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<title>What This Module Does</title>
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<nav role="navigation">
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<h1><a href="../index.html">
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<img src="../docs-assets/Inform.png" height=72">
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</a></h1>
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<ul><li><a href="../index.html">home</a></li>
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</ul><h2>Compiler</h2><ul>
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<li><a href="../structure.html">structure</a></li>
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<li><a href="../inbuildn.html">inbuild</a></li>
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<li><a href="../inform7n.html">inform7</a></li>
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<li><a href="../intern.html">inter</a></li>
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<li><a href="../services.html">services</a></li>
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<li><a href="../secrets.html">secrets</a></li>
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</ul><h2>Other Tools</h2><ul>
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<li><a href="../inblorbn.html">inblorb</a></li>
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<li><a href="../indocn.html">indoc</a></li>
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<li><a href="../inform6.html">inform6</a></li>
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<li><a href="../inpolicyn.html">inpolicy</a></li>
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</ul><h2>Resources</h2><ul>
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<li><a href="../extensions.html">extensions</a></li>
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<li><a href="../kits.html">kits</a></li>
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</ul><h2>Repository</h2><ul>
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<li><a href="https://github.com/ganelson/inform"><img src="../docs-assets/github.png" height=18> github</a></li>
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</ul><h2>Related Projects</h2><ul>
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<li><a href="../../../inweb/index.html">inweb</a></li>
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<li><a href="../../../intest/index.html">intest</a></li>
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</ul>
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</nav>
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<main role="main">
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<!--Weave of 'What This Module Does' 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="../inform7n.html">Inform7</a></li><li><a href="index.html">knowledge</a></li><li><a href="index.html#P">Preliminaries</a></li><li><b>What This Module Does</b></li></ul></div>
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<p class="purpose">An overview of the knowledge module's role and abilities.</p>
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<ul class="toc"><li><a href="P-wtmd.html#SP1">§1. Prerequisites</a></li><li><a href="P-wtmd.html#SP2">§2. The Model</a></li></ul><hr class="tocbar">
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<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>§1. Prerequisites. </b>The knowledge module is a part of the Inform compiler toolset. It is
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presented as a literate program or "web". Before diving in:
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</p>
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<ul class="items"><li>(a) It helps to have some experience of reading webs: see <a href="../../../inweb/index.html" class="internal">inweb</a> for more.
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</li><li>(b) The module is written in C, in fact ANSI C99, but this is disguised by the
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fact that it uses some extension syntaxes provided by the <a href="../../../inweb/index.html" class="internal">inweb</a> literate
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programming tool, making it a dialect of C called InC. See <a href="../../../inweb/index.html" class="internal">inweb</a> for
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full details, but essentially: it's C without predeclarations or header files,
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and where functions have names like <span class="extract"><span class="extract-syntax">Tags::add_by_name</span></span> rather than just <span class="extract"><span class="extract-syntax">add_by_name</span></span>.
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</li><li>(c) This module uses other modules drawn from the compiler (see <a href="../structure.html" class="internal">structure</a>), and also
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uses a module of utility functions called <a href="../../../inweb/foundation-module/index.html" class="internal">foundation</a>.
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For more, see <a href="../../../inweb/foundation-module/P-abgtf.html" class="internal">A Brief Guide to Foundation (in foundation)</a>.
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</li></ul>
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<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>§2. The Model. </b>This module's task is to build the "model world", the initial state of the
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world as it is chosen by an Inform author.<sup id="fnref:1"><a href="#fn:1" rel="footnote">1</a></sup> This consists of:
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</p>
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<ul class="items"><li>● Kinds, such as "vehicle", created in source text.
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</li><li>● Instances of kinds, such as a specific lorry in a fictional world being
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created with Inform.
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</li><li>● Permanently existing variables, which have kinds and initial values.
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</li><li>● Either-or properties, which individual instances might or might not have:
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for example, the lorry might be "parked" or "not parked".
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</li><li>● Properties with values, which some instances will have and some will not,
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and with differing values: for example, the colour of the lorry might be "yellow".
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</li><li>● Relationships between instances. For example, a specific person might
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"know how to drive" the yellow lorry, or might not.
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</li></ul>
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<p class="commentary">But the model does not contain:
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</p>
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<ul class="items"><li>● Kinds, such as "number", built in to Inform.<sup id="fnref:2"><a href="#fn:2" rel="footnote">2</a></sup>
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</li><li>● Values, such as the number 176, which exist without anyone getting to
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choose whether they should exist or not.
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</li><li>● Temporary variables used during phrases or activities, but not existing
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at the start of play.
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</li><li>● Adjectives such as "even" as applied to numbers, or "empty" as applied
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to containers, whose truth or falsity is determined by something other than
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the author's whim. An author cannot choose that 176 is odd, and whether a
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container is empty depends only on whether there is something in it.
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</li><li>● Relationships which are, similarly, determined by an algorithm and not
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by a specific authorial choice. Like it or not, 176 is "greater than" 8.
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</li></ul>
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<ul class="footnotetexts"><li class="footnote" id="fn:1"><p class="inwebfootnote"><sup id="fnref:1"><a href="#fn:1" rel="footnote">1</a></sup> The term "model" is drawn partly from interactive fiction, but also from
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model theory in the sense of logic, where a "model" is a specific solution
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showing that a set of logical propositions can all simultaneously be true.
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<a href="#fnref:1" title="return to text"> ↩</a></p></li><li class="footnote" id="fn:2"><p class="inwebfootnote"><sup id="fnref:2"><a href="#fn:2" rel="footnote">2</a></sup> Properly speaking, kinds created by Neptune files inside kits rather than
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being declared in source text.
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<a href="#fnref:2" title="return to text"> ↩</a></p></li></ul>
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<p class="commentary firstcommentary"><a id="SP3" class="paragraph-anchor"></a><b>§3. </b>The model is constructed entirely from a stream of logical propositions
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sent here by the <a href="../assertions-module/index.html" class="internal">assertions</a> module. Those propositions may be mutually
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inconsistent — either flatly contradictory or just impossible to reconcile.
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</p>
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<p class="commentary">The stream of supposed truthful statements comes to this module through calls
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to either <a href="1-ap.html#SP2" class="internal">Assert::true</a> or <a href="1-ap.html#SP2" class="internal">Assert::true_about</a>. These reduce a
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proposition to a set of facts concerning things in the model; as we have
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seen, the model includes kinds, variables, instances and relations, and so
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we need a unified type for "something you can know a fact about". That
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type is called <a href="4-is.html#SP6" class="internal">inference_subject</a>. Each subject has its own list of known
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facts: we call such a fact an <a href="5-inf.html#SP1" class="internal">inference</a> because it has (usually) been
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inferred from a proposition.
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</p>
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<p class="commentary"><a href="3-prp.html" class="internal">Chapter 3: Properties</a> implements our system of properties: each different value
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or either-or property is a <a href="3-prp.html#SP1" class="internal">property</a> object. There might be a puritan
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case for abolishing this type in favour of regarding either-or properties
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as special cases of <span class="extract"><span class="extract-syntax">unary_predicate</span></span> and value properties as special cases
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of <span class="extract"><span class="extract-syntax">binary_predicate</span></span> (by identifying a predicate with its setting relation),
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but I do not think this would clarify anything.
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</p>
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<p class="commentary"><a href="4-is.html" class="internal">Chapter 4: Subjects</a> gives a general API for dealing with <a href="4-is.html" class="internal">Inference Subjects</a>,
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and then works systematically through the various categories of these.
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Kinds and binary predicates already exist from other modules (see
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<a href="../kinds-module/2-knd.html" class="internal">Chapter 2: Kinds (in kinds)</a> and <a href="../calculus-module/3-bpf.html" class="internal">Chapter 3: Binary Predicates (in calculus)</a>
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respectively), but <a href="2-ins.html" class="internal">Instances</a> and <a href="2-nv.html" class="internal">Nonlocal Variables</a> are new.
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</p>
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<p class="commentary">Finally, <a href="5-inf.html" class="internal">Chapter 5: Modelling</a> deals with the actual inferences, and with how the
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model world is constructed. The core of Inform does nothing very interesting
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here, but features from the <a href="../if-module/index.html" class="internal">if</a> module add some domain-specific savvy.
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</p>
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<nav role="progress"><div class="progresscontainer">
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<ul class="progressbar"><li class="progressprevoff">❮</li><li class="progresscurrentchapter">P</li><li class="progresscurrent">wtmd</li><li class="progresschapter"><a href="1-km.html">1</a></li><li class="progresschapter"><a href="2-ins.html">2</a></li><li class="progresschapter"><a href="3-prp.html">3</a></li><li class="progresschapter"><a href="4-is.html">4</a></li><li class="progresschapter"><a href="5-inf.html">5</a></li><li class="progressnext"><a href="1-km.html">❯</a></li></ul></div>
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