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<!--Weave of 'How To Include This Module' 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="../services.html">Services</a></li><li><a href="index.html">calculus</a></li><li><a href="index.html#P">Preliminaries</a></li><li><b>How To Include This Module</b></li></ul></div>
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<p class="purpose">What to do to make use of the calculus module in a new command-line tool.</p>
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<ul class="toc"><li><a href="P-htitm.html#SP1">§1. Status</a></li><li><a href="P-htitm.html#SP2">§2. Importing the module</a></li><li><a href="P-htitm.html#SP3">§3. Using callbacks</a></li></ul><hr class="tocbar">
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<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>§1. Status. </b>The calculus module provided as one of the "services" suite of modules, which means
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that it was built with a view to potential incorporation in multiple tools.
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</p>
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<p class="commentary">By convention, the modules considered as "services" have no dependencies on
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other modules except for <a href="../../../inweb/foundation-module/index.html" class="internal">foundation</a> and other "services" modules.
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</p>
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<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>§2. Importing the module. </b>We'll use the term "parent" to mean the tool which is importing <a href="index.html" class="internal">calculus</a>,
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that is, which will include its code and be able to use it. As with any
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imported module,
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</p>
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<ul class="items"><li>● The contents page of the parent's web must identify and locate the
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module:
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</li></ul>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="element-syntax">Import</span><span class="plain-syntax">:</span><span class="string-syntax"> somepath/calculus</span>
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</pre>
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<ul class="items"><li>● The parent must call <span class="extract"><span class="extract-syntax">CalculusModule::start()</span></span> just after it starts up, and
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<span class="extract"><span class="extract-syntax">CalculusModule::end()</span></span> just before it shuts down. (But just after, and just
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before, the corresponding calls to <a href="../../../inweb/foundation-module/index.html" class="internal">foundation</a>.)
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</li></ul>
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<p class="commentary firstcommentary"><a id="SP3" class="paragraph-anchor"></a><b>§3. Using callbacks. </b>Shared modules like this one are tweaked in behaviour by defining "callback
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functions". This means that the parent might provide a function of its own
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which would answer a question put to it by the module, or take some action
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on behalf of the module: it's a callback in the sense that the parent is
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normally calling the module, but then the module calls the parent back to
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ask for data or action.
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</p>
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<p class="commentary">The parent must indicate which function to use by defining a constant with
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a specific name as being equal to that function's name. A fictional example
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would be
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax"> </span><span class="function-syntax">@d</span><span class="plain-syntax"> EXPRESS_SURPRISE_CALCULUS_CALLBACK Emotions::zowie</span>
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<span class="plain-syntax"> =</span>
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<span class="plain-syntax"> </span><span class="element-syntax">void Emotions:</span><span class="plain-syntax">:</span><span class="string-syntax">zowie(text_stream *OUT) {</span>
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<span class="plain-syntax"> WRITE("Zowie!\n");</span>
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<span class="plain-syntax"> }</span>
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</pre>
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<p class="commentary">The calculus module has only a few callbacks and all are optional. The
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following alphabetical list has references to fuller explanations:
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</p>
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<ul class="items"><li>● <span class="extract"><span class="extract-syntax">BINARY_PREDICATE_CREATED_CALCULUS_CALLBACK</span></span>, if provided, is called whenever
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a binary predicate is created. (The Inform compiler uses this opportunity to
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register its textual name, if it has one.) See <a href="3-bp.html#SP10" class="internal">BinaryPredicates::make_pair</a>.
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</li><li>● <span class="extract"><span class="extract-syntax">DETECT_NOTHING_CALCULUS_CALLBACK</span></span>. The Inform run-time has a constant called
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<span class="extract"><span class="extract-syntax">nothing</span></span> which acts as a default value for objects, indicating an absence of
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any object. The existence of this enables some propositions to be simplified.
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If provided, the function should test whether the supplied constant is <span class="extract"><span class="extract-syntax">nothing</span></span>.
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See <a href="5-smp.html#SP2" class="internal">Simplifications::nothing_constant</a> and <a href="4-bas.html#SP11" class="internal">Binding::substitute_nothing_in_term</a>.
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</li><li>● <span class="extract"><span class="extract-syntax">PROBLEM_CALCULUS_CALLBACK</span></span> is called when a proposition is mis-constructed,
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and can prevent the resulting warning from being issued to the terminal as an
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error message: see <a href="4-tcp.html#SP11" class="internal">TypecheckPropositions::problem</a>.
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</li><li>● <span class="extract"><span class="extract-syntax">PRODUCE_NOTHING_VALUE_CALCULUS_CALLBACK</span></span> can provide the <span class="extract"><span class="extract-syntax">nothing</span></span> constant;
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see above, and see <a href="5-smp.html#SP14" class="internal">Simplifications::not_related_to_something</a>.
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</li></ul>
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<p class="commentary firstcommentary"><a id="SP4" class="paragraph-anchor"></a><b>§4. </b>In addition, the following value can optionally be defined:
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</p>
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<ul class="items"><li>● <span class="extract"><span class="extract-syntax">VERB_MEANING_UNIVERSAL_CALCULUS_RELATION</span></span> should be the universal relation,
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that is, the binary predicate which defines "relates". This requires specialist
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type-checking: see <a href="4-tcp.html#SP9" class="internal">TypecheckPropositions::type_check_binary_predicate</a>.
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</li></ul>
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