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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>7.3 Linking Units and Creating Compound Units</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.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 Reference</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="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2 </td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3 </td><td><a href="syntax.html" class="tocviewlink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4 </td><td><a href="data.html" class="tocviewlink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6 </td><td><a href="mzlib_class.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">7 </td><td><a href="mzlib_unit.html" class="tocviewselflink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">8 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">9 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18 </td><td><a href="running.html" class="tocviewlink" data-pltdoc="x">Running Racket</a></td></tr><tr><td align="right"></td><td><a href="doc-bibliography.html" class="tocviewlink" 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>7 </td><td><a href="mzlib_unit.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">7.1 </td><td><a href="creatingunits.html" class="tocviewlink" data-pltdoc="x">Creating Units</a></td></tr><tr><td align="right">7.2 </td><td><a href="invokingunits.html" class="tocviewlink" data-pltdoc="x">Invoking Units</a></td></tr><tr><td align="right">7.3 </td><td><a href="compoundunits.html" class="tocviewselflink" data-pltdoc="x">Linking Units and Creating Compound Units</a></td></tr><tr><td align="right">7.4 </td><td><a href="linkinference.html" class="tocviewlink" data-pltdoc="x">Inferred Linking</a></td></tr><tr><td align="right">7.5 </td><td><a href="Generating_A_Unit_from_Context.html" class="tocviewlink" data-pltdoc="x">Generating A Unit from Context</a></td></tr><tr><td align="right">7.6 </td><td><a href="Structural_Matching.html" class="tocviewlink" data-pltdoc="x">Structural Matching</a></td></tr><tr><td align="right">7.7 </td><td><a href="define-sig-form.html" class="tocviewlink" data-pltdoc="x">Extending the Syntax of Signatures</a></td></tr><tr><td align="right">7.8 </td><td><a href="Unit_Utilities.html" class="tocviewlink" data-pltdoc="x">Unit Utilities</a></td></tr><tr><td align="right">7.9 </td><td><a href="unitcontracts.html" class="tocviewlink" data-pltdoc="x">Unit Contracts</a></td></tr><tr><td align="right">7.10 </td><td><a href="single-unit.html" class="tocviewlink" data-pltdoc="x">Single-<wbr></wbr>Unit Modules</a></td></tr><tr><td align="right">7.11 </td><td><a href="Single-Signature_Modules.html" class="tocviewlink" data-pltdoc="x">Single-<wbr></wbr>Signature Modules</a></td></tr><tr><td align="right">7.12 </td><td><a href="Transformer_Helpers.html" class="tocviewlink" data-pltdoc="x">Transformer Helpers</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="compoundunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._compound-unit%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">compound-<wbr></wbr>unit</span></span></a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><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="invokingunits.html" title="backward to "7.2 Invoking Units"" data-pltdoc="x">← prev</a> <a href="mzlib_unit.html" title="up to "7 Units"" data-pltdoc="x">up</a> <a href="linkinference.html" title="forward to "7.4 Inferred Linking"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""compoundunits"">7.3<tt> </tt><a name="(part._compoundunits)"></a>Linking Units and Creating Compound Units</h4><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><table cellspacing="0" cellpadding="0" class="RktBlk RForeground"><tr><td><span class="RktPn">(</span><a name="(form._((lib._racket/unit..rkt)._compound-unit))"></a><span title="Provided from: racket/unit, racket | Package: base"><span class="RktSym"><a href="compoundunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._compound-unit%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">compound-unit</a></span></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._import%29%29" class="RktStxLink" data-pltdoc="x">import</a></span><span class="hspace"> </span><span class="RktVar">link-binding</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._export%29%29" class="RktStxLink" data-pltdoc="x">export</a></span><span class="hspace"> </span><span class="RktVar">tagged-link-id</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._link%29%29" class="RktStxLink" data-pltdoc="x">link</a></span><span class="hspace"> </span><span class="RktVar">linkage-decl</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote></td></tr><tr><td><span class="stt"> </span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="specgrammar"><tr><td align="right" valign="baseline"><span class="RktVar">link-binding</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">(</span><span class="RktVar">link-id</span><span class="hspace"> </span><span class="RktSym">:</span><span class="hspace"> </span><span class="RktVar">tagged-sig-id</span><span class="RktPn">)</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td></tr><tr><td align="right" valign="baseline"><span class="RktVar">tagged-link-id</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">(</span><span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._tag%29%29" class="RktStxLink" data-pltdoc="x">tag</a></span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">link-id</span><span class="RktPn">)</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktVar">link-id</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td></tr><tr><td align="right" valign="baseline"><span class="RktVar">linkage-decl</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktVar">link-binding</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">unit-expr</span><span class="hspace"> </span><span class="RktVar">tagged-link-id</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Links several units into one new compound unit without immediately
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invoking any of the linked units. The <span class="RktVar">unit-expr</span>s in the
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<span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._link%29%29" class="RktStxLink" data-pltdoc="x">link</a></span> clause determine the units to be linked in creating the
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compound unit. The <span class="RktVar">unit-expr</span>s are evaluated when the
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<span class="RktSym"><a href="compoundunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._compound-unit%29%29" class="RktStxLink" data-pltdoc="x">compound-unit</a></span> form is evaluated.</div></p><p>The <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._import%29%29" class="RktStxLink" data-pltdoc="x">import</a></span> clause determines the imports of the compound
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unit. Outside the compound unit, these imports behave as for a plain
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unit; inside the compound unit, they are propagated to some of the
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linked units. The <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._export%29%29" class="RktStxLink" data-pltdoc="x">export</a></span> clause determines the exports of the
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compound unit. Again, outside the compound unit, these exports are
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treated the same as for a plain unit; inside the compound unit, they
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are drawn from the exports of the linked units. Finally, the left-hand
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and right-hand parts of each declaration in the <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._link%29%29" class="RktStxLink" data-pltdoc="x">link</a></span> clause
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specify how the compound unit’s imports and exports are propagated to
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the linked units.</p><p>Individual elements of an imported or exported signature are not
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available within the compound unit. Instead, imports and exports are
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connected at the level of whole signatures. Each specific import or
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export (i.e., an instance of some signature, possibly tagged) is given
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a <span class="RktVar">link-id</span> name. Specifically, a <span class="RktVar">link-id</span> is bound by
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the <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._import%29%29" class="RktStxLink" data-pltdoc="x">import</a></span> clause or the left-hand part of a declaration in
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the <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._link%29%29" class="RktStxLink" data-pltdoc="x">link</a></span> clause. A bound <span class="RktVar">link-id</span> is referenced in
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the right-hand part of a declaration in the <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._link%29%29" class="RktStxLink" data-pltdoc="x">link</a></span> clause or by
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the <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._export%29%29" class="RktStxLink" data-pltdoc="x">export</a></span> clause.</p><p>The left-hand side of a <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._link%29%29" class="RktStxLink" data-pltdoc="x">link</a></span> declaration gives names to each
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expected export of the unit produced by the corresponding
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<span class="RktVar">unit-expr</span>. The actual unit may export additional signatures,
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and it may export an extension of a specific signature instead of just
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the specified one. If the unit does not export one of the specified
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signatures (with the specified tag, if any), the
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<span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract</a></span> exception is raised when the <span class="RktSym"><a href="compoundunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._compound-unit%29%29" class="RktStxLink" data-pltdoc="x">compound-unit</a></span> form is
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evaluated.</p><p>The right-hand side of a <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._link%29%29" class="RktStxLink" data-pltdoc="x">link</a></span> declaration specifies the
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imports to be supplied to the unit produced by the corresponding
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<span class="RktVar">unit-expr</span>. The actual unit may import fewer signatures, and
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it may import a signature that is extended by the specified one. If
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the unit imports a signature (with a particular tag) that is not
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included in the supplied imports, the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract</a></span> exception is raised
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when the <span class="RktSym"><a href="compoundunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._compound-unit%29%29" class="RktStxLink" data-pltdoc="x">compound-unit</a></span> form is evaluated. Each
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<span class="RktVar">link-id</span> supplied as an import must be bound either in the
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<span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._import%29%29" class="RktStxLink" data-pltdoc="x">import</a></span> clause or in some declaration within the <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._link%29%29" class="RktStxLink" data-pltdoc="x">link</a></span>
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clause.</p><p>The order of declarations in the <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._link%29%29" class="RktStxLink" data-pltdoc="x">link</a></span> clause determines the
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order of invocation of the linked units. When the compound unit is
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invoked, the unit produced by the first <span class="RktVar">unit-expr</span> is invoked
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first, then the second, and so on. If the order specified in the
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<span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._link%29%29" class="RktStxLink" data-pltdoc="x">link</a></span> clause is inconsistent with <span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._init-depend%29%29" class="RktStxLink" data-pltdoc="x">init-depend</a></span>
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declarations of the actual units, then the
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<span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract</a></span> exception is raised when the <span class="RktSym"><a href="compoundunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._compound-unit%29%29" class="RktStxLink" data-pltdoc="x">compound-unit</a></span> form is
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evaluated.</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="invokingunits.html" title="backward to "7.2 Invoking Units"" data-pltdoc="x">← prev</a> <a href="mzlib_unit.html" title="up to "7 Units"" data-pltdoc="x">up</a> <a href="linkinference.html" title="forward to "7.4 Inferred Linking"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html> |