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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>14.2 Invoking 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="tocviewlin
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<a href="units.html#%28tech._invoked%29" class="techoutside" data-pltdoc="x"><span class="techinside">invoked</span></a> directly using <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=invokingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._invoke-unit%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">invoke-unit</a></span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktVal">"simple-factory-unit.rkt"</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=invokingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._invoke-unit%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">invoke-unit</a></span><span class="hspace"> </span><span class="RktSym">simple-factory@</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktOut">Factory started.</span></p></td></tr></table></blockquote><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=invokingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._invoke-unit%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">invoke-unit</a></span> form does not make the body definitions
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available, however, so we cannot build any toys with this factory. The
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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=invokingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-values%252Finvoke-unit%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define-values/invoke-unit</a></span> form binds the identifiers of a
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signature to the values supplied by a unit (to be <a href="units.html#%28tech._invoked%29" class="techoutside" data-pltdoc="x"><span class="techinside">invoked</span></a>) that
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implements the signature:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=linkinference.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-values%252Finvoke-unit%252Finfer%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define-values/invoke-unit/infer</a></span><span class="hspace"> </span><span class="RktSym">simple-factory@</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktOut">Factory started.</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">build-toys</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">(list (toy 'blue) (toy 'blue) (toy 'blue))</span></p></td></tr></table></blockquote><p>Since <span class="RktSym">simple-factory@</span> exports the <span class="RktSym">toy-factory^</span>
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signature, each identifier in <span class="RktSym">toy-factory^</span> is defined by the
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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=linkinference.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-values%252Finvoke-unit%252Finfer%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define-values/invoke-unit/infer</a></span> form. The
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<span class="RktSym">/infer</span> part of the form name indicates that the
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identifiers bound by the declaration are inferred from
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<span class="RktSym">simple-factory@</span>.</p><p>Now that the identifiers in <span class="RktSym">toy-factory^</span> are defined, we can
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also invoke <span class="RktSym">toy-store@</span>, which imports <span class="RktSym">toy-factory^</span>
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to produce <span class="RktSym">toy-store^</span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktVal">"toy-store-unit.rkt"</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=linkinference.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-values%252Finvoke-unit%252Finfer%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define-values/invoke-unit/infer</a></span><span class="hspace"> </span><span class="RktSym">toy-store@</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">get-inventory</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'()</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">stock!</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">get-inventory</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">(list (toy 'green) (toy 'green))</span></p></td></tr></table></blockquote><p>Again, the <span class="RktSym">/infer</span> part
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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=linkinference.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-values%252Finvoke-unit%252Finfer%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define-values/invoke-unit/infer</a></span> determines that
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<span class="RktSym">toy-store@</span> imports <span class="RktSym">toy-factory^</span>, and so it supplies
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the top-level bindings that match the names in <span class="RktSym">toy-factory^</span>
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as imports to <span class="RktSym">toy-store@</span>.</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="Signatures_and_Units.html" title="backward to "14.1 Signatures and Units"" data-pltdoc="x">← prev</a> <a href="units.html" title="up to "14 Units (Components)"" data-pltdoc="x">up</a> <a href="Linking_Units.html" title="forward to "14.3 Linking Units"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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