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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>14.7 unit versus module</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="tocvie
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namespace. For example, it allows a code fragment to refer
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specifically to the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">car</a></span> operation from
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<a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=index.html&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a>—<wbr></wbr>the one that extracts the first
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element of an instance of the built-in pair datatype—<wbr></wbr>as
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opposed to any number of other functions with the name
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<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">car</a></span>. In other words, the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=module.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._module%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">module</a></span> construct lets
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you refer to <span class="emph">the</span> binding that you want.</p></li><li><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">unit</a></span> form is for parameterizing a code fragment
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with respect to most any kind of run-time value. For example,
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it allows a code fragment to work with a <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">car</a></span>
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function that accepts a single argument, where the specific
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function is determined later by linking the fragment to
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another. In other words, the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">unit</a></span> construct lets you
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refer to <span class="emph">a</span> binding that meets some specification.</p></li></ul><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=createclass.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fclass-internal..rkt%2529._class%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">class</a></span> forms, among others, also allow
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parameterization of code with respect to values that are chosen
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later. In principle, any of those could be implemented in terms of any
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of the others. In practice, each form offers certain
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conveniences—<wbr></wbr>such as allowing overriding of methods or especially
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simple application to values—<wbr></wbr>that make them suitable for different
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purposes.</p><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=module.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._module%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">module</a></span> form is more fundamental than the others, in a
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sense. After all, a program fragment cannot reliably refer to a
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<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span>, <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=createclass.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fclass-internal..rkt%2529._class%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">class</a></span>, or <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">unit</a></span> form without the
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namespace management provided by <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=module.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._module%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">module</a></span>. At the same time,
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because namespace management is closely related to separate expansion
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and compilation, <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=module.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._module%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">module</a></span> boundaries end up as
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separate-compilation boundaries in a way that prohibits mutual
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dependencies among fragments. For similar reasons, <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=module.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._module%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">module</a></span>
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does not separate interface from implementation.</p><p>Use <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">unit</a></span> when <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=module.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._module%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">module</a></span> by itself almost works, but
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when separately compiled pieces must refer to each other, or when you
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want a stronger separation between <span style="font-style: italic">interface</span> (i.e., the
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parts that need to be known at expansion and compilation time) and
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<span style="font-style: italic">implementation</span> (i.e., the run-time parts). More generally,
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use <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">unit</a></span> when you need to parameterize code over functions,
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datatypes, and classes, and when the parameterized code itself
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provides definitions to be linked with other parameterized code.</p><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://docs.racket-lang.org/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="Contracts_for_Units.html" title="backward to "14.6 Contracts for Units"" data-pltdoc="x">← prev</a> <a href="units.html" title="up to "14 Units (Components)"" data-pltdoc="x">up</a> <a href="reflection.html" title="forward to "15 Reflection and Dynamic Evaluation"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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