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}</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 Guide</span></a></td></tr></table></div><div class="tocviewsublistonly" style="display: block;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="intro.html" class="tocviewlink" data-pltdoc="x">Welcome to Racket</a></td></tr><tr><td align="right">2 </td><td><a href="to-scheme.html" class="tocviewlink" data-pltdoc="x">Racket Essentials</a></td></tr><tr><td align="right">3 </td><td><a href="datatypes.html" class="tocviewlink" data-pltdoc="x">Built-<wbr></wbr>In Datatypes</a></td></tr><tr><td align="right">4 </td><td><a href="scheme-forms.html" class="tocviewlink" data-pltdoc="x">Expressions and Definitions</a></td></tr><tr><td align="right">5 </td><td><a href="define-struct.html" class="tocviewlink" data-pltdoc="x">Programmer-<wbr></wbr>Defined Datatypes</a></td></tr><tr><td align="right">6 </td><td><a href="modules.html" class="tocviewlink" data-pltdoc="x">Modules</a></td></tr><tr><td align="right">7 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">8 </td><td><a href="i_o.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">9 </td><td><a href="regexp.html" class="tocviewlink" data-pltdoc="x">Regular Expressions</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Exceptions and Control</a></td></tr><tr><td align="right">11 </td><td><a href="for.html" class="tocviewlink" data-pltdoc="x">Iterations and Comprehensions</a></td></tr><tr><td align="right">12 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">13 </td><td><a href="classes.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">14 </td><td><a href="units.html" class="tocviewlink" data-pltdoc="x">Units (Components)</a></td></tr><tr><td align="right">15 </td><td><a href="reflection.html" class="tocviewlink" data-pltdoc="x">Reflection and Dynamic Evaluation</a></td></tr><tr><td align="right">16 </td><td><a href="macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">17 </td><td><a href="languages.html" class="tocviewlink" data-pltdoc="x">Creating Languages</a></td></tr><tr><td align="right">18 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Synchronization</a></td></tr><tr><td align="right">19 </td><td><a href="performance.html" class="tocviewlink" data-pltdoc="x">Performance</a></td></tr><tr><td align="right">20 </td><td><a href="parallelism.html" class="tocviewlink" data-pltdoc="x">Parallelism</a></td></tr><tr><td align="right">21 </td><td><a href="running.html" class="tocviewlink" data-pltdoc="x">Running and Creating Executables</a></td></tr><tr><td align="right">22 </td><td><a href="More_Libraries.html" class="tocviewlink" data-pltdoc="x">More Libraries</a></td></tr><tr><td align="right">23 </td><td><a href="dialects.html" class="tocviewlink" data-pltdoc="x">Dialects of Racket and Scheme</a></td></tr><tr><td align="right">24 </td><td><a href="other-editors.html" class="tocviewlink" data-pltdoc="x">Command-<wbr></wbr>Line Tools and Your Editor of Choice</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></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="for.html" title="backward to "11 Iterations and Comprehensions"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "The Racket Guide"" data-pltdoc="x">up</a> <a href="classes.html" title="forward to "13 Classes and Objects"" data-pltdoc="x">next →</a></span> </div><h3 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""match"">12<tt> </tt><a name="(part._match)"></a>Pattern Matching</h3><p>The <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span> form supports pattern matching on arbitrary Racket
values, as opposed to functions like <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span> that
compare regular expressions to byte and character sequences (see
<a href="regexp.html" data-pltdoc="x">Regular Expressions</a>).</p><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVar">target-expr</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></blockquote><p>The <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span> form takes the result of <span class="RktSym">target-expr</span> and
tries to match each <span class="RktVar">pattern</span> in order. As soon as it finds a
match, it evaluates the corresponding <span class="RktVar">expr</span> sequence to
obtain the result for the <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span> form. If <span class="RktVar">pattern</span>
includes <a name="(tech._pattern._variable)"></a><span style="font-style: italic">pattern variables</span>, they are treated like wildcards,
and each variable is bound in the <span class="RktVar">expr</span> to the input
fragments that it matched.</p><p>Most Racket literal expressions can be used as patterns:</p><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">2</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">one</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">two</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">three</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'two</span></p></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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">#f</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">#t</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">yes</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">no</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'no</span></p></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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">"apple"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">'</span><span class="RktVal">apple</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">symbol</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">"apple"</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">string</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">boolean</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'string</span></p></td></tr></table></blockquote><p>Constructors like <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._cons%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">cons</a></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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span>, and <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=vectors.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._vector%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">vector</a></span>
can be used to create patterns that match pairs, lists, and vectors:</p><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVal">0</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">one</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="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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">two</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'two</span></p></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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">. </span><span class="RktVal">2</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">list</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="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._cons%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">pair</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'pair</span></p></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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">#</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">list</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="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=vectors.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._vector%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">vector</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">vector</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'vector</span></p></td></tr></table></blockquote><p>A constructor bound with <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define-struct.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._struct%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">struct</a></span> also can be used as a pattern
constructor:</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=define-struct.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._struct%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">shoe</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">size</span><span class="hspace"> </span><span class="RktSym">color</span><span class="RktPn">)</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=define-struct.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._struct%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">hat</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">size</span><span class="hspace"> </span><span class="RktSym">style</span><span class="RktPn">)</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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hat</span><span class="hspace"> </span><span class="RktVal">23</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">bowler</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="RktSym">shoe</span><span class="hspace"> </span><span class="RktVal">10</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">white</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">"bottom"</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="RktSym">hat</span><span class="hspace"> </span><span class="RktVal">23</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">bowler</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">"top"</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"top"</span></p></td></tr></table></blockquote><p>Unquoted, non-constructor identifiers in a pattern are <a href="#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern
variables</span></a> that are bound in the result expressions, except <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529.__%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">_</a></span>,
which does not bind (and thus is usually used as a catch-all):</p><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</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=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktVal">1</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="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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</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=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">2</span></p></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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</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=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktVal">1</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="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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</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=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">3</span></p></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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hat</span><span class="hspace"> </span><span class="RktVal">23</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">bowler</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="RktSym">shoe</span><span class="hspace"> </span><span class="RktSym">sz</span><span class="hspace"> </span><span class="RktSym">col</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">sz</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="RktSym">hat</span><span class="hspace"> </span><span class="RktSym">sz</span><span class="hspace"> </span><span class="RktSym">stl</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">sz</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">23</span></p></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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hat</span><span class="hspace"> </span><span class="RktVal">11</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">cowboy</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="RktSym">shoe</span><span class="hspace"> </span><span class="RktSym">sz</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">black</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">a-good-shoe</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="RktSym">hat</span><span class="hspace"> </span><span class="RktSym">sz</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">bowler</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">a-good-hat</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=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529.__%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">something-else</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'something-else</span></p></td></tr></table></blockquote><p>An ellipsis, written <span class="RktInBG"><span class="hspace"></span><span class="RktIn">...</span><span class="hspace"></span></span>, acts like a Kleene star within a
list or vector pattern: the preceding sub-pattern can be used to match
any number of times for any number of consecutive elements of the list
or vector. If a sub-pattern followed by an ellipsis includes a pattern
variable, the variable matches multiple times, and it is bound in the
result expression to a list of matches:</p><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVal">1</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=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529._......%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">ones</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=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529.__%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">other</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'ones</span></p></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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVal">1</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=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529._......%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">ones</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=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529.__%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">other</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'other</span></p></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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktVal">4</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktSym">x</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=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529._......%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">...</a></span><span class="hspace"> </span><span class="RktVal">4</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'(2 3)</span></p></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=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</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=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hat</span><span class="hspace"> </span><span class="RktVal">23</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">bowler</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hat</span><span class="hspace"> </span><span class="RktVal">22</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">pork-pie</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="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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hat</span><span class="hspace"> </span><span class="RktSym">sz</span><span class="hspace"> </span><span class="RktSym">styl</span><span class="RktPn">)</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=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529._......%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">...</a></span><span class="RktPn">)</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=procedures.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._apply%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">apply</a></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=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">sz</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">45</span></p></td></tr></table></blockquote><p>Ellipses can be nested to match nested repetitions, and in that case,
pattern variables can be bound to lists of lists of matches:</p><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">(</span><span class="RktVal">!</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktVal">)</span><span class="hspace"> </span><span class="RktVal">(</span><span class="RktVal">!</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktVal">)</span><span class="hspace"> </span><span class="RktVal">(</span><span class="RktVal">!</span><span class="hspace"> </span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktVal">)</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</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=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">!</span><span class="hspace"> </span><span class="RktSym">x</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=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529._......%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">...</a></span><span class="RktPn">)</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=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529._......%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'((1) (2 2) (3 3 3))</span></p></td></tr></table></blockquote><p>The <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=quasiquote.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._quasiquote%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">quasiquote</a></span> form (see <a href="qq.html" data-pltdoc="x">Quasiquoting: <span class="RktSym"><span class="RktStxLink">quasiquote</span></span> and <span class="RktVal">‘</span></a> for more about it) can also be used to build patterns.
While unquoted portions of a normal quasiquoted form mean regular racket evaluation, here unquoted
portions mean go back to regular pattern matching.</p><p>So, in the example below, the with expression is the pattern and it gets rewritten into the
application expression, using quasiquote as a pattern in the first instance and quasiquote
to build an expression in the second.</p><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktVal">`</span><span class="RktVal">{</span><span class="RktVal">with</span><span class="hspace"> </span><span class="RktVal">{</span><span class="RktVal">x</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktVal">}</span><span class="hspace"> </span><span class="RktVal">{</span><span class="RktVal">+</span><span class="hspace"> </span><span class="RktVal">x</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktVal">}</span><span class="RktVal">}</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVal">`</span><span class="RktVal">{</span><span class="RktVal">with</span><span class="hspace"> </span><span class="RktVal">{</span><span class="RktRdr">,</span><span class="RktSym">id</span><span class="hspace"> </span><span class="RktRdr">,</span><span class="RktSym">rhs</span><span class="RktVal">}</span><span class="hspace"> </span><span class="RktRdr">,</span><span class="RktSym">body</span><span class="RktVal">}</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">`</span><span class="RktVal">{</span><span class="RktVal">{</span><span class="RktVal">lambda</span><span class="hspace"> </span><span class="RktVal">{</span><span class="RktRdr">,</span><span class="RktSym">id</span><span class="RktVal">}</span><span class="hspace"> </span><span class="RktRdr">,</span><span class="RktSym">body</span><span class="RktVal">}</span><span class="hspace"> </span><span class="RktRdr">,</span><span class="RktSym">rhs</span><span class="RktVal">}</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'((lambda (x) (+ x 1)) 1)</span></p></td></tr></table></blockquote><p>For information on many more pattern forms, see <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/match</span></a>.</p><p>Forms like <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match-let%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match-let</a></span> and <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match-lambda%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match-lambda</a></span> support
patterns in positions that otherwise must be identifiers. For example,
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match-let%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match-let</a></span> generalizes <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">let</a></span> to a <a name="(idx._(gentag._2._(lib._scribblings/guide/guide..scrbl)))"></a>destructing
bind:</p><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match-let%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match-let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="hspace"> </span><span class="RktSym">z</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktVal">)</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="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._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">z</span><span class="hspace"> </span><span class="RktSym">y</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'(3 2 1)</span></p></td></tr></table></blockquote><p>For information on these additional forms, see <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/match</span></a>.</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="magnify.png" alt="+" width="24" height="24"/></span><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html&version=6.1" class="Sq" data-pltdoc="x">Pattern Matching</a> in <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="Sq" data-pltdoc="x"><span style="font-weight: bold">The Racket Reference</span></a> provides more on pattern matching.</p></blockquote></blockquote></blockquote><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="for.html" title="backward to "11 Iterations and Comprehensions"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "The Racket Guide"" data-pltdoc="x">up</a> <a href="classes.html" title="forward to "13 Classes and Objects"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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