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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>3.5&nbsp;Variable References and #%top</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,&quot;tocview_0&quot;);">&#9658;</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&nbsp;</td><td><a href="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2&nbsp;</td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3&nbsp;</td><td><a href="syntax.html" class="tocviewselflink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4&nbsp;</td><td><a href="data.html" class="tocviewlink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5&nbsp;</td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6&nbsp;</td><td><a href="mzlib_class.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">7&nbsp;</td><td><a href="mzlib_unit.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">8&nbsp;</td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">9&nbsp;</td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">10&nbsp;</td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11&nbsp;</td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12&nbsp;</td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13&nbsp;</td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14&nbsp;</td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15&nbsp;</td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16&nbsp;</td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17&nbsp;</td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18&nbsp;</td><td><a href="running.h
not bound as a transformer (see <a href="syntax-model.html#%28part._expansion%29" data-pltdoc="x">Expansion</a>). At run-time,
the reference evaluates to the value in the <a href="eval-model.html#%28tech._location%29" class="techoutside" data-pltdoc="x"><span class="techinside">location</span></a> associated with
the binding.</div></p><p>When the expander encounters an <span class="RktVar">id</span> that is not bound by a
module-level or local binding, it converts the expression to
<span class="RktPn">(</span><span class="RktSym">#%top</span><span class="stt"> </span><span class="RktPn">. </span><span class="RktVar">id</span><span class="RktPn">)</span> giving <span class="RktSym">#%top</span>
the lexical context of the <span class="RktVar">id</span>; typically, that context refers
to <span class="RktSym"><a href="__top.html#%28form._%28%28quote._~23~25kernel%29._~23~25top%29%29" class="RktStxLink" data-pltdoc="x">#%top</a></span>. See also <a href="syntax-model.html#%28part._expand-steps%29" data-pltdoc="x">Expansion Steps</a>.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">&gt; </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="hspace">&nbsp;</span><span class="RktVal">10</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">&gt; </span><span class="RktSym">x</span></td></tr><tr><td><p><span class="RktRes">10</span></p></td></tr><tr><td><span class="stt">&gt; </span><span class="RktPn">(</span><span class="RktSym"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">x</span><span class="hspace">&nbsp;</span><span class="RktVal">5</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">5</span></p></td></tr><tr><td><span class="stt">&gt; </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace">&nbsp;</span><span class="RktPn">(</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="hspace">&nbsp;</span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">2</span></p></td></tr></table></blockquote></div></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><p class="RForeground"><span class="RktPn">(</span><a name="(form._((quote._~23~25kernel)._~23~25top))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="__top.html#%28form._%28%28quote._~23~25kernel%29._~23~25top%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">#%top</a></span></span><span class="hspace">&nbsp;</span><span class="RktPn">. </span><span class="RktVar">id</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Equivalent to <span class="RktVar">id</span> when <span class="RktVar">id</span> is bound to a module-level
or top-level variable. In a top-level context, <span class="RktPn">(</span><span class="RktSym"><a href="__top.html#%28form._%28%28quote._~23~25kernel%29._~23~25top%29%29" class="RktStxLink" data-pltdoc="x">#%top</a></span><span class="stt"> </span><span class="RktPn">. </span><span class="RktVar">id</span><span class="RktPn">)</span>
always refers to a top-level variable, even if <span class="RktVar">id</span> is
<a href="syntax-model.html#%28tech._unbound%29" class="techoutside" data-pltdoc="x"><span class="techinside">unbound</span></a> or bound to syntax, as long as <span class="RktVar">id</span> does not
have a local binding. In all contexts, <span class="RktPn">(</span><span class="RktSym"><a href="__top.html#%28form._%28%28quote._~23~25kernel%29._~23~25top%29%29" class="RktStxLink" data-pltdoc="x">#%top</a></span><span class="stt"> </span><span class="RktPn">. </span><span class="RktVar">id</span><span class="RktPn">)</span> is
a syntax error if <span class="RktVar">id</span> has a local binding.</div></p><p>Within a <span class="RktSym"><a href="module.html#%28form._%28%28quote._~23~25kernel%29._module%29%29" class="RktStxLink" data-pltdoc="x">module</a></span> form, <span class="RktPn">(</span><span class="RktSym"><a href="__top.html#%28form._%28%28quote._~23~25kernel%29._~23~25top%29%29" class="RktStxLink" data-pltdoc="x">#%top</a></span><span class="stt"> </span><span class="RktPn">. </span><span class="RktVar">id</span><span class="RktPn">)</span> expands to just
<span class="RktVar">id</span> as long as <span class="RktVar">id</span> is defined within
the module and has no local binding in its context. At <a href="syntax-model.html#%28tech._phase._level%29" class="techoutside" data-pltdoc="x"><span class="techinside">phase
level</span></a> 0, <span class="RktPn">(</span><span class="RktSym"><a href="__top.html#%28form._%28%28quote._~23~25kernel%29._~23~25top%29%29" class="RktStxLink" data-pltdoc="x">#%top</a></span><span class="stt"> </span><span class="RktPn">. </span><span class="RktVar">id</span><span class="RktPn">)</span> is an immediate syntax error if
<span class="RktVar">id</span> is not bound. At <a href="syntax-model.html#%28tech._phase._level%29" class="techoutside" data-pltdoc="x"><span class="techinside">phase level</span></a> 1 and higher, a syntax
error is reported if <span class="RktVar">id</span> is not defined at the corresponding
phase by the end of <span class="RktSym"><a href="module.html#%28form._%28%28quote._~23~25kernel%29._module%29%29" class="RktStxLink" data-pltdoc="x">module</a></span>-body <a href="syntax-model.html#%28tech._partial._expansion%29" class="techoutside" data-pltdoc="x"><span class="techinside">partial expansion</span></a>.</p><p>See also <a href="syntax-model.html#%28part._expand-steps%29" data-pltdoc="x">Expansion Steps</a> for information on how the expander
introduces <span class="RktSym">#%top</span> identifiers.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">&gt; </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace">&nbsp;</span><span class="RktSym">x</span><span class="hspace">&nbsp;</span><span class="RktVal">12</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">&gt; </span><span class="RktPn">(</span><span class="RktSym"><a href="__top.html#%28form._%28%28quote._~23~25kernel%29._~23~25top%29%29" class="RktStxLink" data-pltdoc="x">#%top</a></span><span class="hspace">&nbsp;</span><span class="RktPn">. </span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">12</span></p></td></tr></table></blockquote></div></p><p class="SHistory">Changed in version 6.3 of package <span class="stt">base</span>: Changed the introduction of
<span class="RktSym"><a href="__top.html#%28form._%28%28quote._~23~25kernel%29._~23~25top%29%29" class="RktStxLink" data-pltdoc="x">#%top</a></span> in a top-level context
to <a href="syntax-model.html#%28tech._unbound%29" class="techoutside" data-pltdoc="x"><span class="techinside">unbound</span></a> identifiers only.<br/>Changed in version 8.2.0.7: Changed treatment of locally bound <span class="RktVar">id</span> to
always report a syntax error, even outside of a module.</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, &quot;8.6&quot;, &quot;../&quot;);"/></form>&nbsp;&nbsp;<a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot(&quot;8.6&quot;);">top</a><span class="tocsettoggle">&nbsp;&nbsp;<a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright">&nbsp;&nbsp;<a href="__expression.html" title="backward to &quot;3.4 Expression Wrapper: #%expression&quot;" data-pltdoc="x">&larr; prev</a>&nbsp;&nbsp;<a href="syntax.html" title="up to &quot;3 Syntactic Forms&quot;" data-pltdoc="x">up</a>&nbsp;&nbsp;<a href="Locations____variable-reference.html" title="forward to &quot;3.6 Locations: #%variable-reference&quot;" data-pltdoc="x">next &rarr;</a></span>&nbsp;</div></div></div><div id="contextindicator">&nbsp;</div></body></html>
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