1 ==============================
2 William Tell: a tale is born
3 ==============================
9 | *K was King William, once governed the land;*
10 | *L was a lady, who had a white hand.*
14 .. image:: /images/picK.png
21 eeping up the momentum, this chapter (and the three which follow) works
22 steadily through the design of the "William Tell" game that we encountered
23 right at the start of this guide. Many of the principles are the same as
24 the ones we explained when designing Heidi and her forest, so we'll not
25 linger on what should be familiar ground. "William Tell" is a slightly
26 longer and more complex game, so we'll move as swiftly as possible to
27 examine the features which are new.
32 Our starting point is much the same as last time. Here's a basic
36 !===========================================================================
37 Constant Story "William Tell";
39 "^A simple Inform example
40 ^by Roger Firth and Sonja Kesserich.^";
41 Release 3; Serial "040804"; ! for keeping track of public releases
43 Constant MAX_SCORE = 3;
48 !===========================================================================
51 !===========================================================================
54 !===========================================================================
55 ! The player's possessions
57 !===========================================================================
58 ! Entry point routines
62 lookmode = 2; ! like the VERBOSE command
64 move quiver to player; give quiver worn;
66 "You wear the traditional clothing of a Swiss mountaineer.";
68 The place: Altdorf, in the Swiss canton of Uri. The year is 1307,
69 at which time Switzerland is under rule by the Emperor Albert of
70 Habsburg. His local governor -- the vogt -- is the bullying
71 Hermann Gessler, who has placed his hat atop a wooden pole in
72 the centre of the town square; everybody who passes through the
73 square must bow to this hated symbol of imperial might.
75 You have come from your cottage high in the mountains,
76 accompanied by your younger son, to purchase provisions. You are
77 a proud and independent man, a hunter and guide, renowned both
78 for your skill as an archer and, perhaps unwisely (for his soldiers
79 are everywhere), for failing to hide your dislike of the vogt.
81 It's market-day: the town is packed with people from the
82 surrounding villages and settlements.^";
85 !===========================================================================
86 ! Standard and extended grammar
90 !===========================================================================
92 You'll see that we've marked a couple of extra divisions in the file, to
93 help organise the stuff we'll add later, but the overall structure is
94 identical to our first game. Let's quickly point out some extra bits and
97 * If you look at a game's banner, you'll see two pieces of information:
98 "Release" and "Serial number".
100 .. code-block:: transcript
103 A simple Inform example
104 by Roger Firth and Sonja Kesserich.
105 Release 3 / Serial number 040804 / Inform v6.30 Library 6/11 SD
107 These two fields are automatically written by the compiler, which sets by
108 default Release to 1 and the Serial Number to today's date. However, we
109 can explicitly override this behaviour using ``Release`` and ``Serial``,
110 to keep track of different versions of our game. Typically, we will
111 publish several updates of our games over time, each version fixing
112 problems which were found in the previous release. If somebody else
113 reports a problem with a game, we'd like to know exactly which version
114 they were using; so, rather than take the default values, we set our own.
115 When it's time to release a new version, all we have to do is comment out
116 the previous lines and add another below them::
118 !Release 1; Serial "020128"; ! First beta-test release
119 !Release 2; Serial "020217"; ! Second beta-test release
120 Release 3; Serial "020315"; ! IF Library competition entry
122 * We'll be implementing a simple system of awarding points when the player
123 gets something right, so we define top marks::
125 Constant MAX_SCORE = 3;
127 * The ``Initialise`` routine that we wrote last time contained only one
128 statement, to set the player's initial ``location``. We do that here as
129 well, but we also do some other stuff.
131 * The first thing is to assign 2 to the library variable ``lookmode``.
132 Inform's default mode for displaying room descriptions is BRIEF (a
133 description is displayed only when a room is visited for the first time)
134 and, by changing this variable's value, we set it to VERBOSE
135 (descriptions are displayed on *every* visit). Doing this is largely a
136 matter of personal preference, and in any case it's nothing more than a
137 convenience; it just saves having to remember to type VERBOSE each time
138 that we test the game.
140 * At the start of the game, we want Wilhelm to be equipped with his bow and
141 quiver of arrows. The recommended way of making this happen is to
142 perform the necessary object tree rearrangement with a couple of ``move``
143 statements in the ``Initialise`` routine::
146 move quiver to player;
148 and indeed this is the clearest way to place objects in the player's
149 inventory at the beginning of any game.
153 Wait! you say. In the previous chapter, to make an object the child
154 of another object all we needed to do was to define the child object
155 with the internal identification of the parent object at the end of
158 Object bird "baby bird" forest
160 Why not do that with the player? Because the object which represents
161 the player is defined by the library (rather than as part of our
162 game), and actually has an internal ID of ``selfobj``; ``player`` is a
163 variable whose value is that identifier. Rather than worry all about
164 this, it's easier to use the ``move`` statements.
166 There's one other task associated with the quiver; it's an article of
167 clothing which Wilhelm is "wearing", a state denoted by the attribute
168 ``worn``. Normally the interpreter would apply this automatically, while
169 handling a command like WEAR QUIVER, but since we've moved the quiver
170 ourselves, we also need to set the quiver's ``worn`` attribute. The
171 ``give`` statement does the job::
175 (To clear the attribute, by the way, you'd use the statement ``give
176 quiver ~worn`` -- read that as "give the quiver not-worn"; Inform often
177 uses ``~`` to mean "not".)
179 * If the player types EXAMINE ME, the interpreter displays the
180 ``description`` property of the ``player`` object. The default value is
181 "As good-looking as ever", a bit of a cliché in the world of Inform
182 games. It's easy to change, though, once you realise that, since the
183 properties of an object are variables, you can assign new values to them
184 just as you'd assign new values to ``location`` and ``lookmode``. The
185 only problem is getting the syntax right; you can't say just::
187 description = "You wear the traditional clothing of a Swiss mountaineer.";
189 because there are dozens of objects in the game, each with its own
190 ``description`` property; you need to be a little more explicit. Here's
194 "You wear the traditional clothing of a Swiss mountaineer.";
196 * Finally, the ``Initialise`` routine ends with a lengthy ``print_ret``
197 statement. Since the interpreter calls ``Initialise`` right at the start
198 of the game, that's the point at which this material is displayed, so
199 that it acts as a scene-setting preamble before the game gets under way.
200 In fact, everything you want set or done at the very beginning of the
201 game, should go into the ``Initialise`` routine.
203 The game won't compile in this state, because it contains references to
204 objects which we haven't yet defined. In any case, we don't intend to
205 build up the game in layers as we did last time, but rather to talk about
206 it in logically related chunks. To see (and if you wish, to type) the
207 complete source, go to :doc:`/appendices/c`.
212 Remember how we defined the rooms in "Heidi"? Our first attempt started
215 Object "In front of a cottage"
217 "You stand outside a cottage. The forest stretches east.",
220 Object "Deep in the forest"
222 "Through the dense foliage, you glimpse a building to the west.
223 A track heads to the northeast.",
228 and we explained that just about *every* room needs that ``light``
229 attribute, or else the player would be literally in the dark. It's a bit
230 of a nuisance having to specify that same attribute each time; what would
231 be neater would be to say that *all* rooms are illuminated. So we can
237 Room "In front of a cottage"
239 "You stand outside a cottage. The forest stretches east.",
242 Room "Deep in the forest"
244 "Through the dense foliage, you glimpse a building to the west.
245 A track heads to the northeast.",
250 We've done four things:
252 #. We've said that some of the objects in our game are going to be defined
253 by the specialised word ``Room`` rather than the general-purpose word
254 ``Object``. In effect, we've taught Inform a new word specially for
255 defining objects, which we can now use as though it had been part of the
258 #. We've furthermore said that every object which we define using ``Room``
259 is automatically going to have the ``light`` attribute.
261 #. We've changed the way in which we define the four room objects, by
262 starting them with our specialised word ``Room``. The remainder of the
263 definition for these objects -- the header information, the block of
264 properties, the block of attributes and the final semicolon -- remains
265 the same; except that:
267 #. We don't need to explicitly include the ``light`` attribute each time;
268 every ``Room`` object has it automatically.
270 A :term:`class` is a family of closely related objects, all of which behave
271 in the same way. Any properties defined for the class, and any attributes
272 defined for the class, are automatically given to objects which you specify
273 as belonging to that class; this process of acquisition just by being a
274 member of a class is called :term:`inheritance`. In our example, we've
275 defined a ``Room`` class with a ``light`` attribute, and then we've
276 specified four objects each of which is a member of that class, and each of
277 which gets given a ``light`` attribute as a result of that membership.
279 Why have we gone to this trouble? Three main reasons:
281 * By moving the common bits of the definitions from the individual objects
282 to the class definition which they share, those object definitions
283 become shorter and simpler. Even if we had a hundred rooms, we'd still
284 need to specify ``has light`` only once.
286 * By creating a specialised word to identify our class of objects, we make
287 our source file easier to read. Rather than absolutely everything being
288 an anonymous ``Object``, we can now immediately recognise that some are
289 ``Room`` objects (and others belong to the different classes that we'll
292 * By collecting the common definitions into one place, we make it much
293 easier to make widespread modifications in future. If we need to make
294 some change to the definition of all our rooms, we just modify the
295 ``Room`` class, and all of the class members inherit the change.
297 For these reasons, the use of classes is an incredibly powerful technique,
298 easier than it may look, and very well worth mastering. From now on, we'll
299 be defining object classes whenever it makes sense (which is generally when
300 two or more objects are meant to behave in exactly the same way).
302 You may be wondering: suppose I want to define a room which for some reason
303 *doesn't* have ``light``; can I still use the ``Room`` class? Sure you
306 Room cellar "Gloomy cellar"
307 with description "Your torch shows only cobwebby brick walls.",
310 This illustrates another nice feature of inheritance: the object definition
311 can override the class definition. The class says ``has light``, but the
312 object itself says ``has ~light`` (read that as "has no light") and the
313 object wins. The cellar is dark, and the player will need a torch to see
316 In fact, for any object both the block of properties and the block of
317 attributes are optional and can be omitted if there's nothing to be
318 specified. Now that the ``light`` attribute is being provided
319 automatically and there aren't any other attributes to set, the word
320 ``has`` can be left out. Here's the class again::
325 and here is how we could have used it in "Heidi"::
327 Room "In front of a cottage"
329 "You stand outside a cottage. The forest stretches east.";
331 Room "Deep in the forest"
333 "Through the dense foliage, you glimpse a building to the west.
334 A track heads to the northeast.";
338 You'll notice that, if an object has no block of attributes, the semicolon
339 which terminates its definition simply moves to the end of its last
347 We use the ``Room`` class in "William Tell", and a few other classes
348 besides. Here's a ``Prop`` class (that's "Prop" in the sense of a
349 theatrical property rather than a supportive device), useful for scenic
350 items whose only role is to sit waiting in the background on the off-chance
351 that the player might think to EXAMINE them::
358 print_ret "You don't need to worry about ", (the) self, ".";
362 All objects of this class inherit the ``scenery`` attribute, so they're
363 excluded from room descriptions. Also, there's a ``before`` property; one
364 that's more complex than our previous efforts. You'll remember that the
365 first ``before`` we met looked like this::
369 print "It sounds scared and in need of assistance.^";
373 The role of that original ``before`` was to intercept ``Listen`` actions,
374 while leaving all others well alone. The role of the ``before`` in the
375 ``Prop`` class is broader: to intercept (a) ``Examine`` actions, and (b)
376 all the rest. If the action is ``Examine``, then the ``return false``
377 statement means that the action carries on. If the action is ``default``
378 -- none of those explicitly listed, which in this instance means *every*
379 action apart from ``Examine`` -- then the ``print_ret`` statement is
380 executed, after which the interpreter does nothing further. So, a ``Prop``
381 object can be EXAMINEd, but any other action addressed to it results in a
382 "no need to worry" message.
384 That message is also more involved than anything we've so far displayed.
385 The statement which produces it is::
387 print_ret "You don't need to worry about ", (the) self, ".";
389 which you should read as doing this:
391 #. display the string "You don't need to worry about ",
393 #. display a definite article (usually "the") followed by a space and the
394 external name of the object concerned,
396 #. display a period, and
398 #. display a newline and return true in the usual way for a ``print_ret``
401 The interesting things that this statement demonstrates are:
403 * The ``print`` and ``print_ret`` statements aren't restricted to
404 displaying a single piece of information: they can display a list of
405 items which are separated by commas. The statement still ends with a
406 semicolon in the usual way.
408 * As well as displaying strings, you can also display the names of objects:
409 given the ``nest`` object from our first game, ``(the) nest`` would
410 display "the bird's nest", ``(The) nest`` would display "The bird's
411 nest", ``(a) nest`` would display "a bird's nest", ``(A) nest`` would
412 display "A bird's nest" and ``(name) nest`` would display just "bird's
413 nest". This use of a word in parentheses, telling the interpreter how to
414 display the following object's internal ID, is called a :term:`print
417 * There's a library variable ``self`` which always contains the internal ID
418 of the current object, and is really convenient when using a ``Class``.
419 By using this variable in our ``print_ret`` statement, we ensure that the
420 message contains the name of the appropriate object.
422 Let's see an example of this in action; here's a ``Prop`` object from
425 Prop "south gate" street
426 with name 'south' 'southern' 'wooden' 'gate',
427 description "The large wooden gate in the town walls is wide open.",
430 If players type EXAMINE GATE, they'll see "The large wooden gate..."; if
431 they type CLOSE GATE then the gate's ``before`` property will step in and
432 display "You don't need to worry about the south gate", neatly picking up
433 the name of the object from the ``self`` variable.
435 The reason for doing all this, rather than just creating a simple scenery
436 object like Heidi's ``tree`` and ``cottage``, is to support EXAMINE for
437 increased realism, while clearly hinting to players that trying other verbs
438 would be a waste of time.
440 A class for furniture
441 ---------------------
443 The last class for now -- we'll talk about the ``Arrow`` and ``NPC``
444 classes in the next chapter -- is for furniture-like objects. If you label
445 an object with the ``static`` attribute, an attempt to TAKE it results in
446 "That's fixed in place" -- acceptable in the case of Heidi's branch object
447 (which is indeed supposed to be part of the tree), less so for items which
448 are simply large and heavy. This ``Furniture`` class might sometimes be
453 Take,Pull,Push,PushDir:
454 print_ret (The) self, " is too heavy for that.";
456 has static supporter;
458 Its structure is similar to that of our ``Prop`` class: some appropriate
459 attributes, and a ``before`` property to trap actions directed at it.
460 Again, we display a message which is "personalised" for the object
461 concerned by using a ``(The) self`` print rule. This time we're
462 intercepting four actions; we *could* have written the property like this::
465 Take: print_ret (The) self, " is too heavy for that.";
466 Pull: print_ret (The) self, " is too heavy for that.";
467 Push: print_ret (The) self, " is too heavy for that.";
468 PushDir: print_ret (The) self, " is too heavy for that.";
471 but since we're giving exactly the same response each time, it's better to
472 put all of those actions into one list, separated by commas. ``PushDir``,
473 if you were wondering, is the action triggered by a command like PUSH THE
476 Incidentally, another bonus of defining classes like these is that you can
477 probably reuse them in your next game.
479 Now that most of our class definitions are in place, we can get on with
480 defining some real rooms and objects. First, though, if you're typing in
481 the "William Tell" game as you read through the guide, you'd probably like
482 to check that what you've entered so far is correct;
483 :ref:`compile-as-you-go` explains how to compile the game in its current --