7 | *I was an innkeeper, who loved to carouse;*
8 | *J was a joiner, and built up a house.*
12 .. image:: /images/picI.png
19 n even the simplest story, there's bound to be scope for the player to
20 attempt activities that you hadn't anticipated. Sometimes there may be
21 alternative ways of approaching a problem: if you can't be sure which
22 approach the player will take, you really ought to allow for all
23 possibilities. Sometimes the objects you create and the descriptions you
24 provide may suggest to the player that doing such-and-such should be
25 possible, and, within reason, you ought to allow for that also. The basic
26 game design is easy: what takes the time, and makes a game large and
27 complex, is taking care of all the *other* things that the player may think
30 Here, we try to illustrate what this means by addressing a few of the more
31 glaring deficiencies in our first game.
36 Here's a fragment of the game being played:
38 .. code-block:: transcript
41 Through the dense foliage, you glimpse a building to the west. A track heads
44 You can see a baby bird here.
47 Too young to fly, the nestling tweets helplessly.
50 You hear nothing unexpected.
54 That's not too smart, is it? Our description specifically calls the
55 player's attention to the sound of the bird -- and then she finds out that
56 we've got nothing special to say about its helpless tweeting.
58 The library has a stock of actions and responses for each of the game's
59 defined verbs, so it can handle most of the player's input with a default,
60 standard behaviour instead of remaining impertinently silent or saying that
61 it doesn't understand what the player intends. "You hear nothing
62 unexpected" is the library's standard LISTEN response, good enough after
63 LISTEN TO NEST or LISTEN TO TREE, but fairly inappropriate here; we really
64 need to substitute a more relevant response after LISTEN TO BIRD. Here's
67 .. code-block:: inform
69 Object bird "baby bird" forest
70 with description "Too young to fly, the nestling tweets helplessly.",
71 name 'baby' 'bird' 'nestling',
74 print "It sounds scared and in need of assistance.^";
79 We'll go through this a step at a time:
81 #. We've added a new ``before`` property to our bird object. The
82 interpreter looks at the property *before* attempting to perform any
83 action which is directed specifically at this object::
87 #. The value of the property is an embedded routine, containing a label and
91 print "It sounds scared and in need of assistance.^";
94 #. The label is the name of an action, in this case ``Listen``. What we're
95 telling the interpreter is: if the action that you're about to perform
96 on the bird is a ``Listen``, execute these statements first; if it's any
97 other action, carry on as normal. So, if the player types EXAMINE BIRD,
98 PICK UP BIRD, PUT BIRD IN NEST, HIT BIRD or FONDLE BIRD, then she'll get
99 the standard response. If she types LISTEN TO BIRD, then our two
100 statements get executed before anything else happens. We call this
101 "trapping" or "intercepting" the action of Listening to the bird.
103 #. The two statements that we execute are, first::
105 print "It sounds scared and in need of assistance.^";
107 which causes the interpreter to display the string given in double
108 quotes; remember that a ``^`` character in a string appears as a
109 newline. Second, we execute::
113 which tells the interpreter that it doesn't need to do anything else,
114 because we've handled the ``Listen`` action ourselves. And the game now
115 behaves like this -- perfect:
117 .. code-block:: transcript
120 It sounds scared and in need of assistance.
124 The use of the ``return true`` statement probably needs a bit more
125 explanation. An object's ``before`` property traps an action aimed at that
126 object right at the start, before the interpreter has started to do
127 anything. That's the point at which the statements in the embedded routine
128 are executed. If the last of those statements is ``return true`` then the
129 interpreter assumes that the action has been dealt with by those
130 statements, and so there's nothing left to do: no action, no message;
131 nothing. On the other hand, if the last of the statements is ``return
132 false`` then the interpreter carries on to perform the default action as
133 though it hadn't been intercepted. Sometimes that's what you want it to
134 do, but not here: if instead we'd written this:
136 .. code-block:: inform
138 Object bird "baby bird" forest
139 with description "Too young to fly, the nestling tweets helplessly.",
140 name 'baby' 'bird' 'nestling',
143 print "It sounds scared and in need of assistance.^";
148 then the interpreter would first have displayed our string, and then
149 carried on with its normal response, which is to display the standard
152 .. code-block:: transcript
155 It sounds scared and in need of assistance.
156 You hear nothing unexpected.
160 This technique -- intercepting an action aimed at a particular object in
161 order to do something appropriate for that object -- is one that we'll use
167 At the start of the game the player character stands "outside a cottage", which
168 might lead her to believe that she can go inside:
170 .. code-block:: transcript
172 In front of a cottage
173 You stand outside a cottage. The forest stretches east.
176 You can't go that way.
180 Again, that isn't perhaps the most appropriate response, but it's easy to
183 .. code-block:: inform
185 Object before_cottage "In front of a cottage"
187 "You stand outside a cottage. The forest stretches east.",
189 in_to "It's such a lovely day -- much too nice to go inside.",
190 cant_go "The only path lies to the east.",
193 The ``in_to`` property would normally link to another room, in the same way
194 as the ``e_to`` property contain the internal ID of the ``forest`` object.
195 However, if instead you set its value to be a string, the interpreter
196 displays that string when the player tries the IN direction. Other --
197 unspecified -- directions like NORTH and UP still elicit the standard "You
198 can't go that way" response, but we can change that too, by supplying a
199 ``cant_go`` property whose value is a suitable string. We then get this
200 friendlier behaviour:
202 .. code-block:: transcript
204 In front of a cottage
205 You stand outside a cottage. The forest stretches east.
208 It's such a lovely day -- much too nice to go inside.
211 The only path lies to the east.
218 There's another issue here; since we haven't actually implemented an object
219 to represent the cottage, a perfectly reasonable EXAMINE COTTAGE command
220 receives the obviously nonsensical reply "You can't see any such thing".
221 That's easy to fix; we can add a new ``cottage`` object, making it a piece
222 of ``scenery`` just like the ``tree``:
224 .. code-block:: inform
226 Object cottage "tiny cottage" before_cottage
227 with description "It's small and simple, but you're very happy here.",
228 name 'tiny' 'cottage' 'home' 'house' 'hut' 'shed' 'hovel',
231 This solves the problem, but promptly gives us another unreasonable
234 .. code-block:: transcript
236 In front of a cottage
237 You stand outside a cottage. The forest stretches east.
240 That's not something you can enter.
244 The situation here is similar to our LISTEN TO BIRD problem, and the
245 solution we adopt is similar as well:
247 .. code-block:: inform
249 Object cottage "tiny cottage" before_cottage
250 with description "It's small and simple, but you're very happy here.",
251 name 'tiny' 'cottage' 'home' 'house' 'hut' 'shed' 'hovel',
254 print_ret "It's such a lovely day -- much too nice to go inside.";
258 We use a ``before`` property to intercept the ``Enter`` action applied to
259 the cottage object, so that we can display a more appropriate message.
260 This time, however, we've done it using one statement rather than two. It
261 turns out that the sequence "``print`` a string which ends with a newline
262 character, and then ``return true``" is so frequently needed that there's a
263 special statement which does it all. That is, this single statement (where
264 you'll note that the string *doesn't* need to end in ``^``)::
266 print_ret "It's such a lovely day -- much too nice to go inside.";
268 works exactly the same as this pair of statements::
270 print "It's such a lovely day -- much too nice to go inside.^";
273 We could have used the shorter form when handling LISTEN TO BIRD, and we
274 *will* use it from now on.
279 In the clearing, holding the nest and looking at the tree, the player is
280 meant to type UP. Just as likely, though, she'll try CLIMB TREE (which
281 currently gives the completely misleading response "I don't think much is
282 to be achieved by that"). Yet another opportunity to use a ``before``
283 property, but now with a difference.
285 .. code-block:: inform
287 Object tree "tall sycamore tree" clearing
289 "Standing proud in the middle of the clearing,
290 the stout tree looks easy to climb.",
291 name 'tall' 'sycamore' 'tree' 'stout' 'proud',
294 PlayerTo(top_of_tree);
299 This time, when we intercept the ``Climb`` action applied to the ``tree``
300 object, it's not in order to display a better message; it's because we want
301 to move the player character to another room, just as if she'd typed UP.
302 Relocating the player character is actually quite a complex business, but
303 fortunately all of that complexity is hidden: there's a standard
304 :term:`library routine` to do the job, not one that we've written, but one
305 that's provided as part of the Inform system.
307 You'll remember that, when we first mentioned routines (see
308 :ref:`standalone-routines`), we used the example of ``Initialise()`` and
309 said that "the routine's name followed by opening and closing parentheses
310 is all that it takes to call a routine". That was true for
311 ``Initialise()``, but not quite the whole story. To move the player
312 character, we've got to specify where we want her to go, and we do that by
313 supplying the internal ID of the destination room within the opening and
314 closing parentheses. That is, instead of just ``PlayerTo()`` we call
315 ``PlayerTo(top_of_tree)``, and we describe ``top_of_tree`` as the routine's
318 Although we've moved the player character to another room, we're still in
319 the middle of the intercepted ``Climb`` action. As previously, we need to
320 tell the interpreter that we've dealt with the action, and so we don't want
321 the standard rejection message to be displayed. The ``return true``
322 statement does that, as usual.
324 Dropping objects from the tree
325 ==============================
327 In a normal room like the ``forest`` or the ``clearing``, the player can
328 DROP something she's carrying and it'll effectively fall to the ground at
329 her feet. Simple, convenient, predictable -- except when the player is at
330 the top of the tree. Should she DROP something from up there, having it
331 land nearby might seem a bit improbable; much more likely that it would
332 fall to the clearing below.
334 It looks like we might want to intercept the ``Drop`` action, but not quite
335 in the way we've been doing up until now. For one thing, we don't want to
336 complicate the definitions of the ``bird`` and the ``nest`` and any other
337 objects we may introduce: much better to find a general solution that will
338 work for all objects. And second, we need to recognise that not all
339 objects are droppable; the player can't, for example, DROP THE BRANCH.
341 The best approach to the second problem is to intercept the ``Drop`` action
342 *after* it has occurred, rather than beforehand. That way, we let the
343 library take care of objects which aren't being held or which can't be
344 dropped, and only become involved once a ``Drop`` has been successful. And
345 the best approach to the first problem is to do this particular
346 interception not on an object-by-object basis, as we have been doing so
347 far, but instead for every ``Drop`` which takes place in our troublesome
348 ``top_of_tree`` room. This is what we have to write:
350 .. code-block:: inform
352 Object top_of_tree "At the top of the tree"
353 with description "You cling precariously to the trunk.",
357 move noun to clearing;
362 Let's again take it a step at a time:
364 #. We've added a new ``after`` property to our ``top_of_tree`` object. The
365 interpreter looks at the property *subsequent to* performing any action in
370 #. The value of the property is an embedded routine, containing a label and
374 move noun to clearing;
377 #. The label is the name of an action, in this case ``Drop``. What we're
378 telling the interpreter is: if the action that you've just performed
379 here is a ``Drop``, execute these statements before telling the player
380 what you've done; if it's any other action, carry on as normal.
382 #. The two statements that we execute are first::
384 move noun to clearing;
386 which takes the object which has just been moved from the ``player``
387 object to the ``top_of_tree`` object (by the successful ``Drop`` action)
388 and moves it again so that its parent becomes the ``clearing`` object.
389 That ``noun`` is a library variable that always contains the internal ID
390 of the object which is the target of the current action. If the player
391 types DROP NEST, ``noun`` contains the internal ID of the ``nest``
392 object; if she types DROP NESTLING then ``noun`` contains the internal
393 ID of the ``bird`` object. Second, we execute::
397 which tells the interpreter that it should now let the player know
398 what's happened. Here's the result of all this:
400 .. code-block:: transcript
402 At the top of the tree
403 You cling precariously to the trunk.
405 You can see a wide firm bough here.
412 At the top of the tree
413 You cling precariously to the trunk.
415 You can see a wide firm bough here.
420 A tall sycamore stands in the middle of this clearing. The path winds
421 southwest through the trees.
423 You can see a bird's nest (in which is a baby bird) here.
427 Of course, you might think that the standard message "Dropped" is slightly
428 unhelpful in these non-standard circumstances. If you prefer to hint at
429 what's just happened, you could use this alternative solution:
431 .. code-block:: inform
433 Object top_of_tree "At the top of the tree"
434 with description "You cling precariously to the trunk.",
438 move noun to clearing;
439 print_ret "Dropped... to the ground far below.";
443 The ``print_ret`` statement does two things for us: displays a more
444 informative message, and returns ``true`` to tell the interpreter that
445 there's no need to let the player know what's happened -- we've handled
448 Is the bird in the nest?
449 ========================
451 The game ends when the player character puts the nest onto the branch. Our
452 assumption here is that the bird is inside the nest, but this might not be
453 so; the player may have first taken up the bird and then gone back for the
454 nest, or vice versa. It would be better not to end the game until we'd
455 checked for the bird actually being in the nest; fortunately, that's easy
458 .. code-block:: inform
460 Object branch "wide firm bough" top_of_tree
461 with description "It's flat enough to support a small object.",
462 name 'wide' 'firm' 'flat' 'bough' 'branch',
463 each_turn [; if (bird in nest && nest in branch) deadflag = 2; ],
464 has static supporter;
466 The extended ``if`` statement::
468 if (bird in nest && nest in branch) deadflag = 2;
470 should now be read as: "Test whether the ``bird`` is currently in (or on)
471 the ``nest``, *and* whether the ``nest`` is currently on (or in) the
472 ``branch``; if both parts are ``true``, set the value of ``deadflag`` to 2;
473 otherwise, do nothing".
478 You should by now have some appreciation of the need not only to handle the
479 obvious actions which were at the forefront of your mind when designing the
480 game, but also as many as you can of the other possible ways that a player
481 may choose to interact with the objects presented to her. Some of those
482 ways will be highly intelligent, some downright dumb; in either case you
483 should try to ensure that the game's response is at least sensible, even
484 when you're telling the player "sorry, you can't do that".
486 The new topics that we've encountered here include these:
491 Objects can have a ``before`` property -- if there is one, the interpreter
492 looks at it *before* performing an action which in some way involves that
493 object. Similarly, you can provide an ``after`` property, which the
494 interpreter looks at *after* performing an action but before telling the
495 player what's happened. Both ``before`` and ``after`` properties can be
496 used not only with tangible objects like the ``bird``, ``cottage`` and
497 ``tree`` (when they intercept actions aimed at that particular object) but
498 also with rooms (when they intercept actions aimed at any object in that
501 The value of each ``before`` and ``after`` property is an embedded routine.
502 If such a routine ends with ``return false``, the interpreter then carries
503 on with the next stage of the action which has been intercepted; if it ends
504 with ``return true``, the interpreter does nothing further for that action.
505 By combining these possibilities, you can supplement the work done by a
506 standard action with statements of your own, or you can replace a standard
509 Previously, we've seen connection properties used with the internal ID of
510 the room to which they lead. In this chapter, we showed that the value
511 could also be a string (explaining why movement in that direction isn't
512 possible). Here are examples of both, and also of the ``cant_go`` property
513 which provides just such an explanation for *all* connections that aren't
517 in_to "It's such a lovely day -- much too nice to go inside.",
518 cant_go "The only path lies to the east.",
522 Routines and arguments
523 ----------------------
525 The library includes a number of useful routines, available to perform
526 certain common tasks if you require them; there's a list in
527 :ref:`library-routines`. We used the ``PlayerTo`` routine, which moves the
528 player character from her current room to another one -- not necessarily
529 adjacent to the first room.
531 When calling ``PlayerTo``, we had to tell the library which room is the
532 destination. We did this by supplying that room's internal ID within
537 A value given in parentheses like that is called an :term:`argument` of the
538 routine. In fact, a routine can have more than one argument; if so,
539 they're separated by commas. For example, to move the player character to
540 a room *without* displaying that room's description, we could have supplied
543 PlayerTo(clearing,1);
545 In this example, the effect of the ``1`` is to prevent the description
551 We encountered several new statements:
556 We used these at the end of embedded routines to control what the
557 interpreter did next.
561 ``print_ret "string";``
562 The ``print`` statement simply displays the string of characters
563 represented here by *string*. The ``print_ret`` statement also does
564 that, then outputs a newline character, and finally executes a ``return
567 ``if (condition && condition ) ...``
568 We extended the simple ``if`` statement that we met before. The ``&&``
569 (to be read as "and") is an operator commonly used when testing for
570 more than one condition at the same time. It means "if this condition
571 is true *and* this condition is also true *and* ..." There's also a
572 ``||`` operator, to be read as "or", and a "not" operator ``~~``, which
573 turns true into false and vice versa.
577 In addition, there are ``&``, ``|`` and ``~`` operators, but they do
578 a rather different job and are much less common. Take care not to
581 ``move obj_id to parent_obj_id;``
582 The ``move`` statement rearranges the object tree, by making the first
583 ``obj_id`` a child of the ``parent_obj_id``.
587 We've talked a lot about intercepting actions like ``Listen``, ``Enter``,
588 ``Climb`` and ``Drop``. An action is a generalised representation of
589 something to be done, determined by the verb which the player types. For
590 example, the verbs HEAR and LISTEN are ways of saying much the same thing,
591 and so both result in the same action: ``Listen``. Similarly, verbs like
592 ENTER, GET INTO, SIT ON and WALK INSIDE all lead to an action of ``Enter``,
593 CLIMB and SCALE lead to Climb, and DISCARD, DROP, PUT DOWN and THROW all
594 lead to ``Drop``. This makes life much easier for the designer; although
595 Inform defines quite a lot of actions, there are many fewer than there are
596 ways of expressing those same actions using English verbs.
598 Each action is represented internally by a number, and the value of the
599 current action is stored in a library variable called, erm, ``action``.
600 Two more variables are also useful here: ``noun`` holds the internal ID of
601 the object which is the focus of the action, and ``second`` holds the
602 internal ID of the secondary object (if there is one). Here are some
605 =============================== ====== ======= =======
606 Player types action noun second
607 ------------------------------- ------ ------- -------
608 LISTEN Listen nothing nothing
609 LISTEN TO THE BIRD Listen bird nothing
610 PICK UP THE BIRD Take bird nothing
611 PUT BIRD IN NEST Insert bird nest
612 DROP THE NEST Drop nest nothing
613 PUT NEST ON BRANCH PutOn nest branch
614 =============================== ====== ======= =======
616 The value ``nothing`` is a built-in constant (like ``true`` and ``false``)
617 which means, well, there isn't any object to refer to. There's a list of
618 standard library actions in :ref:`group-1-actions`, :ref:`group-2-actions`
619 and :ref:`group-3-actions`.
621 We've now reached the end of our first game. In these three chapters we've
622 shown you the basic principles on which almost all games are based, and
623 introduced you to many of the components that you'll need when creating
624 more interesting IF. We suggest that you take one last look at the source
625 code (see :doc:`/appendices/b`), and then move on to the next stage.