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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>7.2 Invoking 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" 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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="" class="tocviewselflink" data-pltdoc="x">Invoking Units</a></td></tr><tr><td align="right">7.3 </td><td><a href="compoundunits.html" class="tocviewlink" 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="#%28form._%28%28lib._racket%2Funit..rkt%29._invoke-unit%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">invoke-<wbr></wbr>unit</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Funit..rkt%29._define-values%2Finvoke-unit%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">define-<wbr></wbr>values/<span class="mywbr"> </span>invoke-<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" 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="creatingunits.html" title="backward to "7.1 Creating Units"" data-pltdoc="x">← prev</a> <a href="mzlib_unit.html" title="up to "7 Units"" data-pltdoc="x">up</a> <a href="compoundunits.html" title="forward to "7.3 Linking Units and Creating Compound Units"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-part-tag=""invokingunits"">7.2<tt> </tt><a name="(part._invokingunits)"></a>Invoking 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><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/unit..rkt)._invoke-unit))"></a><span title="Provided from: racket/unit, racket | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Funit..rkt%29._invoke-unit%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">invoke-unit</a></span></span><span class="hspace"> </span><span class="RktVar">unit-expr</span><span class="RktPn">)</span></p></blockquote></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Funit..rkt%29._invoke-unit%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">invoke-unit</a></span><span class="hspace"> </span><span class="RktVar">unit-expr</span><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">tagged-sig-spec</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote></div><div class="SIntrapara">Invokes the unit produced by <span class="RktVar">unit-expr</span>. For each of the
unit’s imports, the <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Funit..rkt%29._invoke-unit%29%29" class="RktStxLink" data-pltdoc="x">invoke-unit</a></span> expression must contain a
<span class="RktVar">tagged-sig-spec</span> in 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; see
<span class="RktSym"><a href="creatingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._unit%29%29" class="RktStxLink" data-pltdoc="x">unit</a></span> for the grammar of <span class="RktVar">tagged-sig-spec</span>. If the unit
has no imports, 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 can be omitted.</div></p><p>When no <span class="RktVar">tagged-sig-spec</span>s are provided, <span class="RktVar">unit-expr</span>
must produce a unit that expects no imports. To invoke the unit, all
bindings are first initialized to the <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=guide&rel=void_undefined.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="nobreak"><span class="RktRes">#<undefined></span></span></a> value. Next,
the unit’s body definitions and expressions are evaluated in order; in
the case of a definition, evaluation sets the value of the
corresponding variable(s). Finally, the result of the last expression
in the unit is the result of the <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Funit..rkt%29._invoke-unit%29%29" class="RktStxLink" data-pltdoc="x">invoke-unit</a></span> expression.</p><p>Each supplied <span class="RktVar">tagged-sig-spec</span> takes bindings from the
surrounding context and turns them into imports for the invoked unit.
The unit need not declare an import for every provided
<span class="RktVar">tagged-sig-spec</span>, but one <span class="RktVar">tagged-sig-spec</span> must be
provided for each declared import of the unit. For each variable
identifier in each provided <span class="RktVar">tagged-sig-spec</span>, the value of the
identifier’s binding in the surrounding context is used for the
corresponding import in the invoked unit.</p><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)._define-values/invoke-unit))"></a><span title="Provided from: racket/unit, racket | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Funit..rkt%29._define-values%2Finvoke-unit%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">define-values/invoke-unit</a></span></span><span class="hspace"> </span><span class="RktVar">unit-expr</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">tagged-sig-spec</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-sig-spec</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Funit..rkt%29._invoke-unit%29%29" class="RktStxLink" data-pltdoc="x">invoke-unit</a></span>, but the values of the unit’s exports are
copied to new bindings.</div></p><p>The unit produced by <span class="RktVar">unit-expr</span> is linked and invoked as for
<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Funit..rkt%29._invoke-unit%29%29" class="RktStxLink" data-pltdoc="x">invoke-unit</a></span>. In addition, 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 is
treated as a kind of import into the local definition context. That
is, for every binding that would be available in a unit that used 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’s <span class="RktVar">tagged-sig-spec</span> as an import, a
definition is generated for the context of the
<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Funit..rkt%29._define-values%2Finvoke-unit%29%29" class="RktStxLink" data-pltdoc="x">define-values/invoke-unit</a></span> form.</p><div class="navsetbottom"><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="creatingunits.html" title="backward to "7.1 Creating Units"" data-pltdoc="x">← prev</a> <a href="mzlib_unit.html" title="up to "7 Units"" data-pltdoc="x">up</a> <a href="compoundunits.html" title="forward to "7.3 Linking Units and Creating Compound Units"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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