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<HTML><HEAD><TITLE>Section 21: Extending and redefining the Library</TITLE></HEAD>
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<BODY BGCOLOR="#FFFFFF">
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<TR><TD Valign="top"><A HREF="contents.html">Contents</A><BR><A HREF="section20.html">Back</A><BR><A HREF="chapter5.html">Forward</A><TD bgcolor="#F5DEB3"><BLOCKQUOTE><H3>21. Extending and redefining the Library</H3></BLOCKQUOTE><TR><TD><TD>
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A circulating library in a town is as an ever-green
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tree of diabolical knowledge! It blossoms through the year!
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<P>...R. B. Sheridan (<B>1751</B>--<B>1816</B>), <I>The Rivals</I></BLOCKQUOTE>
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Most large games will need to enrich the 'model world': for
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instance, by creating a new concept such as "magic amulets''. The game
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might contain a dozen of these, each with the power to cast a different
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spell. So it will need routines which can tell whether or not a given
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object is an amulet, and what to do when the spell is cast.
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To do this, a game should make a class definition for amulets:
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called <TT>Amulet</TT>, say. Then
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if (noun ofclass Amulet) ...
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will test to see if <TT>noun</TT> is one of the amulets, for instance.
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The amulet's spell will be represented by the property <TT>amulet_spell</TT>.
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Typical values for this might be:
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amulet_spell "The spell fizzles out with a dull phut! sound.",
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[; if (location == thedark)
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{ give real_location light;
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"There is a burst of magical light!";
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amulet_spell HiddenVault,
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[; return random(LeadRoom, SilverRoom, GoldRoom);
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Then the process of casting the spell for amulet <TT>X</TT> is a matter of
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which will reply with either: false, meaning nothing has happened;
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true, meaning that something did happen; or an object, a room to
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teleport the player to. Here is a routine which deals with it all:
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[ CastSub destination;
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if (noun ofclass Amulet)
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{ if (~~(noun provides amulet_spell))
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"[Ooops. Forgot to program this amulet_spell.]";
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destination = noun.amulet_spell();
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{ false: "Nothing happens.";
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default: print "You are magically teleported to...^";
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PlayerTo(destination);
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else "You only know how to cast spells with amulets.";
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An elaborate library extension will end up defining many classes,
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grammar, actions and verb definitions. These may neatly be packaged
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up into an <TT>Include</TT> file and placed with the other library
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<P><TR><TD Valign="top"><IMG SRC="icons/ddbend.gif" ALT="/\/\"><TD bgcolor="#EEEEEE"><SMALL> If this file contains the directive <TT>System_file;</TT> then it will
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even be possible for games to <TT>Replace</TT> routines from it
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<P><TR><TD Valign="top"><IMG SRC="icons/dbend.gif" ALT="/\"><TD bgcolor="#EEEEEE"><SMALL> The ordinary Library's own properties, such as <TT>description</TT>
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or <TT>e_to</TT>, are called "common properties''. They are special for the
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following reason: if an object <TT>O</TT> does not give any value for common
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property <TT>P</TT>, then <TT>O.P</TT> can still be looked up, though it can't be
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set to something else. (If you tried this with a property of your
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own invention, such as <TT>amulet_spell</TT> above, an error would be printed
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out at run-time.) The value of <TT>O.P</TT> is just the "default value''
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provided by the Library for property <TT>P</TT>: for example, the default
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value of <TT>cant_go</TT> is "You can't go that way.''
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<P><TR><TD Valign="top"><IMG SRC="icons/dbend.gif" ALT="/\"><TD bgcolor="#EEEEEE"><SMALL> But you can change this default value during play, using the
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library's <TT>ChangeDefault</TT> routine. For instance, at a late stage
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ChangeDefault(cant_go, "You're a Master Adventurer now, and still
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you walk into walls!");
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Of course this cannot change defaults for properties of your own
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invention, because they haven't got default values.
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<P><TR><TD Valign="top"><IMG SRC="icons/ddbend.gif" ALT="/\/\"><TD bgcolor="#EEEEEE"><SMALL> Common properties are also slightly faster to perform
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calculations with: the down side is that there's a strictly limited
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supply of them (63 in all), of which the library uses up half
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already. To indicate that a property needs to be a common property,
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use the <TT>Property</TT> directive. For example:
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Property capacity 100;
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Property cant_go "You can't go that way.";
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In the latter cases we are giving default values: in the former case,
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the default value will just be 0.
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Major library extensions are rarely needed.
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More often, one would like simply to change the stock of
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standard messages, such as the "Nothing is on sale.'' which tends
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to be printed when the player asks to buy something, or the "Taken.''
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printed when something is picked up.
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This facility is available as follows. Provide a special
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object called <TT>LibraryMessages</TT>, which must be defined <I> between</I>
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the inclusion of the "Parser" and "VerbLib" library files.
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This object should have just one property, a <TT>before</TT> rule. For
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Object LibraryMessages
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[; Jump: "You jump and float uselessly for a while in
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the zero gravity here on Space Station Alpha.";
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{ print "You power up ", (the) lm_o, "."; }
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The object never physically appears in the game, of course. The idea
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is that the <TT>before</TT> rule is consulted before any message is printed:
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if it returns false, the standard message is printed; if true, then
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nothing is printed, as it's assumed that this has already happened.
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The <TT>Jump</TT> action only ever prints one message (usually "You jump on
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the spot.''), but more elaborate actions such as <TT>SwitchOn</TT> have
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several (the extreme case is <TT>Take</TT>, with 13). <TT>lm_n</TT> holds the
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message number, which counts upwards from 1. The messages and
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numbers are given in <A HREF="sectionA9.html">Appendix A9</A>. New message numbers may possibly be
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added in future, but old ones will not be renumbered.
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An especially useful library message to change is the prompt, normally
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set to <TT>"^>"</TT> (new-line followed by <TT>></TT>). This is printed under the
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action <TT>Prompt</TT> (actually a fake action existing for exactly this
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purpose). In this way, the game's prompt can be made context-sensitive,
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or the "skipped line on screen each turn'' convention can be removed.
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<P><TR><TD Valign="top"><IMG SRC="icons/dbend.gif" ALT="/\"><TD bgcolor="#EEEEEE"><SMALL>
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This prompt is only used in ordinary game play, and not at such keyboard
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inputs as yes/no questions or the RESTART/RESTORE/QUIT game over
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<P><TR><TD Valign="top"><IMG SRC="icons/exercise.gif" ALT="??"><TD bgcolor="#FBB9AC"><A NAME="ex47"><B>EXERCISE 47:</B><BR>(link to <A HREF="answers1/answer47.html">the answer</A>)<TR><TD><TD> Infocom's game 'The Witness' has the prompt "What should you,
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the detective, do next?'' on turn one and "What next?'' subsequently.
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<P><TR><TD Valign="top"><IMG SRC="icons/ddbend.gif" ALT="/\/\"><TD bgcolor="#EEEEEE"><SMALL> An amusing way to see the system in action is to put
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Object LibraryMessages
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[; print "[", sw__var, ", ", lm_n, "] ";
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into your game (arcane note: <TT>sw__var</TT>, the "switch variable'', in
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this case holds the action number). Another amusing effect is to
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simply write <TT>rtrue;</TT> for the <TT>before</TT> routine, which results in an
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alarmingly silent game -- blindfold Adventure, perhaps.
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<P><TR><TD Valign="top"><IMG SRC="icons/ddbend.gif" ALT="/\/\"><TD bgcolor="#EEEEEE"><SMALL>
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Note that <TT>LibraryMessages</TT> can be used as a sneaky way to add extra
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rules onto the back of actions, since there's nothing to stop you
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doing real processing in a call to it; or, more happily,
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to make messages more sensitive to game context, so that "Nothing is
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on sale.'' might become "That's not one of the goods on sale.''
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inside a shopping mall.
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<P><TR><TD Valign="top"><IMG SRC="icons/ddexercise.gif" ALT="??/\/\"><TD bgcolor="#FBB9AC"><A NAME="ex48"><B>EXERCISE 48:</B><BR>(link to <A HREF="answers1/answer48.html">the answer</A>)<TR><TD><TD>
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Write an Inform game in Occitan (a dialect of medieval French
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The Library is itself written in Inform, and with experience it's not
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too hard to alter it if need be. But this is an inconvenience and an
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inelegant way to carry on. So here is the last resort in library
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modification: work out which routine is giving trouble, and <TT>Replace</TT>
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it. For example, if the directive
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is placed in your file <I> before the library files are included</I>,
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Inform ignores the definition of <TT>BurnSub</TT> in the library files.
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You then have to define a routine called <TT>BurnSub</TT> yourself.
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It would be normal to copy the definition of <TT>BurnSub</TT> out of the
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library files into your own code, and then modify that copy as needed.
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The most popular routine to replace is <TT>DrawStatusLine</TT>: see <A HREF="section33.html">Section 33</A> for
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<P><TR><TD Valign="top"><IMG SRC="icons/ddbend.gif" ALT="/\/\"><TD bgcolor="#EEEEEE"><SMALL>
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Inform even allows you to <TT>Replace</TT> "hardware'' functions like <TT>random</TT>,
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which would normally be translated directly to machine opcodes. Obviously,
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replacing something like <TT>child</TT> with a software routine will impose an
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appreciable speed penalty and slightly increase object code size. Replacing
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<TT>random</TT> may however be useful when fixing the random number generator for
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game-testing purposes.
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<P><TR><TD Valign="top"><IMG SRC="icons/refs.gif" ALT="*"><TD bgcolor="#EEEEEE"><B>REFERENCES:</B><BR><SMALL>
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'Balances' contains a section of code (easily extractable to
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other games) implementing the 'Enchanter' trilogy's magic
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system by methods like the above.
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There are several formal library extension files in existence,
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mostly small: see the Inform home page on the WWW.
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by Andrew Clover makes large-scale use of <TT>LibraryMessages</TT>
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to ensure that the library always uses words like "those''
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instead of "that'' when talking about objects with names like
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"a heap of magazines''.
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<HR><A HREF="contents.html">Contents</A> / <A HREF="section20.html">Back</A> / <A HREF="chapter5.html">Forward</A> <BR>
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<A HREF="chapter1.html">Chapter I</A> / <A HREF="chapter2.html">Chapter II</A> / <A HREF="chapter3.html">Chapter III</A> / <A HREF="chapter4.html">Chapter IV</A> / <A HREF="chapter5.html">Chapter V</A> / <A HREF="chapter6.html">Chapter VI</A> / <A HREF="chapterA.html">Appendix</A><HR><SMALL><I>Mechanically translated to HTML from third edition as revised 16 May 1997. Copyright © Graham Nelson 1993, 1994, 1995, 1996, 1997: all rights reserved.</I></SMALL></BODY></HTML>
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