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<TR><TD Valign="top"><A HREF="contents.html">Contents</A><BR><A HREF="sectionA3.html">Back</A><BR><A HREF="sectionA5.html">Forward</A><TD bgcolor="#F5DEB3"><BLOCKQUOTE><H3>A4. Grammar</H3></BLOCKQUOTE><TR><TD><TD>

A 'verb' is a set of possible initial words in keyboard command, which are

treated synonymously (for example, "wear'' and "don'') together with

a 'grammar'. A grammar is a list of 'lines' which the parser tries to match,

one at a time, and accepts the first one which matches. The directive

Verb [<TT>meta</TT>] <I><B><verb-word-1></B></I> ... <I><B><verb-word-<I>n</I>></B></I> <I><B><grammar></B></I>

</BLOCKQUOTE>

creates a new verb. If it is said to be <TT>meta</TT> then it will count as 'out

of the game': for instance "score'' or "save''. New synonyms can be added

to an old verb with:

Verb <I><B><new-word-1></B></I> ... <I><B><new-word-<I>n</I>></B></I> <TT>=</TT> <I><B><existing-verb-word></B></I>

</BLOCKQUOTE>

An old verb can be modified with the directive

Extend [<TT>only</TT>] <I><B><existing-word-1></B></I> ... <I><B><existing-word-<I>n</I>></B></I> [<I><B><priority></B></I>] <I><B><grammar></B></I>

</BLOCKQUOTE>

If <TT>only</TT> is specified, the existing words given (which must all be from the same

existing verb) are split off into a new independent copy of the verb.

If not, the directive extends the whole existing verb.

The priority can be <TT>first</TT> (insert

this grammar at the head of the list), <TT>last</TT> (insert it at the end) or <TT>replace</TT>

(throw away the old list and use this instead); the default is <TT>last</TT>.

<P>

A line is a list of 'tokens' together with the action generated if each

token matches so that the line is accepted. The syntax of a line is

</BLOCKQUOTE>

where

0 <= n <= 31.

The action is named without initial <TT>##</TT> signs and if

an action which isn't in the standard library set is named then an action

routine (named with the action name followed by <TT>Sub</TT>) must be defined

somewhere in the game.

<P>

A grammar line can optionally be followed by the word

<TT>reverse</TT>. This signals that the action to be generated has

two parameters, but which have been parsed in the wrong

order and need to swapped over. (Note that a <TT>topic</TT> is

not a parameter, and nor is a preposition.)

<P>

A token matches a single particle of what has been typed. The possible

tokens are:

<BR><TABLE Border><TR><TD> <TABLE Border><TR><TD><TT>"</TT><I><B><word></B></I><TT>"</TT></TABLE> <TD> that literal word only

<TR><TD> <TABLE Border><TR><TD><TT>noun</TT></TABLE> <TD> any object in scope

<TR><TD> <TABLE Border><TR><TD><TT>held</TT></TABLE> <TD> object held by the player

<TR><TD> <TABLE Border><TR><TD><TT>multi</TT></TABLE> <TD> one or more objects in scope

<TR><TD> <TABLE Border><TR><TD><TT>multiheld</TT></TABLE> <TD> one or more held objects

<TR><TD> <TABLE Border><TR><TD><TT>multiexcept</TT></TABLE> <TD> one or more in scope, except the other

<TR><TD> <TABLE Border><TR><TD><TT>multiinside</TT></TABLE> <TD> one or more in scope, inside the other

<TR><TD> <TABLE Border><TR><TD><I><B><attribute></B></I></TABLE> <TD> any object in scope which has the attribute

<TR><TD> <TABLE Border><TR><TD><TT>creature</TT></TABLE> <TD> an object in scope which is <TT>animate</TT>

<TR><TD> <TABLE Border><TR><TD><TT>noun = </TT><I><B><Routine></B></I></TABLE> <TD> any object in scope passing the given test

<TR><TD> <TABLE Border><TR><TD><TT>scope = </TT><I><B><Routine></B></I></TABLE> <TD> an object in this definition of scope

<TR><TD> <TABLE Border><TR><TD><TT>number</TT></TABLE> <TD> a number only

<TR><TD> <TABLE Border><TR><TD><I><B><Routine></B></I></TABLE> <TD> refer to this general parsing routine

<TR><TD> <TABLE Border><TR><TD><TT>topic</TT></TABLE> <TD> any text at all

<TR><TD> <TABLE Border><TR><TD><TT>special</TT></TABLE> <TD> any single word or number

</TABLE>

<P>

Two or more literal words (only) can be written with slash

signs <TT>/</TT> between them as alternatives. E.g., <TT>"in"/"on"</TT>

matches either the word "in'' or the word "on''.

<P>

For the <TABLE Border><TR><TD><TT>noun = </TT><I><B><Routine></B></I></TABLE> token,

the test routine must decide whether or not the

object in the <TT>noun</TT> variable is acceptable and return true or false.

<P>

For the <TABLE Border><TR><TD><TT>scope = </TT><I><B><Routine></B></I></TABLE> token,

the routine must look at the variable <TT>scope_stage</TT>.

If this is 1, then it must decide whether or not to allow a multiple object

(such as "all'') here and return true or false. If 2, then the routine may

put objects into scope by calling either <TT>PlaceInScope(obj)</TT> to put just <TT>obj</TT>

in, or <TT>ScopeWithin(obj)</TT> to put the contents of <TT>obj</TT> into scope. It must

then return either true (to prevent any other objects from entering scope) or

false (to let the parser put in all the usual objects). If <TT>scope_stage=3</TT>,

it must print a suitable message to tell the player that this token was

misunderstood.

<P>

A general parsing routine can match any text it likes. It should use <TT>wn</TT>,

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the variable holding the number of the word currently being parsed (counting

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from the verb being word 1) and the routine <TT>NextWord()</TT> to read the next

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word and move <TT>wn</TT> on by 1. The routine returns:

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-1<SAMP> </SAMP> if the user's input isn't understood,<BR>

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0<SAMP> </SAMP> if it's understood but doesn't refer to anything,<BR>

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1<SAMP> </SAMP> if there is a numerical value resulting, or<BR>

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<I>n</I><SAMP> </SAMP> if object <I>n</I> is understood.<BR>

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</BLOCKQUOTE>

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In the case of a number, the actual value should be put into the variable

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<TT>parsed_number</TT>.

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On an unsuccessful match (returning -1) it doesn't matter what the final

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value of <TT>wn</TT> is. Otherwise it should be left pointing to the next

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thing <I> after</I> what the routine understood.

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<P>

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</TABLE>

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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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