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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>14.3 Linking 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
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that cooperate with stores, creating toys that do not have to be
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repainted. Instead, the toys are always created using the store’s
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color, which the factory gets by importing <span class="RktSym">toy-store^</span>:</p><blockquote class="SCodeFlow"><blockquote class="Rfilebox"><p class="Rfiletitle"><span class="Rfilename"><span class="stt">"store-specific-factory-unit.rkt"</span></span></p><blockquote class="Rfilecontent"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><a href="Module_Syntax.html#%28part._hash-lang%29" class="RktModLink" data-pltdoc="x"><span class="RktMod">#lang</span></a><span class="hspace"> </span><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</span></a></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><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-sig.rkt"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">"toy-factory-sig.rkt"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><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-unit%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define-unit</a></span><span class="hspace"> </span><span class="RktSym">store-specific-factory@</span></td></tr><tr><td><span class="hspace"> </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=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._import%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">import</a></span><span class="hspace"> </span><span class="RktSym">toy-store^</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </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=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._export%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">export</a></span><span class="hspace"> </span><span class="RktSym">toy-factory^</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="hspace"> </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=define-struct.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._struct%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">toy</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:transparent</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="hspace"> </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=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">toy-color</span><span class="hspace"> </span><span class=
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<span class="RktSym">toy-store^</span> bindings to supply to the unit. But to get
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<span class="RktSym">toy-store^</span> bindings by invoking <span class="RktSym">toy-store@</span>, we will
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need a toy factory! The unit implementations are mutually dependent,
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and we cannot invoke either before the other.</p><p>The solution is to <a name="(tech._link)"></a><span style="font-style: italic">link</span> the units together, and then we can
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invoke the combined units. The <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-compound-unit%252Finfer%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define-compound-unit/infer</a></span> form
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links any number of units to form a combined unit. It can propagate
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imports and exports from the linked units, and it can satisfy each
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unit’s imports using the exports of other linked units.</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-factory-sig.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=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-sig.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=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">"store-specific-factory-unit.rkt"</span><span class="RktPn">)</span></td></tr><tr><td><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-compound-unit%252Finfer%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define-compound-unit/infer</a></span><span class="hspace"> </span><span class="RktSym">toy-store+factory@</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </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=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._import%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">import</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </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=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._export%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">export</a></span><span class="hspace"> </span><span class="RktSym">toy-factory^</span><span class="hspace"> </span><span class="RktSym">toy-store^</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </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=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._link%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">link</a></span><span class="hspace"> </span><span class="RktSym">store-specific-factory@</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym">toy-store@</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote><p>The overall result above is a unit <span class="RktSym">toy-store+factory@</span> that
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exports both <span class="RktSym">toy-factory^</span> and <span class="RktSym">toy-store^</span>. The
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connection between <span class="RktSym">store-specific-factory@</span> and
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<span class="RktSym">toy-store@</span> is inferred from the signatures that each imports
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and exports.</p><p>This unit has no imports, so we can always invoke it:</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">toy-store+factory@</span><span class="RktPn">)</span></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) (toy))</span></p></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=pairs.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmap..rkt%2529._map%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">map</a></span><span class="hspace"> </span><span class="RktSym">toy-color</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">get-inventory</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'(green green)</span></p></td></tr></table></blockquote><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="Invoking_Units.html" title="backward to "14.2 Invoking Units"" data-pltdoc="x">← prev</a> <a href="units.html" title="up to "14 Units (Components)"" data-pltdoc="x">up</a> <a href="firstclassunits.html" title="forward to "14.4 First-Class Units"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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