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1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_2");">▼</a></td><td>16.2 </td><td><a href="proc-macros.html" class="tocviewlink" data-pltdoc="x">General Macro Transformers</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_2"><table cellspacing="0" cellpadding="0"><tr><td align="right">16.2.1 </td><td><a href="stx-obj.html" class="tocviewlink" data-pltdoc="x">Syntax Objects</a></td></tr><tr><td align="right">16.2.2 </td><td><a href="macro-transformers.html" class="tocviewlink" data-pltdoc="x">Macro Transformer Procedures</a></td></tr><tr><td align="right">16.2.3 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Mixing Patterns and Expressions:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">syntax-<wbr></wbr>case</span></span></a></td></tr><tr><td align="right">16.2.4 </td><td><a href="with-syntax.html" class="tocviewlink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">with-<wbr></wbr>syntax</span></span> and <span class="RktSym"><span class="RktValLink">generate-<wbr></wbr>temporaries</span></span></a></td></tr><tr><td align="right">16.2.5 </td><td><a href="stx-phases.html" class="tocviewlink" data-pltdoc="x">Compile and Run-<wbr></wbr>Time Phases</a></td></tr><tr><td align="right">16.2.6 </td><td><a href="phases.html" class="tocviewlink" data-pltdoc="x">General Phase Levels</a></td></tr><tr><td align="right">16.2.7 </td><td><a href="stx-certs.html" class="tocviewlink" data-pltdoc="x">Syntax Taints</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="macro-transformers.html" title="backward to "16.2.2 Macro Transformer Procedures"" data-pltdoc="x">← prev</a> <a href="proc-macros.html" title="up to "16.2 General Macro Transformers"" data-pltdoc="x">up</a> <a href="with-syntax.html" title="forward to "16.2.4 with-syntax and generate-temporaries"" data-pltdoc="x">next →</a></span> </div><h5 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""syntax-case"">16.2.3<tt> </tt><a name="(part._syntax-case)"></a>Mixing Patterns and Expressions: <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._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span></h5><p>The procedure generated by <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._syntax-rules%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-rules</a></span> internally uses
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=stxops.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._syntax-e%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">syntax-e</a></span> to deconstruct the given syntax object, and it uses
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=stxops.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._datum-%7E3esyntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">datum->syntax</a></span> to construct the result. The
<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._syntax-rules%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-rules</a></span> form doesn’t provide a way to escape from
pattern-matching and template-construction mode into an arbitrary
Racket expression.</p><p>The <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._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span> form lets you mix pattern matching, template
construction, and arbitrary expressions:</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=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktVar">stx-expr</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">literal-id</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></blockquote><p>Unlike <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._syntax-rules%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-rules</a></span>, the <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._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span> form does not
produce a procedure. Instead, it starts with a <span class="RktVar">stx-expr</span>
expression that determines the syntax object to match against the
<span class="RktVar">pattern</span>s. Also, each <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._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span> clause has a
<span class="RktVar">pattern</span> and <span class="RktVar">expr</span>, instead of a <span class="RktVar">pattern</span>
and <span class="RktVar">template</span>. Within an <span class="RktVar">expr</span>, the <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._syntax%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax</a></span>
form—<wbr></wbr>usually abbreviated with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#'</span><span class="hspace"></span></span>—<wbr></wbr>shifts into
template-construction mode; if the <span class="RktVar">expr</span> of a clause starts
with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#'</span><span class="hspace"></span></span>, then we have something like a <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._syntax-rules%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-rules</a></span>
form:</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=stxops.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._syntax-%7E3edatum%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">syntax->datum</a></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._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktRdr">#'</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="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="hspace"> </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">op</span><span class="hspace"> </span><span class="RktSym">n1</span><span class="hspace"> </span><span class="RktSym">n2</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktRdr">#'</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._-%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x"><span class="nobreak">-</span></a></span><span class="hspace"> </span><span class="RktSym">n1</span><span class="hspace"> </span><span class="RktSym">n2</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'(- 1 2)</span></p></td></tr></table></blockquote><p>We could write the <span class="RktSym">swap</span> macro using <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._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span>
instead of <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%252Fmisc..rkt%2529._define-syntax-rule%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-syntax-rule</a></span> or <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._syntax-rules%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-rules</a></span>:</p><blockquote class="SCodeFlow"><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=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define-syntax%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">swap</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><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._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">swap</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="RktRdr">#'</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=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><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">tmp</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="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">set!</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</span></td></tr><tr><td><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=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">set!</a></span><span class="hspace"> </span><span class="RktSym">y</span><span class="hspace"> </span><span class="RktSym">tmp</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>One advantage of using <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._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span> is that we can provide
better error reporting for <span class="RktSym">swap</span>. For example, with the
<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%252Fmisc..rkt%2529._define-syntax-rule%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-syntax-rule</a></span> definition of <span class="RktSym">swap</span>, then
<span class="RktPn">(</span><span class="RktSym">swap</span><span class="stt"> </span><span class="RktSym">x</span><span class="stt"> </span><span class="RktVal">2</span><span class="RktPn">)</span> produces a syntax error in terms of <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">set!</a></span>,
because <span class="RktVal">2</span> is not an identifier. We can refine our
<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._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span> implementation of <span class="RktSym">swap</span> to explicitly
check the sub-forms:</p><blockquote class="SCodeFlow"><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=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define-syntax%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">swap</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><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._syntax-case%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">swap</span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</span></td></tr><tr><td><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=if.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._if%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">if</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=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._and%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">and</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=stxops.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fstx..rkt%2529._identifier%7E3f%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">identifier?</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr><tr><td><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=stxops.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fstx..rkt%2529._identifier%7E3f%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">identifier?</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">y</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktRdr">#'</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=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><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">tmp</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="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">set!</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</span></td></tr><tr><td><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=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">set!</a></span><span class="hspace"> </span><span class="RktSym">y</span><span class="hspace"> </span><span class="RktSym">tmp</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><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=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._raise-syntax-error%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">raise-syntax-error</a></span><span class="hspace"> </span><span class="RktVal">#f</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">"not an identifier"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">stx</span></td></tr><tr><td><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=if.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._if%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">if</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=stxops.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fstx..rkt%2529._identifier%7E3f%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">identifier?</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">y</span></td></tr><tr><td><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>With this definition, <span class="RktPn">(</span><span class="RktSym">swap</span><span class="stt"> </span><span class="RktSym">x</span><span class="stt"> </span><span class="RktVal">2</span><span class="RktPn">)</span> provides a syntax error
originating from <span class="RktSym">swap</span> instead of <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">set!</a></span>.</p><p>In the above definition of <span class="RktSym">swap</span>, <span class="RktRdr">#'</span><span class="RktSym">x</span> and
<span class="RktRdr">#'</span><span class="RktSym">y</span> are templates, even though they are not used as the
result of the macro transformer. This example illustrates how
templates can be used to access pieces of the input syntax, in this
case for checking the form of the pieces. Also, the match for
<span class="RktRdr">#'</span><span class="RktSym">x</span> or <span class="RktRdr">#'</span><span class="RktSym">y</span> is used in the call to
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._raise-syntax-error%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">raise-syntax-error</a></span>, so that the syntax-error message can
point directly to the source location of the non-identifier.</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="macro-transformers.html" title="backward to "16.2.2 Macro Transformer Procedures"" data-pltdoc="x">← prev</a> <a href="proc-macros.html" title="up to "16.2 General Macro Transformers"" data-pltdoc="x">up</a> <a href="with-syntax.html" title="forward to "16.2.4 with-syntax and generate-temporaries"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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