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Copyright updated.
Annotate for file /docs/html/unifying-substitutions.html
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<title>CL Unifying Substitutions</title>
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style="font-family:=Verdana,Arial,Helvetica; font-size: 18px; color: #41286f;">
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<strong><i>CL Extensions: UNIFYING SUBSTITUTIONS</i><string>
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<h1>Unifying Substitutions</h1>
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<p>The central notion of every unification machinery is that of
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<em>unifying substitution</em>. A unifying substitution - or
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<em>environment</em> - is a
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mapping from <em>variable names</em> (or <em>variables</em>) to
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<em>values</em> (which can be variables themselves.)</p>
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<p>The generic function UNIFY, and several other functions and macros
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in the package accept unifying substitutions as parameters, and
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return them as values.</p>
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<p>There are several notations to describe such unifying
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substitutions. The one chosen here is simply the following:</p>
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<p>
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<pre>
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{x<sub>1</sub> -> y<sub>1</sub>, x<sub>2</sub> -> y<sub>2</sub>, ..., x<sub>k</sub> -> y<sub>k</sub>}
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</pre>
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</p>
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<p>The UNIFY library has a number of functions and data structures
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that can be used to construct and manipulate unifying substitutions.</p>
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<p>In order to facilitate the construction of interpreter-like
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programs the UNIFICATION library provides data structures
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representing <em>bindings</em>, <em>frames</em>, and
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<em>environments</em> - i.e. the unifying substitions proper.</p>
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<ul>
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<li><em>Bindings</em>
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<p>A <em>binding</em> simply represent the mapping between a variable and a
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value.</p>
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<p>
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<pre>
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<i>binding</i>: <i>variable</i> --> <i>value</i>
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</pre>
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</p>
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<li><em>Frames</em>
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<p>A <em>frame</em> is simply a collection of bindings.</p>
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<p>
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<pre>
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<i>frame</i>: {<i>binding</i><sub>i</sub>}
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</pre>
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</p>
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<li><em>Environments</em>
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<p>An <em>environment</em> is simply a stack of frames.</p>
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<p>
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<pre>
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<i>environment</i>: <<i>frame</i><sub>0</sub>, <i>frame</i><sub>1</sub>, ..., <i>frame</i><sub>k</sub>>
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</pre>
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</p>
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</ul>
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<p>The collection of operators described hereafter manipulates these
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opaque data structures.</p>
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<h1>Unifying Substitutions Dictionary</h1>
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<ul>
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<li><a href="find-variable-value-function.html">FIND-VARIABLE-VALUE</a></li>
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<li><a href="make-empty-environment-function.html">MAKE-EMTPY-ENVIRONMENT</a></li>
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<li><a href="make-shared-environment-function.html">MAKE-SHARED-ENVIRONMENT</a></li>
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</ul>
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<h1>Notes</h1>
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<h2>Current Implementation Details</h2>
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<p>The current implementation is rather straightforward. A binding
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is a CONS, a frame is a wrapper around an A-LIST, and an environment
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is a wrapper around a LIST (stack) of frames.</p>
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<h2>Functional Style Unifying Substitutions</h2>
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<p>There are very elegant implementations of unification and
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substitutions based on curried functions. A typical exercise in
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functional programming using ML or a similar language is to write a
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simplified Milner Type Checker. Writing the unifying substitution
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support can be achieved by using curried functions in that context.</p>
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<!--
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;;; Copyright (c) 2004 Marco Antoniotti, All rigths reserved.
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<h2>Site Map</h2>
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<p>Enjoy!</p>
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<hr>
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<p>Questions? Queries? Suggestions? Comments? Please direct them
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at <a href="mailto:marcoxa_PROVA_A_SPAMMARME@alu.org">me</a>.
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</p>
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2011-04-02 mantoniotti
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<ul>
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<li><strong>2004-10-30</strong><br>
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Document created
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© 2003-2011, Marco Antoniotti, all rights reserved.
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