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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>3.23 Blocks: block</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"><span style="font-weight: bold">The Racket Reference</span></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="tocviewselflink" 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="tocviewlink" 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>3 </td><td><a href="syntax.html" class="tocviewlink" data-pltdoc="x">Syntactic Forms</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">3.1 </td><td><a href="module.html" class="tocviewlink" data-pltdoc="x">Modules:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">module</span></span>, <span class="RktSym"><span class="RktStxLink">module*</span></span>, ...</a></td></tr><tr><td align="right">3.2 </td><td><a href="require.html" class="tocviewlink" data-pltdoc="x">Importing and Exporting:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">require</span></span> and <span class="RktSym"><span class="RktStxLink">provide</span></span></a></td></tr><tr><td align="right">3.3 </td><td><a href="quote.html" class="tocviewlink" data-pltdoc="x">Literals:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">quote</span></span> and <span class="RktSym"><span class="RktStxLink">#%datum</span></span></a></td></tr><tr><td align="right">3.4 </td><td><a href="__expression.html" class="tocviewlink" data-pltdoc="x">Expression Wrapper:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">#%expression</span></span></a></td></tr><tr><td align="right">3.5 </td><td><a href="__top.html" class="tocviewlink" data-pltdoc="x">Variable References and <span class="RktSym"><span class="RktStxLink">#%top</span></span></a></td></tr><tr><td align="right">3.6 </td><td><a href="Locations____variable-reference.html" class="tocviewlink" data-pltdoc="x">Locations:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">#%variable-<wbr></wbr>reference</span></span></a></td></tr><tr><td align="right">3.7 </td><td><a href="application.html" class="tocviewlink" data-pltdoc="x">Procedure Applications and <span class="RktSym"><span class="RktStxLink">#%app</span></span></a></td></tr><tr><td align="right">3.8 </td><td><a href="lambda.html" class="tocviewlink" data-pltdoc="x">Procedure Expressions:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">lambda</span></span> and <span class="RktSym"><span class="RktStxLink">case-<wbr></wbr>lambda</span></span></a></td></tr><tr><td align="right">3.9 </td><td><a href="let.html" class="tocviewlink" data-pltdoc="x">Local Binding:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">let</span></span>, <span class="RktSym"><span class="RktStxLink">let*</span></span>, <span class="RktSym"><span class="RktStxLink">letrec</span></span>, ...</a></td></tr><tr><td align="right">3.10 </td><td><a href="local.html" class="tocviewlink" data-pltdoc="x">Local Definitions:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">local</span></span></a></td></tr><tr><td align="right">3.11 </td><td><a href="shared.html" class="tocviewlink" data-pltdoc="x">Constructing Graphs:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">shared</span></span></a></td></tr><tr><td align="right">3.12 </td><td><a href="if.html" class="tocviewlink" data-pltdoc="x">Conditionals:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">if</span></span>, <span class="RktSym"><span class="RktStxLink">cond</span></span>, <span class="RktSym"><span class="RktStxLink">and</span></span>, and <span class="RktSym"><span class="RktStxLink">or</span></span></a></td></tr><tr><td align="right">3.13 </td><td><a href="case.html" class="tocviewlink" data-pltdoc="x">Dispatch:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">case</span></span></a></td></tr><tr><td align="right">3.14 </td><td><a href="define.html" class="tocviewlink" data-pltdoc="x">Definitions:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">define</span></span>, <span class="RktSym"><span class="RktStxLink">define-<wbr></wbr>syntax</span></span>, ...</a></td></tr><tr><td align="right">3.15 </td><td><a href="begin.html" class="tocviewlink" data-pltdoc="x">Sequencing:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">begin</span></span>, <span class="RktSym"><span class="RktStxLink">begin0</span></span>, and <span class="RktSym"><span class="RktStxLink">begin-<wbr></wbr>for-<wbr></wbr>syntax</span></span></a></td></tr><tr><td align="right">3.16 </td><td><a href="when_unless.html" class="tocviewlink" data-pltdoc="x">Guarded Evaluation:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">when</span></span> and <span class="RktSym"><span class="RktStxLink">unless</span></span></a></td></tr><tr><td align="right">3.17 </td><td><a href="set_.html" class="tocviewlink" data-pltdoc="x">Assignment:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">set!</span></span> and <span class="RktSym"><span class="RktStxLink">set!-<wbr></wbr>values</span></span></a></td></tr><tr><td align="right">3.18 </td><td><a href="for.html" class="tocviewlink" data-pltdoc="x">Iterations and Comprehensions:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">for</span></span>, <span class="RktSym"><span class="RktStxLink">for/<span class="mywbr"> </span>list</span></span>, ...</a></td></tr><tr><td align="right">3.19 </td><td><a href="wcm.html" class="tocviewlink" data-pltdoc="x">Continuation Marks:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">with-<wbr></wbr>continuation-<wbr></wbr>mark</span></span></a></td></tr><tr><td align="right">3.20 </td><td><a href="quasiquote.html" class="tocviewlink" data-pltdoc="x">Quasiquoting:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">quasiquote</span></span>, <span class="RktSym"><span class="RktStxLink">unquote</span></span>, and <span class="RktSym"><span class="RktStxLink">unquote-<wbr></wbr>splicing</span></span></a></td></tr><tr><td align="right">3.21 </td><td><a href="Syntax_Quoting__quote-syntax.html" class="tocviewlink" data-pltdoc="x">Syntax Quoting:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">quote-<wbr></wbr>syntax</span></span></a></td></tr><tr><td align="right">3.22 </td><td><a href="__top-interaction.html" class="tocviewlink" data-pltdoc="x">Interaction Wrapper:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">#%top-<wbr></wbr>interaction</span></span></a></td></tr><tr><td align="right">3.23 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Blocks:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">block</span></span></a></td></tr><tr><td align="right">3.24 </td><td><a href="stratified-body.html" class="tocviewlink" data-pltdoc="x">Internal-<wbr></wbr>Definition Limiting:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">#%stratified-<wbr></wbr>body</span></span></a></td></tr><tr><td align="right">3.25 </td><td><a href="performance-hint.html" class="tocviewlink" data-pltdoc="x">Performance Hints:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">begin-<wbr></wbr>encourage-<wbr></wbr>inline</span></span></a></td></tr><tr><td align="right">3.26 </td><td><a href="lazy-require.html" class="tocviewlink" data-pltdoc="x">Importing Modules Lazily:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">lazy-<wbr></wbr>require</span></span></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%2Fblock..rkt%29._block%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">block</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="__top-interaction.html" title="backward to "3.22 Interaction Wrapper: #%top-interaction"" data-pltdoc="x">← prev</a> <a href="syntax.html" title="up to "3 Syntactic Forms"" data-pltdoc="x">up</a> <a href="stratified-body.html" title="forward to "3.24 Internal-Definition Limiting: #%stratified-body"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-part-tag=""block"">3.23<tt> </tt><a name="(part._block)"></a><a name="(mod-path._racket/block)"></a>Blocks: <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fblock..rkt%29._block%29%29" class="RktStxLink" data-pltdoc="x">block</a></span></h4><p><div class="SIntrapara"><table cellspacing="0" cellpadding="0" class="defmodule"><tr><td align="left"><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="stt"> </span><a href="" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/block</span></a><span class="RktPn">)</span></td><td align="right"><span class="RpackageSpec"><span class="Smaller"> package:</span> <span class="stt">base</span></span></td></tr></table></div><div class="SIntrapara">The bindings documented in this section are provided by the <a href="" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/block</span></a> library, not <a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/base</span></a> or <a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket</span></a>.</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._((lib._racket/block..rkt)._block))"></a><span title="Provided from: racket/block | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fblock..rkt%29._block%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">block</a></span></span><span class="hspace"> </span><span class="RktVar">defn-or-expr</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Supports a mixture of expressions and mutually recursive definitions,
as in 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> body. Unlike an <a href="syntax-model.html#%28tech._internal._definition._context%29" class="techoutside" data-pltdoc="x"><span class="techinside">internal-definition
context</span></a>, the last <span class="RktVar">defn-or-expr</span> need not be an expression.</div></p><p>The result of the <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fblock..rkt%29._block%29%29" class="RktStxLink" data-pltdoc="x">block</a></span> form is the result
of the last <span class="RktVar">defn-or-expr</span> if it is an expression,
<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">#<void></span></span></a> otherwise. If no <span class="RktVar">defn-or-expr</span> is provided
(after flattening <span class="RktSym"><a href="begin.html#%28form._%28%28quote._~23~25kernel%29._begin%29%29" class="RktStxLink" data-pltdoc="x">begin</a></span> forms), the result is <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">#<void></span></span></a>.</p><p>The final <span class="RktVar">defn-or-expr</span> is executed in tail position, if it is
an expression.</p><p><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><p>Examples:</p></td></tr><tr><td><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </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"> </span><span class="RktPn">(</span><span class="RktSym">f</span><span class="hspace"> </span><span class="RktSym">x</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="#%28form._%28%28lib._racket%2Fblock..rkt%29._block%29%29" class="RktStxLink" data-pltdoc="x">block</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </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"> </span><span class="RktSym">y</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._add1%29%29" class="RktValLink" data-pltdoc="x">add1</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</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="Writing.html#%28def._%28%28lib._racket%2Fprivate%2Fmisc..rkt%29._displayln%29%29" class="RktValLink" data-pltdoc="x">displayln</a></span><span class="hspace"> </span><span class="RktSym">y</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="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">z</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2A%29%29" class="RktValLink" data-pltdoc="x">*</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</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="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktSym">z</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><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">f</span><span class="hspace"> </span><span class="RktVal">12</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktOut">13</span></p></td></tr><tr><td><p><span class="RktRes">29</span></p></td></tr></table></blockquote></td></tr></table></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="__top-interaction.html" title="backward to "3.22 Interaction Wrapper: #%top-interaction"" data-pltdoc="x">← prev</a> <a href="syntax.html" title="up to "3 Syntactic Forms"" data-pltdoc="x">up</a> <a href="stratified-body.html" title="forward to "3.24 Internal-Definition Limiting: #%stratified-body"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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