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<!--Weave of 'Introduction to Grammar' generated by Inweb-->
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<div class="breadcrumbs">
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<ul class="crumbs"><li><a href="../index.html">Home</a></li><li><a href="../compiler.html">Inform7</a></li><li><a href="index.html">if</a></li><li><a href="index.html#5">Chapter 5: Command Grammar</a></li><li><b>Introduction to Grammar</b></li></ul></div>
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<p class="purpose">An exposition of the data structures and basic method used to deal with the command-parsing grammar implied by Understand sentences in the source text.</p>
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<ul class="toc"><li><a href="5-itg.html#SP2">§2. Phase I: Slash Grammar</a></li><li><a href="5-itg.html#SP3">§3. Phase II: Determining Grammar</a></li><li><a href="5-itg.html#SP4">§4. Phase III: Sort Grammar</a></li><li><a href="5-itg.html#SP5">§5. Phase IV: Compile Grammar</a></li></ul><hr class="tocbar">
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<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>§1. </b>This is grammar in the sense of the parsing structures used at run-time,
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and it occupies a chapter of its own in the source code since it is to some
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extent detached from the rest of Inform: what we create in this chapter is
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almost an independent compiler in its own right, but of a much simpler
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language. Although we use many higher-level features of Inform in the process,
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none use this.
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</p>
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<p class="commentary">Grammar is organised in a three-level hierarchy:
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</p>
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<ul class="items"><li>(a) A grammar verb (GV) is a small independent grammar of alternative
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formulations for some concept: for instance, the possible commands beginning
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TAKE, or the possible verbal forms of numbers. Each GV is a list of GLs, and
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an individual GL must belong to exactly one GV. There are five different
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types of GV, differentiated mostly by the purpose to which the GV is put:
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<ul class="items"><li>(1) <span class="extract"><span class="extract-syntax">GV_IS_COMMAND</span></span>. An imperative verbal command at run-time.
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</li><li>(2) <span class="extract"><span class="extract-syntax">GV_IS_TOKEN</span></span>. A square-bracketed token in other grammar.
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</li><li>(3) <span class="extract"><span class="extract-syntax">GV_IS_OBJECT</span></span>. A noun phrase at run time: a name for an object.
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</li><li>(4) <span class="extract"><span class="extract-syntax">GV_IS_VALUE</span></span>. A noun phrase at run time: a name for a value.
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</li><li>(5) <span class="extract"><span class="extract-syntax">GV_IS_CONSULT</span></span>. A pattern to match in part of a command (such as "consult").
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</li><li>(6) <span class="extract"><span class="extract-syntax">GV_IS_PROPERTY_NAME</span></span>. A noun phrase at run time: a name for one
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possibility for an either/or property, say "open" or "fixed in place".
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</li></ul>
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<li>(b) A grammar line (GL) is a single possibility within a GV: for
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example, the line matching <span class="extract"><span class="extract-syntax">"take [something]"</span></span> in the GV for the TAKE
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command. Each GL is a list of tokens, and an individual token must belong
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to exactly one GL.
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</li><li>(c) A grammar token (GTOK) is a single particle of a GL: for
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example, <span class="extract"><span class="extract-syntax">'take'</span></span> and <span class="extract"><span class="extract-syntax">something</span></span> are tokens.
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</li></ul>
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<p class="commentary">The picture is not quite so hierarchical as it looks, though, because
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a GV naming a token can be used as a token inside other GVs. We need to
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be careful that this does not lead to infinite regress: see below.
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</p>
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<p class="commentary">Much of what we do with grammar involves recursing down this hierarchy,
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in some cases allowing results to percolate back upwards. What happens
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takes place in four chronological phases. (This division into phases is
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convenient because Inform 6 requires that all general parsing routines
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and noun filter routines already exist when a <span class="extract"><span class="extract-syntax">Verb</span></span> directive is reached
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which uses them.)
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</p>
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<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>§2. Phase I: Slash Grammar. </b>Slashing is the process of dealing with slashes <span class="extract"><span class="extract-syntax">/</span></span> used in grammar
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to indicate alternatives.
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</p>
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<p class="commentary firstcommentary"><a id="SP3" class="paragraph-anchor"></a><b>§3. Phase II: Determining Grammar. </b>We check that the grammar is well-founded and find the types of values
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expressed by it, if any.
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</p>
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<p class="commentary">Determining well-foundedness means checking that no two grammar tokens
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each require the use of the other, and that when a grammar token takes
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several alternative forms, they have compatible results: so, for instance,
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you can't have one version resulting in a number and another in a thing.
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(This check is only meaningful for grammar verbs of type <span class="extract"><span class="extract-syntax">GV_IS_TOKEN</span></span>.)
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</p>
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<p class="commentary">The result of a <span class="extract"><span class="extract-syntax">GV_IS_TOKEN</span></span> is a single specification, which is the union of the
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kinds resulting from its grammar lines. This is a more sophisticated approach
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than we really need here, but might be useful for future expansion.
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</p>
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<p class="commentary">Of the determining traverse the following can be said:
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</p>
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<ul class="items"><li>(a) either errors are produced, or it is verified that no token's
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definition depends directly or indirectly on already knowing itself;
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</li><li>(b) also that no grammar line attached to a <span class="extract"><span class="extract-syntax">GV_IS_COMMAND</span></span> produces
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more than 2 values, and that no grammar line attached to anything else
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produces more than one; and
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</li><li>(c) also that the grammar lines attached to a <span class="extract"><span class="extract-syntax">GV_IS_TOKEN</span></span> are
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compatible in that there is a type to which they can all always be cast.
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</li></ul>
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<p class="commentary">We note of the determining routines that:
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</p>
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<ul class="items"><li>(a) <span class="extract"><span class="extract-syntax">PL::Parsing::Verbs::determine</span></span> runs at least once for each GV;
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</li><li>(b) <span class="extract"><span class="extract-syntax">PL::Parsing::Lines::gl_determine</span></span> runs exactly once on each GL;
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</li><li>(c) <span class="extract"><span class="extract-syntax">PL::Parsing::Tokens::determine</span></span> runs exactly once on each token.
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</li></ul>
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<p class="commentary firstcommentary"><a id="SP4" class="paragraph-anchor"></a><b>§4. Phase III: Sort Grammar. </b>We must ensure that if grammar line L1 is logically impossible once L2
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has been parsed, then L1 must be checked before L2, regardless of the
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ordering in the source code. Since the data sets are very small and
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time is not of the essence, we simply insertion-sort the original
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definition-order list into a second linked list.
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</p>
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<p class="commentary firstcommentary"><a id="SP5" class="paragraph-anchor"></a><b>§5. Phase IV: Compile Grammar. </b>The final run-through, which uses the sorted order and not the original
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declaration order, actually compiles the necessary I6 <span class="extract"><span class="extract-syntax">Verb</span></span> and
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<span class="extract"><span class="extract-syntax">Extend</span></span> directives.
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</p>
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