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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>14.4 First-Class 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" 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data-pltdoc="x">Bibliography</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_1");">▼</a></td><td>14 </td><td><a href="units.html" class="tocviewlink" data-pltdoc="x">Units (Components)</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">14.1 </td><td><a href="Signatures_and_Units.html" class="tocviewlink" data-pltdoc="x">Signatures and Units</a></td></tr><tr><td align="right">14.2 </td><td><a href="Invoking_Units.html" class="tocviewlink" data-pltdoc="x">Invoking Units</a></td></tr><tr><td align="right">14.3 </td><td><a href="Linking_Units.html" class="tocviewlink" data-pltdoc="x">Linking Units</a></td></tr><tr><td align="right">14.4 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">First-<wbr></wbr>Class Units</a></td></tr><tr><td align="right">14.5 </td><td><a href="Whole-module_Signatures_and_Units.html" class="tocviewlink" data-pltdoc="x">Whole-<span class="RktSym"><span class="RktStxLink">module</span></span> Signatures and Units</a></td></tr><tr><td align="right">14.6 </td><td><a href="Contracts_for_Units.html" class="tocviewlink" data-pltdoc="x">Contracts for Units</a></td></tr><tr><td align="right">14.7 </td><td><a href="unit_versus_module.html" class="tocviewlink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">unit</span></span> versus <span class="RktSym"><span class="RktStxLink">module</span></span></a></td></tr></table></div></div></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="Linking_Units.html" title="backward to "14.3 Linking Units"" data-pltdoc="x">← prev</a> <a href="units.html" title="up to "14 Units (Components)"" data-pltdoc="x">up</a> <a href="Whole-module_Signatures_and_Units.html" title="forward to "14.5 Whole-module Signatures and Units"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""firstclassunits"">14.4<tt> </tt><a name="(part._firstclassunits)"></a>First-Class Units</h4><p>The <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=linkinference.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-unit</a></span> form combines <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span> with a
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">unit</a></span> form, similar to the way that <span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym">f</span><span class="stt"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="stt"> </span><span class="RktSym">....</span><span class="RktPn">)</span> combines <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span> followed by an identifier with an
implicit <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span>.</p><p>Expanding the shorthand, the definition of <span class="RktSym">toy-store@</span> could
almost be written as</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">toy-store@</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">unit</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._import%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">import</a></span><span class="hspace"> </span><span class="RktSym">toy-factory^</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._export%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">export</a></span><span class="hspace"> </span><span class="RktSym">toy-store^</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">inventory</span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._null%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">null</a></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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">store-color</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">green</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">....</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>A difference between this expansion and <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=linkinference.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-unit</a></span> is that
the imports and exports of <span class="RktSym">toy-store@</span> cannot be
inferred. That is, besides combining <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span> and
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">unit</a></span>, <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=linkinference.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-unit</a></span> attaches static information to the
defined identifier so that its signature information is available
statically to <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/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=6.1" class="RktStxLink Sq" data-pltdoc="x">define-values/invoke-unit/infer</a></span> and other
forms.</p><p>Despite the drawback of losing static signature information,
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">unit</a></span> can be useful in combination with other forms that work
with first-class values. For example, we could wrap a <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">unit</a></span>
that creates a toy store in a <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span> to supply the store’s
color:</p><blockquote class="SCodeFlow"><blockquote class="Rfilebox"><p class="Rfiletitle"><span class="Rfilename"><span class="stt">"toy-store-maker.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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" 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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" 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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">toy-store@-maker</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">the-color</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">unit</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._import%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">import</a></span><span class="hspace"> </span><span class="RktSym">toy-factory^</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._export%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">export</a></span><span class="hspace"> </span><span class="RktSym">toy-store^</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">inventory</span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._null%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">null</a></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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">store-color</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">the-color</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">the rest is the same as before</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">maybe-repaint</span><span class="hspace"> </span><span class="RktSym">t</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._if%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">if</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eq%7E3f%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">eq?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">toy-color</span><span class="hspace"> </span><span class="RktSym">t</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">store-color</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">t</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">repaint</span><span class="hspace"> </span><span class="RktSym">t</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">store-color</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">stock!</span><span class="hspace"> </span><span class="RktSym">n</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">set!</a></span><span class="hspace"> </span><span class="RktSym">inventory</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._append%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">append</a></span><span class="hspace"> </span><span class="RktSym">inventory</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fmap..rkt%2529._map%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">map</a></span><span class="hspace"> </span><span class="RktSym">maybe-repaint</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">build-toys</span><span class="hspace"> </span><span class="RktSym">n</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">get-inventory</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">inventory</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._provide%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">provide</a></span><span class="hspace"> </span><span class="RktSym">toy-store@-maker</span><span class="RktPn">)</span></td></tr></table></blockquote></blockquote></blockquote><p>To invoke a unit created by <span class="RktSym">toy-store@-maker</span>, we must use
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=invokingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-values%252Finvoke-unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-values/invoke-unit</a></span>, instead of the
<span class="RktSym">/infer</span> variant:</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" 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><table cellspacing="0" cellpadding="0"><tr><td></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/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=6.1" 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><table cellspacing="0" cellpadding="0"><tr><td></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktVal">"toy-store-maker.rkt"</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td></td></tr></table></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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=invokingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-values%252Finvoke-unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-values/invoke-unit</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">toy-store@-maker</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">purple</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._import%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">import</a></span><span class="hspace"> </span><span class="RktSym">toy-factory^</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._export%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">export</a></span><span class="hspace"> </span><span class="RktSym">toy-store^</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td></td></tr></table></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><table cellspacing="0" cellpadding="0"><tr><td></td></tr></table></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 'purple) (toy 'purple))</span></p></td></tr></table></blockquote><p>In the <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=invokingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-values%252Finvoke-unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-values/invoke-unit</a></span> form, the <span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._import%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">import</a></span><span class="stt"> </span><span class="RktSym">toy-factory^</span><span class="RktPn">)</span> line takes bindings from the current context that match
the names in <span class="RktSym">toy-factory^</span> (the ones that we created by
invoking <span class="RktSym">simple-factory@</span>), and it supplies them as imports to
<span class="RktSym">toy-store@</span>. The <span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._export%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">export</a></span><span class="stt"> </span><span class="RktSym">toy-store^</span><span class="RktPn">)</span> clause indicates
that the unit produced by <span class="RktSym">toy-store@-maker</span> will export
<span class="RktSym">toy-store^</span>, and the names from that signature are defined
after invoking the unit.</p><p>To link a unit from <span class="RktSym">toy-store@-maker</span>, we can use the
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=compoundunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._compound-unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">compound-unit</a></span> form:</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" 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"><tr><td></td></tr></table></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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=compoundunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._compound-unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">compound-unit</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._import%2529%2529&version=6.1" 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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._export%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">export</a></span><span class="hspace"> </span><span class="RktSym">TF</span><span class="hspace"> </span><span class="RktSym">TS</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._link%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">link</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym">TF</span><span class="hspace"> </span><span class="RktSym">:</span><span class="hspace"> </span><span class="RktSym">toy-factory^</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">store-specific-factory@</span><span class="hspace"> </span><span class="RktSym">TS</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym">TS</span><span class="hspace"> </span><span class="RktSym">:</span><span class="hspace"> </span><span class="RktSym">toy-store^</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">toy-store@</span><span class="hspace"> </span><span class="RktSym">TF</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td></td></tr></table></td></tr></table></blockquote><p>This <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=compoundunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._compound-unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">compound-unit</a></span> form packs a lot of information into one
place. The left-hand-side <span class="RktSym">TF</span> and <span class="RktSym">TS</span> in the
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._link%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">link</a></span> clause are binding identifiers. The identifier
<span class="RktSym">TF</span> is essentially bound to the elements of
<span class="RktSym">toy-factory^</span> as implemented by
<span class="RktSym">store-specific-factory@</span>. The identifier <span class="RktSym">TS</span> is
similarly bound to the elements of <span class="RktSym">toy-store^</span> as implemented
by <span class="RktSym">toy-store@</span>. Meanwhile, the elements bound to <span class="RktSym">TS</span>
are supplied as imports for <span class="RktSym">store-specific-factory@</span>, since
<span class="RktSym">TS</span> follows <span class="RktSym">store-specific-factory@</span>. The elements
bound to <span class="RktSym">TF</span> are similarly supplied to
<span class="RktSym">toy-store@</span>. Finally, <span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=creatingunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._export%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">export</a></span><span class="stt"> </span><span class="RktSym">TF</span><span class="stt"> </span><span class="RktSym">TS</span><span class="RktPn">)</span> indicates that
the elements bound to <span class="RktSym">TF</span> and <span class="RktSym">TS</span> are exported from
the compound unit.</p><p>The above <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=compoundunits.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._compound-unit%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">compound-unit</a></span> form uses
<span class="RktSym">store-specific-factory@</span> as a first-class unit, even though
its information could be inferred. Every unit can be used as a
first-class unit, in addition to its use in inference contexts. Also,
various forms let a programmer bridge the gap between inferred and
first-class worlds. For example, <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=linkinference.html%23%2528form._%2528%2528lib._racket%252Funit..rkt%2529._define-unit-binding%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-unit-binding</a></span> binds a
new identifier to the unit produced by an arbitrary expression; it
statically associates signature information to the identifier, and it
dynamically checks the signatures against the first-class unit
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