1 ==============================
2 William Tell: a tale is born
3 ==============================
9 | |CENTER| *K was King William, once governed the land;*
10 | |CENTER| *L was a lady, who had a white hand.*
14 .. image:: /images/picK.png
17 |K|\eeping up the momentum, this chapter (and the three which follow) works
18 steadily through the design of the "William Tell" game that we encountered
19 right at the start of this guide. Many of the principles are the same as
20 the ones we explained when designing Heidi and her forest, so we'll not
21 linger on what should be familiar ground. "William Tell" is a slightly
22 longer and more complex game, so we'll move as swiftly as possible to
23 examine the features which are new.
28 Our starting point is much the same as last time. Here's a basic
32 !===========================================================================
33 Constant Story "William Tell";
35 "^A simple Inform example
36 ^by Roger Firth and Sonja Kesserich.^";
37 Release 3; Serial "040804"; ! for keeping track of public releases
39 Constant MAX_SCORE = 3;
44 !===========================================================================
47 !===========================================================================
50 !===========================================================================
51 ! The player's possessions
53 !===========================================================================
54 ! Entry point routines
58 lookmode = 2; ! like the VERBOSE command
60 move quiver to player; give quiver worn;
62 "You wear the traditional clothing of a Swiss mountaineer.";
64 The place: Altdorf, in the Swiss canton of Uri. The year is 1307,
65 at which time Switzerland is under rule by the Emperor Albert of
66 Habsburg. His local governor -- the vogt -- is the bullying
67 Hermann Gessler, who has placed his hat atop a wooden pole in
68 the centre of the town square; everybody who passes through the
69 square must bow to this hated symbol of imperial might.
71 You have come from your cottage high in the mountains,
72 accompanied by your younger son, to purchase provisions. You are
73 a proud and independent man, a hunter and guide, renowned both
74 for your skill as an archer and, perhaps unwisely (for his soldiers
75 are everywhere), for failing to hide your dislike of the vogt.
77 It's market-day: the town is packed with people from the
78 surrounding villages and settlements.^";
81 !===========================================================================
82 ! Standard and extended grammar
86 !===========================================================================
88 You'll see that we've marked a couple of extra divisions in the file, to
89 help organise the stuff we'll add later, but the overall structure is
90 identical to our first game. Let's quickly point out some extra bits and
93 * If you look at a game's banner, you'll see two pieces of information:
94 "Release" and "Serial number".
96 .. code-block:: transcript
99 A simple Inform example
100 by Roger Firth and Sonja Kesserich.
101 Release 3 / Serial number 040804 / Inform v6.30 Library 6/11 SD
103 These two fields are automatically written by the compiler, which sets by
104 default Release to 1 and the Serial Number to today's date. However, we
105 can explicitly override this behaviour using ``Release`` and ``Serial``,
106 to keep track of different versions of our game. Typically, we will
107 publish several updates of our games over time, each version fixing
108 problems which were found in the previous release. If somebody else
109 reports a problem with a game, we'd like to know exactly which version
110 they were using; so, rather than take the default values, we set our own.
111 When it's time to release a new version, all we have to do is comment out
112 the previous lines and add another below them::
114 !Release 1; Serial "020128"; ! First beta-test release
115 !Release 2; Serial "020217"; ! Second beta-test release
116 Release 3; Serial "020315"; ! IF Library competition entry
118 * We'll be implementing a simple system of awarding points when the player
119 gets something right, so we define top marks::
121 Constant MAX_SCORE = 3;
123 * The ``Initialise`` routine that we wrote last time contained only one
124 statement, to set the player's initial ``location``. We do that here as
125 well, but we also do some other stuff.
127 * The first thing is to assign 2 to the library variable ``lookmode``.
128 Inform's default mode for displaying room descriptions is BRIEF (a
129 description is displayed only when a room is visited for the first time)
130 and, by changing this variable's value, we set it to VERBOSE
131 (descriptions are displayed on *every* visit). Doing this is largely a
132 matter of personal preference, and in any case it's nothing more than a
133 convenience; it just saves having to remember to type VERBOSE each time
134 that we test the game.
136 * At the start of the game, we want Wilhelm to be equipped with his bow and
137 quiver of arrows. The recommended way of making this happen is to
138 perform the necessary object tree rearrangement with a couple of ``move``
139 statements in the ``Initialise`` routine::
142 move quiver to player;
144 and indeed this is the clearest way to place objects in the player's
145 inventory at the beginning of any game.
149 Wait! you say. In the previous chapter, to make an object the child
150 of another object all we needed to do was to define the child object
151 with the internal identification of the parent object at the end of
154 Object bird "baby bird" forest
156 Why not do that with the player? Because the object which represents
157 the player is defined by the library (rather than as part of our
158 game), and actually has an internal ID of ``selfobj``; ``player`` is a
159 variable whose value is that identifier. Rather than worry all about
160 this, it's easier to use the ``move`` statements.
162 There's one other task associated with the quiver; it's an article of
163 clothing which Wilhelm is "wearing", a state denoted by the attribute
164 ``worn``. Normally the interpreter would apply this automatically, while
165 handling a command like WEAR QUIVER, but since we've moved the quiver
166 ourselves, we also need to set the quiver's ``worn`` attribute. The
167 ``give`` statement does the job::
171 (To clear the attribute, by the way, you'd use the statement ``give
172 quiver ~worn`` -- read that as "give the quiver not-worn"; Inform often
173 uses ``~`` to mean "not".)
175 * If the player types EXAMINE ME, the interpreter displays the
176 ``description`` property of the ``player`` object. The default value is
177 "As good-looking as ever", a bit of a cliché in the world of Inform
178 games. It's easy to change, though, once you realise that, since the
179 properties of an object are variables, you can assign new values to them
180 just as you'd assign new values to ``location`` and ``lookmode``. The
181 only problem is getting the syntax right; you can't say just::
183 description = "You wear the traditional clothing of a Swiss mountaineer.";
185 because there are dozens of objects in the game, each with its own
186 ``description`` property; you need to be a little more explicit. Here's
190 "You wear the traditional clothing of a Swiss mountaineer.";
192 * Finally, the ``Initialise`` routine ends with a lengthy ``print_ret``
193 statement. Since the interpreter calls ``Initialise`` right at the start
194 of the game, that's the point at which this material is displayed, so
195 that it acts as a scene-setting preamble before the game gets under way.
196 In fact, everything you want set or done at the very beginning of the
197 game, should go into the ``Initialise`` routine.
199 The game won't compile in this state, because it contains references to
200 objects which we haven't yet defined. In any case, we don't intend to
201 build up the game in layers as we did last time, but rather to talk about
202 it in logically related chunks. To see (and if you wish, to type) the
203 complete source, go to :doc:`/appendices/c`.
208 Remember how we defined the rooms in "Heidi"? Our first attempt started
211 Object "In front of a cottage"
213 "You stand outside a cottage. The forest stretches east.",
216 Object "Deep in the forest"
218 "Through the dense foliage, you glimpse a building to the west.
219 A track heads to the northeast.",
224 and we explained that just about *every* room needs that ``light``
225 attribute, or else the player would be literally in the dark. It's a bit
226 of a nuisance having to specify that same attribute each time; what would
227 be neater would be to say that *all* rooms are illuminated. So we can
233 Room "In front of a cottage"
235 "You stand outside a cottage. The forest stretches east.",
238 Room "Deep in the forest"
240 "Through the dense foliage, you glimpse a building to the west.
241 A track heads to the northeast.",
246 We've done four things:
248 #. We've said that some of the objects in our game are going to be defined
249 by the specialised word ``Room`` rather than the general-purpose word
250 ``Object``. In effect, we've taught Inform a new word specially for
251 defining objects, which we can now use as though it had been part of the
254 #. We've furthermore said that every object which we define using ``Room``
255 is automatically going to have the ``light`` attribute.
257 #. We've changed the way in which we define the four room objects, by
258 starting them with our specialised word ``Room``. The remainder of the
259 definition for these objects -- the header information, the block of
260 properties, the block of attributes and the final semicolon -- remains
261 the same; except that:
263 #. We don't need to explicitly include the ``light`` attribute each time;
264 every ``Room`` object has it automatically.
266 A :term:`class` is a family of closely related objects, all of which behave
267 in the same way. Any properties defined for the class, and any attributes
268 defined for the class, are automatically given to objects which you specify
269 as belonging to that class; this process of acquisition just by being a
270 member of a class is called :term:`inheritance`. In our example, we've
271 defined a ``Room`` class with a ``light`` attribute, and then we've
272 specified four objects each of which is a member of that class, and each of
273 which gets given a ``light`` attribute as a result of that membership.
275 Why have we gone to this trouble? Three main reasons:
277 * By moving the common bits of the definitions from the individual objects
278 to the class definition which they share, those object definitions
279 become shorter and simpler. Even if we had a hundred rooms, we'd still
280 need to specify ``has light`` only once.
282 * By creating a specialised word to identify our class of objects, we make
283 our source file easier to read. Rather than absolutely everything being
284 an anonymous ``Object``, we can now immediately recognise that some are
285 ``Room`` objects (and others belong to the different classes that we'll
288 * By collecting the common definitions into one place, we make it much
289 easier to make widespread modifications in future. If we need to make
290 some change to the definition of all our rooms, we just modify the
291 ``Room`` class, and all of the class members inherit the change.
293 For these reasons, the use of classes is an incredibly powerful technique,
294 easier than it may look, and very well worth mastering. From now on, we'll
295 be defining object classes whenever it makes sense (which is generally when
296 two or more objects are meant to behave in exactly the same way).
298 You may be wondering: suppose I want to define a room which for some reason
299 *doesn't* have ``light``; can I still use the ``Room`` class? Sure you
302 Room cellar "Gloomy cellar"
303 with description "Your torch shows only cobwebby brick walls.",
306 This illustrates another nice feature of inheritance: the object definition
307 can override the class definition. The class says ``has light``, but the
308 object itself says ``has ~light`` (read that as "has no light") and the
309 object wins. The cellar is dark, and the player will need a torch to see
312 In fact, for any object both the block of properties and the block of
313 attributes are optional and can be omitted if there's nothing to be
314 specified. Now that the ``light`` attribute is being provided
315 automatically and there aren't any other attributes to set, the word
316 ``has`` can be left out. Here's the class again::
321 and here is how we could have used it in "Heidi"::
323 Room "In front of a cottage"
325 "You stand outside a cottage. The forest stretches east.";
327 Room "Deep in the forest"
329 "Through the dense foliage, you glimpse a building to the west.
330 A track heads to the northeast.";
334 You'll notice that, if an object has no block of attributes, the semicolon
335 which terminates its definition simply moves to the end of its last
343 We use the ``Room`` class in "William Tell", and a few other classes
344 besides. Here's a ``Prop`` class (that's "Prop" in the sense of a
345 theatrical property rather than a supportive device), useful for scenic
346 items whose only role is to sit waiting in the background on the off-chance
347 that the player might think to EXAMINE them::
354 print_ret "You don't need to worry about ", (the) self, ".";
358 All objects of this class inherit the ``scenery`` attribute, so they're
359 excluded from room descriptions. Also, there's a ``before`` property; one
360 that's more complex than our previous efforts. You'll remember that the
361 first ``before`` we met looked like this::
365 print "It sounds scared and in need of assistance.^";
369 The role of that original ``before`` was to intercept ``Listen`` actions,
370 while leaving all others well alone. The role of the ``before`` in the
371 ``Prop`` class is broader: to intercept (a) ``Examine`` actions, and (b)
372 all the rest. If the action is ``Examine``, then the ``return false``
373 statement means that the action carries on. If the action is ``default``
374 -- none of those explicitly listed, which in this instance means *every*
375 action apart from ``Examine`` -- then the ``print_ret`` statement is
376 executed, after which the interpreter does nothing further. So, a ``Prop``
377 object can be EXAMINEd, but any other action addressed to it results in a
378 "no need to worry" message.
380 That message is also more involved than anything we've so far displayed.
381 The statement which produces it is::
383 print_ret "You don't need to worry about ", (the) self, ".";
385 which you should read as doing this:
387 #. display the string "You don't need to worry about ",
389 #. display a definite article (usually "the") followed by a space and the
390 external name of the object concerned,
392 #. display a period, and
394 #. display a newline and return true in the usual way for a ``print_ret``
397 The interesting things that this statement demonstrates are:
399 * The ``print`` and ``print_ret`` statements aren't restricted to
400 displaying a single piece of information: they can display a list of
401 items which are separated by commas. The statement still ends with a
402 semicolon in the usual way.
404 * As well as displaying strings, you can also display the names of objects:
405 given the ``nest`` object from our first game, ``(the) nest`` would
406 display "the bird's nest", ``(The) nest`` would display "The bird's
407 nest", ``(a) nest`` would display "a bird's nest", ``(A) nest`` would
408 display "A bird's nest" and ``(name) nest`` would display just "bird's
409 nest". This use of a word in parentheses, telling the interpreter how to
410 display the following object's internal ID, is called a :term:`print
413 * There's a library variable ``self`` which always contains the internal ID
414 of the current object, and is really convenient when using a ``Class``.
415 By using this variable in our ``print_ret`` statement, we ensure that the
416 message contains the name of the appropriate object.
418 Let's see an example of this in action; here's a ``Prop`` object from
421 Prop "south gate" street
422 with name 'south' 'southern' 'wooden' 'gate',
423 description "The large wooden gate in the town walls is wide open.",
426 If players type EXAMINE GATE, they'll see "The large wooden gate..."; if
427 they type CLOSE GATE then the gate's ``before`` property will step in and
428 display "You don't need to worry about the south gate", neatly picking up
429 the name of the object from the ``self`` variable.
431 The reason for doing all this, rather than just creating a simple scenery
432 object like Heidi's ``tree`` and ``cottage``, is to support EXAMINE for
433 increased realism, while clearly hinting to players that trying other verbs
434 would be a waste of time.
436 A class for furniture
437 ---------------------
439 The last class for now -- we'll talk about the ``Arrow`` and ``NPC``
440 classes in the next chapter -- is for furniture-like objects. If you label
441 an object with the ``static`` attribute, an attempt to TAKE it results in
442 "That's fixed in place" -- acceptable in the case of Heidi's branch object
443 (which is indeed supposed to be part of the tree), less so for items which
444 are simply large and heavy. This ``Furniture`` class might sometimes be
449 Take,Pull,Push,PushDir:
450 print_ret (The) self, " is too heavy for that.";
452 has static supporter;
454 Its structure is similar to that of our ``Prop`` class: some appropriate
455 attributes, and a ``before`` property to trap actions directed at it.
456 Again, we display a message which is "personalised" for the object
457 concerned by using a ``(The) self`` print rule. This time we're
458 intercepting four actions; we *could* have written the property like this::
461 Take: print_ret (The) self, " is too heavy for that.";
462 Pull: print_ret (The) self, " is too heavy for that.";
463 Push: print_ret (The) self, " is too heavy for that.";
464 PushDir: print_ret (The) self, " is too heavy for that.";
467 but since we're giving exactly the same response each time, it's better to
468 put all of those actions into one list, separated by commas. ``PushDir``,
469 if you were wondering, is the action triggered by a command like PUSH THE
472 Incidentally, another bonus of defining classes like these is that you can
473 probably reuse them in your next game.
475 Now that most of our class definitions are in place, we can get on with
476 defining some real rooms and objects. First, though, if you're typing in
477 the "William Tell" game as you read through the guide, you'd probably like
478 to check that what you've entered so far is correct;
479 :ref:`compile-as-you-go` explains how to compile the game in its current --