1 ======================================
2 Appendix C -- "William Tell" story
3 ======================================
7 .. image:: /images/picW.png
14 illiam Tell, our second game, is also very straightforward. See "William
15 Tell: a tale is born" on page 69, "William Tell: the early years" on
16 page 79, "William Tell: in his prime" on page 91 and "William Tell: the
17 end is nigh" on page 103.
22 .. literalinclude:: /examples/Tell.txt
28 .. literalinclude:: /examples/Tell.inf
34 Your understanding of how the "William Tell" game works will be
35 considerably enhanced if you type in the code for yourself as you read
36 through the guide. However, it takes us four chapters to describe the
37 game, which isn't complete and playable until the end of Chapter 9. Even
38 if you make no mistakes in your typing, the game won't compile without
39 errors before that point, because of references in earlier chapters to
40 objects which aren't presented until later chapters (for example,
41 Chapter 6 mentions the ``bow`` and ``quiver`` objects, but we don't
42 define them until Chapter 7). This is a bit of a nuisance, because as a
43 general rule we advise you to compile frequently -- more or less after
44 every change you make to a game -- in order to detect syntax and
45 spelling mistakes as soon as possible.
47 Fortunately, there's a fairly easy way round the difficulty, though it
48 involves a little bit of cheating. The trick is temporarily to add
49 minimal definitions -- often called "stubs" -- of the objects whose full
50 definitions have yet to be provided.
52 For example, if you try to compile the game in the state that it's
53 reached by the end of Chapter 6, you’ll get this::
55 Tell.inf(16): Warning: Class "Room" declared but not used
56 Tell.inf(19): Warning: Class "Prop" declared but not used
57 Tell.inf(27): Warning: Class "Furniture" declared but not used
58 Tell.inf(44): Error: No such constant as "street"
59 Tell.inf(46): Error: No such constant as "bow"
60 Tell.inf(47): Error: No such constant as "quiver"
61 Compiled with 3 errors and 3 warnings
63 However, by adding these lines to the end of your game file::
65 ! ===============================================================
66 ! TEMPORARY DEFINITIONS NEEDED TO COMPILE AT THE END OF CHAPTER 6
72 a compilation should now give only this::
74 Tell.inf(19): Warning: Class "Prop" declared but not used
75 Tell.inf(27): Warning: Class "Furniture" declared but not used
76 Compiled with 2 warnings
78 That's a lot better. It's not worth worrying about those warnings, since
79 it's easy to understand where they come from; anyway, they'll go away
80 shortly. More important, there are no errors, which means that you've
81 probably not made any major typing mistakes. It also means that the
82 compiler has created a story file, so you can try "playing" the game. If
83 you do, though, you'll get this::
86 A simple Inform example
87 by Roger Firth and Sonja Kesserich.
88 Release 3 / Serial number 040804 / Inform v6.30 Library 6/11 SD
91 ** Library error 11 (27,0) **
92 ** The room "(street)" has no "description" property **
95 Whoops! We've fallen foul of Inform's rule saying that every room must
96 have a ``description`` property, to be displayed by the interpreter when
97 you enter that room. Our ``street`` stub hasn't got a ``description``,
98 so although the game compiles successfully, it still causes an error to
99 be reported at run-time.
101 The best way round this is to extend the definition of our ``Room``
105 with description "UNDER CONSTRUCTION",
108 By doing this, we ensure that *every* room has a description of some
109 form; normally we'd override this default value with something
110 meaningful -- "The narrow street runs north towards the town square..."
111 and so on -- by including a ``description`` property in the object's
112 definition. However, in a stub object used only for testing, a default
113 description is sufficient (and less trouble)::
116 A simple Inform example
117 by Roger Firth and Sonja Kesserich.
118 Release 3 / Serial number 040804 / Inform v6.30 Library 6/11 SD
125 a (quiver) (being worn)
129 You can't see any such thing.
133 You'll notice a couple of interesting points. Because we didn't supply
134 external names with our ``street`` , ``bow`` and ``quiver`` stubs, the
135 compiler has provided some for us -- ``(street)`` , ``(bow)`` and
136 ``(quiver)`` -- simply by adding parentheses around the internal IDs
137 which we used. And, because our ``bow`` and ``quiver`` stubs have no
138 ``name`` properties, we can't actually refer to those objects when
139 playing the game. Neither of these points would be acceptable in a
140 finished game, but for testing purposes at this early stage -- they'll
143 So far, we've seen how the addition of three temporary object
144 definitions enables us to compile the incomplete game, in its state at
145 the end of Chapter 6. But once we reach the end of Chapter 7, things
146 have moved on, and we now need a different set of stub objects. For a
147 test compilation at this point, remove the previous set of stubs, and
148 instead add these -- ``south_square`` and ``apple`` objects, and a dummy
149 action handler to satisfy the ``Talk`` action in Helga’s life property::
151 ! ===============================================================
152 ! TEMPORARY DEFINITIONS NEEDED TO COMPILE AT THE END OF CHAPTER 7
159 Similarly, at the end of Chapter 8, replace the previous stubs by these
160 if you wish to check that the game compiles::
162 ! ===============================================================
163 ! TEMPORARY DEFINITIONS NEEDED TO COMPILE AT THE END OF CHAPTER 8
172 Finally, by the end of Chapter 9 the game is complete, so you can delete
173 the stubs altogether.
175 Used with care, this technique of creating a few minimal stub objects
176 can be convenient -- it enables you to "sketch" a portion of your game
177 in outline form, and to compile and test the game in that state, without
178 needing to create complete object definitions. However, you've got very
179 little interaction with your stubs, so don't create too many of them.
180 And of course, never forget to flesh out the stubs into full definitions