complete, but there's an additional complexity
653 with the nest: we need to be able to put the bird into it. We do this by
654 labelling the nest as a :attr:`container` -- able to hold other objects --
655 so that the player can type PUT (or INSERT) BIRD IN (or INTO) NEST.
656 Furthermore, we label it as :attr:`open`; this prevents the interpreter
657 from asking us to open it before putting in the bird.
659 .. code-block:: inform
661 Object nest "bird's nest"
662 with description "The nest is carefully woven of twigs and moss.",
663 name 'bird^s' 'nest' 'twigs' 'moss',
666 Both objects are now defined, and we can incorporate them into the game.
667 To do this, we need to choose the locations where the player will find
668 them. Let's say that the bird is found in the forest, while the nest is in
669 the clearing. This is how we set this up:
671 .. code-block:: inform
673 Object bird "baby bird" forest
674 with description "Too young to fly, the nestling tweets helplessly.",
675 name 'baby' 'bird' 'nestling',
678 Object nest "bird's nest" clearing
679 with description "The nest is carefully woven of twigs and moss.",
680 name 'bird^s' 'nest' 'twigs' 'moss',
683 Read that first line as: "Here's the definition of an object which is
684 identified within this file as ``bird``, which is known to the player as
685 ``baby bird``, and which is initially located inside the object identified
686 within this file as ``forest``."
688 Where in the source file do these new objects fit? Well, anywhere really,
689 but you'll find it convenient to insert them following the rooms where
690 they're found. This means adding the bird just after the forest, and the
691 nest just after the clearing. Here's the middle piece of the source file:
693 .. code-block:: inform
695 !============================================================================
698 Object before_cottage "In front of a cottage"
700 "You stand outside a cottage. The forest stretches east.",
704 Object forest "Deep in the forest"
706 "Through the dense foliage, you glimpse a building to the west.
707 A track heads to the northeast.",
712 Object bird "baby bird" forest
713 with description "Too young to fly, the nestling tweets helplessly.",
714 name 'baby' 'bird' 'nestling',
717 Object clearing "A forest clearing"
719 "A tall sycamore stands in the middle of this clearing.
720 The path winds southwest through the trees.",
725 Object nest "bird's nest" clearing
726 with description "The nest is carefully woven of twigs and moss.",
727 name 'bird^s' 'nest' 'twigs' 'moss',
730 Object top_of_tree "At the top of the tree"
731 with description "You cling precariously to the trunk.",
735 !============================================================================
737 Make those changes, recompile the game, play it and you'll see this:
739 .. code-block:: transcript
742 Through the dense foliage, you glimpse a building to the west. A track heads
745 You can see a baby bird here.
749 Adding the tree and the branch
750 ==============================
752 The description of the clearing mentions a tall sycamore tree, up which the
753 player character supposedly "climbs". We'd better define it:
755 .. code-block:: inform
757 Object tree "tall sycamore tree" clearing
759 "Standing proud in the middle of the clearing,
760 the stout tree looks easy to climb.",
761 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
764 Everything there should be familiar, apart from that :attr:`scenery` at the
765 end. We've already mentioned the tree in the description of the forest
766 clearing, so we don't want the interpreter adding "You can see a tall
767 sycamore tree here" afterwards, as it does for the bird and the nest. By
768 labelling the tree as :attr:`scenery` we suppress that, and also prevent it
769 from being picked up by the player character. One final object: the branch
770 at the top of the tree. Again, not many surprises in this definition:
772 .. code-block:: inform
774 Object branch "wide firm bough" top_of_tree
775 with description "It's flat enough to support a small object.",
776 name 'wide' 'firm' 'flat' 'bough' 'branch',
777 has static supporter;
779 The only new things are those two labels. :attr:`static` is similar to
780 :attr:`scenery`: it prevents the branch from being picked up by the player
781 character, but *doesn't* suppress mention of it when describing the
782 setting. And :attr:`supporter` is rather like the :attr:`container` that
783 we used for the nest, except that this time the player character can put
784 other objects *onto* the branch. (In passing, we'll mention that an object
785 can't normally be both a :attr:`container` *and* a :attr:`supporter`.) And
786 so here are our objects again:
788 .. code-block:: inform
790 !============================================================================
793 Object before_cottage "In front of a cottage"
795 "You stand outside a cottage. The forest stretches east.",
799 Object forest "Deep in the forest"
801 "Through the dense foliage, you glimpse a building to the west.
802 A track heads to the northeast.",
807 Object bird "baby bird" forest
808 with description "Too young to fly, the nestling tweets helplessly.",
809 name 'baby' 'bird' 'nestling',
812 Object clearing "A forest clearing"
814 "A tall sycamore stands in the middle of this clearing.
815 The path winds southwest through the trees.",
820 Object nest "bird's nest" clearing
821 with description "The nest is carefully woven of twigs and moss.",
822 name 'bird^s' 'nest' 'twigs' 'moss',
825 Object tree "tall sycamore tree" clearing
827 "Standing proud in the middle of the clearing,
828 the stout tree looks easy to climb.",
829 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
832 Object top_of_tree "At the top of the tree"
833 with description "You cling precariously to the trunk.",
837 Object branch "wide firm bough" top_of_tree
838 with description "It's flat enough to support a small object.",
839 name 'wide' 'firm' 'flat' 'bough' 'branch',
840 has static supporter;
842 !============================================================================
844 Once again, make the changes, recompile, and investigate what you can do in
850 Our first pass at the game is nearly done; just two more changes to
851 describe. The first is easy: Heidi wouldn't be able to climb the tree
852 carrying the bird and the nest separately: we want the player character to
853 put the bird into the nest first. One easy way to enforce this is by
854 adding a line near the top of the file:
856 .. code-block:: inform
858 !============================================================================
859 Constant Story "Heidi";
861 "^A simple Inform example
862 ^by Roger Firth and Sonja Kesserich.^";
864 Constant MAX_CARRIED 1;
866 The value of ``MAX_CARRIED`` limits the number of objects that the player
867 character can be holding at any one time; by setting it to 1, we're saying
868 that she can carry the bird or the nest, but not both. However, the limit
869 ignores the contents of :attr:`container` or :attr:`supporter` objects, so
870 the nest with the bird inside it is still counted as one object.
872 The other change is slightly more complex and more important: there's
873 currently no way to "win" the game! The goal is for the player character
874 to put the bird in the nest, take the nest to the top of the tree, and
875 place it on the branch; when that happens, the game should be over. This
876 is one way of making it happen:
878 .. code-block:: inform
880 Object branch "wide firm bough" top_of_tree
881 with description "It's flat enough to support a small object.",
882 name 'wide' 'firm' 'flat' 'bough' 'branch',
883 each_turn [; if (nest in branch) deadflag = 2; ],
884 has static supporter;
888 Here's an explanation of what's going on. If you find this difficult to
889 grasp, don't worry. It's the hardest bit so far, and it introduces
890 several new concepts all at once. Later in the guide, we'll explain
891 those concepts more clearly, so you can just skip this bit if you want.
893 .. Generated by autoindex
895 pair: deadflag; library variable
897 The variable :var:`deadflag`, part of the library, is normally 0. If
898 you set its value to 2, the interpreter notices and ends the game with
899 "You have won". The statement::
901 if (nest in branch) deadflag = 2;
903 should be read as: "Test whether the ``nest`` is currently in the
904 ``branch`` (if the branch is a :attr:`container`) or on it (if the
905 ``branch`` is a supporter); if it is, set the value of :var:`deadflag`
906 to 2; if it isn't, do nothing." The surrounding part::
910 should be read as: "At the end of each turn (when the player is in the
911 same room as the branch), do whatever is written inside the square
912 brackets". So, putting that all together:
914 * At the end of each turn (after the player has typed something and
915 pressed the Enter key, and the interpreter has done whatever was
916 requested) the interpreter checks whether the player and the
917 ``branch`` are in the same room. If not, nothing happens. If they're
918 together, it looks to see where the nest is. Initially it's in the
919 ``clearing``, so nothing happens.
921 * Also at the end of each turn, the interpreter checks the value of
922 :var:`deadflag`. Usually it's 0, so nothing happens.
924 * Finally the player character puts the ``nest`` on the ``branch``.
925 "Aha!" says the interpreter (to itself, of course), and sets the
926 value of :var:`deadflag` to 2.
928 * Immediately afterwards, (another part of) the interpreter checks and
929 finds that the value of :var:`deadflag` has changed to 2, which means
930 that the game is successfully completed; so, it says to the player,
933 That's as far as we'll take this example for now. Make those final
934 changes, recompile, and test what you've achieved. You'll probably find a
935 few things that could be done better -- even on a simple game like this
936 there's considerable scope for improvement -- so we'll revisit Heidi in her
937 forest shortly. First, though, we'll recap what we've learnt so far.