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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>14 Units</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">►</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">The Racket Guide</a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="intro.html" class="tocviewlink" data-pltdoc="x">Welcome to Racket</a></td></tr><tr><td align="right">2 </td><td><a href="to-scheme.html" class="tocviewlink" data-pltdoc="x">Racket Essentials</a></td></tr><tr><td align="right">3 </td><td><a href="datatypes.html" class="tocviewlink" data-pltdoc="x">Built-<wbr></wbr>In Datatypes</a></td></tr><tr><td align="right">4 </td><td><a href="scheme-forms.html" class="tocviewlink" data-pltdoc="x">Expressions and Definitions</a></td></tr><tr><td align="right">5 </td><td><a href="define-struct.html" class="tocviewlink" data-pltdoc="x">Programmer-<wbr></wbr>Defined Datatypes</a></td></tr><tr><td align="right">6 </td><td><a href="modules.html" class="tocviewlink" data-pltdoc="x">Modules</a></td></tr><tr><td align="right">7 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">8 </td><td><a href="i_o.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">9 </td><td><a href="regexp.html" class="tocviewlink" data-pltdoc="x">Regular Expressions</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Exceptions and Control</a></td></tr><tr><td align="right">11 </td><td><a href="for.html" class="tocviewlink" data-pltdoc="x">Iterations and Comprehensions</a></td></tr><tr><td align="right">12 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">13 </td><td><a href="classes.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">14 </td><td><a href="units.html" class="tocviewselflink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">15 </td><td><a href="reflection.html" class="tocviewlink" data-pltdoc="x">Reflection and Dynamic Evaluation</a></td></tr><tr><td align="right">16 </td><td><a href="macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">17 </td><td><a href="languages.html" class="tocviewlink" data-pltdoc="x">Creating Languages</a></td></tr><tr><td align="right">18 </td><td><a href="concurrency.html" class="tocviewlink" data-plt
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reusable <a name="(tech._component)"></a><span style="font-style: italic">components</span>. A unit resembles a procedure in that
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both are first-class values that are used for abstraction. While
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procedures abstract over values in expressions, units abstract over
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names in collections of definitions. Just as a procedure is called to
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evaluate its expressions given actual arguments for its formal
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parameters, a unit is <a name="(tech._invoked)"></a><span style="font-style: italic">invoked</span> to evaluate its definitions
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given actual references for its imported variables. Unlike a
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procedure, however, a unit’s imported variables can be partially
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linked with the exported variables of another unit <span style="font-style: italic">prior to
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invocation</span>. Linking merges multiple units together into a single
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compound unit. The compound unit itself imports variables that will be
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propagated to unresolved imported variables in the linked units, and
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re-exports some variables from the linked units for further linking.</p><table cellspacing="0" cellpadding="0"><tr><td><p><span class="hspace"> </span><a href="Signatures_and_Units.html" class="toclink" data-pltdoc="x">14.1<span class="hspace"> </span>Signatures and Units</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="Invoking_Units.html" class="toclink" data-pltdoc="x">14.2<span class="hspace"> </span>Invoking Units</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="Linking_Units.html" class="toclink" data-pltdoc="x">14.3<span class="hspace"> </span>Linking Units</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="firstclassunits.html" class="toclink" data-pltdoc="x">14.4<span class="hspace"> </span>First-Class Units</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="Whole-module_Signatures_and_Units.html" class="toclink" data-pltdoc="x">14.5<span class="hspace"> </span>Whole-<span class="RktSym"><span class="RktStxLink">module</span></span> Signatures and Units</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="Contracts_for_Units.html" class="toclink" data-pltdoc="x">14.6<span class="hspace"> </span>Contracts for Units</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="Contracts_for_Units.html#%28part._.Adding_.Contracts_to_.Signatures%29" class="toclink" data-pltdoc="x">14.6.1<span class="hspace"> </span>Adding Contracts to Signatures</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="Contracts_for_Units.html#%28part._.Adding_.Contracts_to_.Units%29" class="toclink" data-pltdoc="x">14.6.2<span class="hspace"> </span>Adding Contracts to Units</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="unit_versus_module.html" class="toclink" data-pltdoc="x">14.7<span class="hspace"> </span><span class="RktSym"><span class="RktStxLink">unit</span></span> versus <span class="RktSym"><span class="RktStxLink">module</span></span></a></p></td></tr></table><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://download.racket-lang.org/releases/8.6/doc/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="classes.html" title="backward to "13 Classes and Objects"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "The Racket Guide"" data-pltdoc="x">up</a> <a href="Signatures_and_Units.html" title="forward to "14.1 Signatures and Units"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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