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121 lines
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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8" /><title>4 Pattern matching: syntax-case and syntax-rules</title><link rel="stylesheet" type="text/css" href="scribble.css" title="default" /><link rel="stylesheet" type="text/css" href="racket.css" title="default" /><link rel="stylesheet" type="text/css" href="scribble-style.css" title="default" /><link rel="stylesheet" type="text/css" href="gh.css" title="default" /><script type="text/javascript" src="scribble-common.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--><meta name="keywords" content="Racket,macros,Scheme"><meta name="description" content="Practical Racket macros"><meta name="author" content="Greg Hendershott"><meta name="charset" content="utf-8"><link href='http://fonts.googleapis.com/css?family=Fenix' rel='stylesheet' type='text/css'><script type="text/javascript">var _gaq = _gaq || [];_gaq.push(['_setAccount', 'UA-29709446-1']);_gaq.push(['_setDomainName', 'greghendershott.com']);_gaq.push(['_trackPageview']);(function() {var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);})();</script></head><body id="scribble-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist" style="margin-bottom: 1em;"><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">Fear of Macros</a></td></tr></table></div><div class="tocviewsublisttop" style="display: block;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="Preface.html" class="tocviewlink" data-pltdoc="x">Preface</a></td></tr><tr><td align="right">2 </td><td><a href="Our_plan_of_attack.html" class="tocviewlink" data-pltdoc="x">Our plan of attack</a></td></tr><tr><td align="right">3 </td><td><a href="Transform_.html" class="tocviewlink" data-pltdoc="x">Transform!</a></td></tr><tr><td align="right">4 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Pattern matching:<span class="mywbr"> </span> syntax-<wbr></wbr>case and syntax-<wbr></wbr>rules</a></td></tr><tr><td align="right">5 </td><td><a href="Syntax_parameters.html" class="tocviewlink" data-pltdoc="x">Syntax parameters</a></td></tr><tr><td align="right">6 </td><td><a href="What_s_the_point_of_splicing-let_.html" class="tocviewlink" data-pltdoc="x">What’s the point of <span class="RktSym"><span class="RktStxLink">splicing-<wbr></wbr>let</span></span>?</a></td></tr><tr><td align="right">7 </td><td><a href="Robust_macros__syntax-parse.html" class="tocviewlink" data-pltdoc="x">Robust macros:<span class="mywbr"> </span> syntax-<wbr></wbr>parse</a></td></tr><tr><td align="right">8 </td><td><a href="References_and_Acknowledgments.html" class="tocviewlink" data-pltdoc="x">References and Acknowledgments</a></td></tr><tr><td align="right">9 </td><td><a href="Epilogue.html" class="tocviewlink" data-pltdoc="x">Epilogue</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>4 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Pattern matching:<span class="mywbr"> </span> syntax-<wbr></wbr>case and syntax-<wbr></wbr>rules</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">4.1 </td><td><a href="#%28part._.Pattern_variable_vs__template---fight_%29" class="tocviewlink" data-pltdoc="x">Pattern variable vs. template—<wbr></wbr>fight!</a></td></tr><tr><td align="right">4.2 </td><td><a href="#%28part._.Making_our_own_struct%29" class="tocviewlink" data-pltdoc="x">Making our own <span class="RktSym"><span class="RktStxLink">struct</span></span></a></td></tr><tr><td align="right">4.3 </td><td><a href="#%28part._hash..refs%29" class="tocviewlink" data-pltdoc="x">Using dot notation for nested hash lookups</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><span class="tocsublinknumber">4.1<tt> </tt></span><a href="#(part._.Pattern_variable_vs__template---fight_)" class="tocsubseclink" data-pltdoc="x">Pattern variable vs. template—<wbr></wbr>fight!</a></td></tr><tr><td><span class="tocsublinknumber">4.1.1<tt> </tt></span><a href="#(part._with-syntax)" class="tocsubseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">with-<wbr></wbr>syntax</span></span></a></td></tr><tr><td><span class="tocsublinknumber">4.1.2<tt> </tt></span><a href="#(part._with-syntax_)" class="tocsubseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">with-<wbr></wbr>syntax*</span></span></a></td></tr><tr><td><span class="tocsublinknumber">4.1.3<tt> </tt></span><a href="#(part._format-id)" class="tocsubseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">format-<wbr></wbr>id</span></span></a></td></tr><tr><td><span class="tocsublinknumber">4.1.4<tt> </tt></span><a href="#(part._.Another_example)" class="tocsubseclink" data-pltdoc="x">Another example</a></td></tr><tr><td><span class="tocsublinknumber">4.2<tt> </tt></span><a href="#(part._.Making_our_own_struct)" class="tocsubseclink" data-pltdoc="x">Making our own <span class="RktSym"><span class="RktStxLink">struct</span></span></a></td></tr><tr><td><span class="tocsublinknumber">4.3<tt> </tt></span><a href="#(part._hash..refs)" class="tocsubseclink" data-pltdoc="x">Using dot notation for nested hash lookups</a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"> </span><span class="navright"> <a href="Transform_.html" title="backward to "3 Transform!"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "Fear of Macros"" data-pltdoc="x">up</a> <a href="Syntax_parameters.html" title="forward to "5 Syntax parameters"" data-pltdoc="x">next →</a></span> </div><h3>4<tt> </tt><a name="(part._pattern-matching)"></a>Pattern matching: syntax-case and syntax-rules</h3><p>Most useful syntax transformers work by taking some input syntax, and
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rearranging the pieces into something else. As we saw, this is
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possible but tedious using list accessors such as
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._cadddr%29%29" class="RktValLink" data-pltdoc="x">cadddr</a></span>. It’s more convenient and less error-prone to use
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/match.html#%28form._%28%28lib._racket%2Fmatch..rkt%29._match%29%29" class="RktStxLink" data-pltdoc="x">match</a></span> to do pattern-matching.</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>Historically, <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> and
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-rules%29%29" class="RktStxLink" data-pltdoc="x">syntax-rules</a></span> pattern matching came first. <span class="RktSym"><a href="http://docs.racket-lang.org/reference/match.html#%28form._%28%28lib._racket%2Fmatch..rkt%29._match%29%29" class="RktStxLink" data-pltdoc="x">match</a></span> was
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added to Racket later.</p></blockquote></blockquote></blockquote><p>It turns out that pattern-matching was one of the first improvements
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to be added to the Racket macro system. It’s called
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span>, and has a shorthand for simple situations called
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fmisc..rkt%29._define-syntax-rule%29%29" class="RktStxLink" data-pltdoc="x">define-syntax-rule</a></span>.</p><p>Recall our previous example:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span><span class="hspace"> </span><span class="RktSym">racket/match</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">our-if-using-match-v2</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://docs.racket-lang.org/reference/match.html#%28form._%28%28lib._racket%2Fmatch..rkt%29._match%29%29" class="RktStxLink" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3elist%29%29" class="RktValLink" data-pltdoc="x">syntax->list</a></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="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._list%29%29" class="RktValLink" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">condition</span><span class="hspace"> </span><span class="RktSym">true-expr</span><span class="hspace"> </span><span class="RktSym">false-expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktVal">`</span><span class="RktVal">(</span><span class="RktVal">cond</span><span class="hspace"> </span><span class="RktVal">[</span><span class="RktRdr">,</span><span class="RktSym">condition</span><span class="hspace"> </span><span class="RktRdr">,</span><span class="RktSym">true-expr</span><span class="RktVal">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">[</span><span class="RktVal">else</span><span class="hspace"> </span><span class="RktRdr">,</span><span class="RktSym">false-expr</span><span class="RktVal">]</span><span class="RktVal">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Here’s what it looks like using <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">our-if-using-syntax-case</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">condition</span><span class="hspace"> </span><span class="RktSym">true-expr</span><span class="hspace"> </span><span class="RktSym">false-expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._cond%29%29" class="RktStxLink" data-pltdoc="x">cond</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">condition</span><span class="hspace"> </span><span class="RktSym">true-expr</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._else%29%29" class="RktStxLink" data-pltdoc="x">else</a></span><span class="hspace"> </span><span class="RktSym">false-expr</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">our-if-using-syntax-case</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="hspace"> </span><span class="RktVal">"true"</span><span class="hspace"> </span><span class="RktVal">"false"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"true"</span></p></td></tr></table></blockquote><p>Pretty similar, huh? The pattern matching part looks almost exactly
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the same. The way we specify the new syntax is simpler. We don’t need
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to do quasi-quoting and unquoting. We don’t need to use
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span>. Instead, we supply a "template", which uses
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variables from the pattern.</p><p>There is a shorthand for simple pattern-matching cases, which expands
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into <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span>. It’s called <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fmisc..rkt%29._define-syntax-rule%29%29" class="RktStxLink" data-pltdoc="x">define-syntax-rule</a></span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fmisc..rkt%29._define-syntax-rule%29%29" class="RktStxLink" data-pltdoc="x">define-syntax-rule</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">our-if-using-syntax-rule</span><span class="hspace"> </span><span class="RktSym">condition</span><span class="hspace"> </span><span class="RktSym">true-expr</span><span class="hspace"> </span><span class="RktSym">false-expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._cond%29%29" class="RktStxLink" data-pltdoc="x">cond</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">condition</span><span class="hspace"> </span><span class="RktSym">true-expr</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._else%29%29" class="RktStxLink" data-pltdoc="x">else</a></span><span class="hspace"> </span><span class="RktSym">false-expr</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">our-if-using-syntax-rule</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="hspace"> </span><span class="RktVal">"true"</span><span class="hspace"> </span><span class="RktVal">"false"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"true"</span></p></td></tr></table></blockquote><p>Here’s the thing about <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fmisc..rkt%29._define-syntax-rule%29%29" class="RktStxLink" data-pltdoc="x">define-syntax-rule</a></span>. Because it’s so
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simple, <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fmisc..rkt%29._define-syntax-rule%29%29" class="RktStxLink" data-pltdoc="x">define-syntax-rule</a></span> is often the first thing people are
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taught about macros. But it’s almost deceptively simple. It looks so
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much like defining a normal run time function—<wbr></wbr>yet it’s not. It’s
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working at compile time, not run time. Worse, the moment you want to
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do more than <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fmisc..rkt%29._define-syntax-rule%29%29" class="RktStxLink" data-pltdoc="x">define-syntax-rule</a></span> can handle, you can fall off
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a cliff into what feels like complicated and confusing
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territory. Hopefully, because we started with a basic syntax
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transformer, and worked up from that, we won’t have that problem. We
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can appreciate <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fmisc..rkt%29._define-syntax-rule%29%29" class="RktStxLink" data-pltdoc="x">define-syntax-rule</a></span> as a convenient shorthand,
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but not be scared of, or confused about, that for which it’s
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shorthand.</p><p>Most of the materials I found for learning macros, including the
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Racket <span style="font-style: italic">Guide</span>, do a very good job explaining
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<a href="http://docs.racket-lang.org/guide/pattern-macros.html">how
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patterns and templates work</a>. So I won’t regurgitate that here.</p><p>Sometimes, we need to go a step beyond the pattern and template. Let’s
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look at some examples, how we can get confused, and how to get it
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working.</p><h4>4.1<tt> </tt><a name="(part._.Pattern_variable_vs__template---fight_)"></a>Pattern variable vs. template—<wbr></wbr>fight!</h4><p>Let’s say we want to define a function with a hyphenated name, a-b,
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but we supply the a and b parts separately. The Racket <span class="RktSym"><a href="http://docs.racket-lang.org/reference/define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span>
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macro does something like this: <span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="stt"> </span><span class="RktSym">foo</span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym">field1</span><span class="stt"> </span><span class="RktSym">field2</span><span class="RktPn">)</span><span class="RktPn">)</span>
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automatically defines a number of functions whose names are variations
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on the name <span class="RktSym">foo</span>—<wbr></wbr>such as <span class="RktSym">foo-field1</span>,
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<span class="RktSym">foo-field2</span>, <span class="RktSym">foo?</span>, and so on.</p><p>So let’s pretend we’re doing something like that. We want to transform
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the syntax <span class="RktPn">(</span><span class="RktSym">hyphen-define</span><span class="stt"> </span><span class="RktSym">a</span><span class="stt"> </span><span class="RktSym">b</span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym">args</span><span class="RktPn">)</span><span class="stt"> </span><span class="RktSym">body</span><span class="RktPn">)</span> to the syntax
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|
<span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym">a-b</span><span class="stt"> </span><span class="RktSym">args</span><span class="RktPn">)</span><span class="stt"> </span><span class="RktSym">body</span><span class="RktPn">)</span>.</p><p>A wrong first attempt is:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/wrong1</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._string-~3esymbol%29%29" class="RktValLink" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/Writing.html#%28def._%28%28quote._~23~25kernel%29._format%29%29" class="RktValLink" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktVal">"~a-~a"</span><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">name</span><span class="hspace"> </span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</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="RktErr">eval:47:0: a: pattern variable cannot be used outside of a</span></p></td></tr><tr><td><p><span class="RktErr">template</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: a</span></p></td></tr></table></blockquote><p>Huh. We have no idea what this error message means. Well, let’s try to
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work it out. The "template" the error message refers to is the
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<span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym">name</span><span class="stt"> </span><span class="RktSym">args</span><span class="stt"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="stt"> </span><span class="RktSym">body0</span><span class="stt"> </span><span class="RktSym">body</span><span class="stt"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span> portion. The
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span> isn’t part of that template. It sounds like we can’t use
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<span class="RktSym">a</span> (or <span class="RktSym">b</span>) in the <span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span> part.</p><p>In fact, <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> can have as many templates as you
|
|
want. The obvious, required template is the final expression supplying
|
|
the output syntax. But you can use <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%29%29" class="RktStxLink" data-pltdoc="x">syntax</a></span> (a.k.a. #’) on a
|
|
pattern variable. This makes another template, albeit a small, "fun
|
|
size" template. Let’s try that:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/wrong1.1</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._string-~3esymbol%29%29" class="RktValLink" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/Writing.html#%28def._%28%28quote._~23~25kernel%29._format%29%29" class="RktValLink" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktVal">"~a-~a"</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">b</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">name</span><span class="hspace"> </span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote><p>No more error—<wbr></wbr>good! Let’s try to use it:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/wrong1.1</span><span class="hspace"> </span><span class="RktSym">foo</span><span class="hspace"> </span><span class="RktSym">bar</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">foo-bar</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">foo-bar: undefined;</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">cannot reference an identifier before its definition</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in module: 'program</span></p></td></tr></table></blockquote><p>Apparently our macro is defining a function with some name other than
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|
<span class="RktSym">foo-bar</span>. Huh.</p><p><div class="SIntrapara">This is where the Macro Stepper in DrRacket is
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|
invaluable. </div><div class="SIntrapara"><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>Even if you prefer mostly to use Emacs, this
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|
is a situation where it’s definitely worth temporarily using DrRacket
|
|
for its Macro Stepper.</p></blockquote></blockquote></blockquote></div></p><p><img src="macro-stepper.png" alt="" width="598" height="564" /></p><p>The Macro Stepper says that the use of our macro:</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym">hyphen-define/wrong1.1</span><span class="hspace"> </span><span class="RktSym">foo</span><span class="hspace"> </span><span class="RktSym">bar</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="RktPn">)</span></p></blockquote><p>expanded to:</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">name</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="RktPn">)</span></p></blockquote><p>Well that explains it. Instead, we wanted to expand to:</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">foo-bar</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="RktPn">)</span></p></blockquote><p>Our template is using the symbol <span class="RktSym">name</span> but we wanted its
|
|
value, such as <span class="RktSym">foo-bar</span> in this use of our macro.</p><p>Is there anything we already know that behaves like this—<wbr></wbr>where using
|
|
a variable in the template yields its value? Yes: Pattern
|
|
variables. Our pattern doesn’t include <span class="RktSym">name</span> because we don’t
|
|
expect it in the original syntax—<wbr></wbr>indeed the whole point of this
|
|
macro is to create it. So <span class="RktSym">name</span> can’t be in the main
|
|
pattern. Fine—<wbr></wbr>let’s make an <span style="font-style: italic">additional</span> pattern. We can do
|
|
that using an additional, nested <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/wrong1.2</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span><span class="hspace"> </span><span class="RktSym">stx</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._string-~3esymbol%29%29" class="RktValLink" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/Writing.html#%28def._%28%28quote._~23~25kernel%29._format%29%29" class="RktValLink" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktVal">"~a-~a"</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">b</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">name</span><span class="hspace"> </span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></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></td></tr></table></blockquote><p>Looks weird? Let’s take a deep breath. Normally our transformer
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function is given syntax by Racket, and we pass that syntax to
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span>. But we can also create some syntax of our own,
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on the fly, and pass <span style="font-style: italic">that</span> to <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span>. That’s all
|
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we’re doing here. The whole <span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span><span class="stt"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span> expression is
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syntax that we’re creating on the fly. We can give that to
|
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span>, and match it using a pattern variable named
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<span class="RktSym">name</span>. Voila, we have a new pattern variable. We can use it in
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a template, and its value will go in the template.</p><p>We might have one more—<wbr></wbr>just one, I promise!—<wbr></wbr>small problem left.
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Let’s try to use our new version:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/wrong1.2</span><span class="hspace"> </span><span class="RktSym">foo</span><span class="hspace"> </span><span class="RktSym">bar</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">foo-bar</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">foo-bar: undefined;</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">cannot reference an identifier before its definition</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in module: 'program</span></p></td></tr></table></blockquote><p>Hmm. <span class="RktSym">foo-bar</span> is <span style="font-style: italic">still</span> not defined. Back to the Macro
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Stepper. It says now we’re expanding to:</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">|#<syntax:11:24foo>-#<syntax:11:28 bar>|</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="RktPn">)</span></p></blockquote><p>Oh right: <span class="RktRdr">#'</span><span class="RktSym">a</span> and <span class="RktRdr">#'</span><span class="RktSym">b</span> are syntax objects. Therefore</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._string-~3esymbol%29%29" class="RktValLink" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/Writing.html#%28def._%28%28quote._~23~25kernel%29._format%29%29" class="RktValLink" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktVal">"~a-~a"</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">b</span><span class="RktPn">)</span><span class="RktPn">)</span></p></blockquote><p>is the printed form of both syntax objects, joined by a hyphen:</p><blockquote class="SCodeFlow"><p><span class="RktSym">|#<syntax:11:24foo>-#<syntax:11:28 bar>|</span></p></blockquote><p>Instead we want the datum in the syntax objects, such as the symbols
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<span class="RktSym">foo</span> and <span class="RktSym">bar</span>. Which we get using
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax->datum</a></span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/ok1</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span><span class="hspace"> </span><span class="RktSym">stx</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._string-~3esymbol%29%29" class="RktValLink" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/Writing.html#%28def._%28%28quote._~23~25kernel%29._format%29%29" class="RktValLink" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktVal">"~a-~a"</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax->datum</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">a</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax->datum</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">b</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">name</span><span class="hspace"> </span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></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></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/ok1</span><span class="hspace"> </span><span class="RktSym">foo</span><span class="hspace"> </span><span class="RktSym">bar</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">foo-bar</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#t</span></p></td></tr></table></blockquote><p>And now it works!</p><p>Next, some shortcuts.</p><h5>4.1.1<tt> </tt><a name="(part._with-syntax)"></a><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span></h5><p>Instead of an additional, nested <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span>, we could use
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="refelem"><span class="refcolumn"><span class="refcontent">Another name for
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span> could be, "with new pattern variable".</span></span></span>. This
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rearranges the <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> to look more like a <span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span>
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statement—<wbr></wbr>first the name, then the value. Also it’s more convenient
|
|
if we need to define more than one pattern variable.</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/ok2</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span><span class="hspace"> </span><span class="RktSym">stx</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._string-~3esymbol%29%29" class="RktValLink" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/Writing.html#%28def._%28%28quote._~23~25kernel%29._format%29%29" class="RktValLink" data-pltdoc="x">format</a></span><span class="hspace"> </span><span class="RktVal">"~a-~a"</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax->datum</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">a</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax->datum</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">b</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><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">name</span><span class="hspace"> </span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/ok2</span><span class="hspace"> </span><span class="RktSym">foo</span><span class="hspace"> </span><span class="RktSym">bar</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">foo-bar</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#t</span></p></td></tr></table></blockquote><p>Again, <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span> is simply <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> rearranged:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span style="font-style: italic"><syntax></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">[</span><span style="font-weight: bold"><pattern></span><span class="hspace"> </span><span class="RktSym"><body></span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span style="font-weight: bold"><pattern></span><span class="hspace"> </span><span style="font-style: italic"><syntax></span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><body></span><span class="RktPn">)</span></td></tr></table></blockquote><p>Whether you use an additional <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> or use
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span>, either way you are simply defining additional
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pattern variables. Don’t let the terminology and structure make it
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seem mysterious.</p><h5>4.1.2<tt> </tt><a name="(part._with-syntax_)"></a><span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28form._%28%28lib._racket%2Fsyntax..rkt%29._with-syntax%2A%29%29" class="RktStxLink" data-pltdoc="x">with-syntax*</a></span></h5><p>We know that <span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span> doesn’t let us use a binding in a subsequent
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one:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktSym">a</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">b</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktErr">a: undefined;</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">cannot reference an identifier before its definition</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in module: 'program</span></p></td></tr></table></blockquote><p>Instead we can nest <span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span>s:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktSym">a</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">b</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">0</span></p></td></tr></table></blockquote><p>Or use a shorthand for nesting, <span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%2A%29%29" class="RktStxLink" data-pltdoc="x">let*</a></span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%2A%29%29" class="RktStxLink" data-pltdoc="x">let*</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktSym">a</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">b</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">0</span></p></td></tr></table></blockquote><p>Similarly, instead of writing nested <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span>s, we can use
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28form._%28%28lib._racket%2Fsyntax..rkt%29._with-syntax%2A%29%29" class="RktStxLink" data-pltdoc="x">with-syntax*</a></span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span><span class="hspace"> </span><span class="RktSym">racket/syntax</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">foo</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28form._%28%28lib._racket%2Fsyntax..rkt%29._with-syntax%2A%29%29" class="RktStxLink" data-pltdoc="x">with-syntax*</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">a</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">c</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">b</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">c</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote><p>One gotcha is that <span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28form._%28%28lib._racket%2Fsyntax..rkt%29._with-syntax%2A%29%29" class="RktStxLink" data-pltdoc="x">with-syntax*</a></span> isn’t provided by
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<span class="RktSym">racket/base</span>. We must <span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span><span class="stt"> </span><span class="RktSym">racket/syntax</span><span class="RktPn">)</span><span class="RktPn">)</span>. Otherwise we may get a rather bewildering error
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message:</p><p><span style="font-style: italic"><span class="stt">...: ellipses not allowed as an expression in: ...</span></span>.</p><h5>4.1.3<tt> </tt><a name="(part._format-id)"></a><span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28def._%28%28lib._racket%2Fsyntax..rkt%29._format-id%29%29" class="RktValLink" data-pltdoc="x">format-id</a></span></h5><p>There is a utility function in <span class="RktSym">racket/syntax</span> called
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28def._%28%28lib._racket%2Fsyntax..rkt%29._format-id%29%29" class="RktValLink" data-pltdoc="x">format-id</a></span> that lets us format identifier names more
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succinctly than what we did above:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span><span class="hspace"> </span><span class="RktSym">racket/syntax</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/ok3</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28def._%28%28lib._racket%2Fsyntax..rkt%29._format-id%29%29" class="RktValLink" data-pltdoc="x">format-id</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktVal">"~a-~a"</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">b</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">name</span><span class="hspace"> </span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define/ok3</span><span class="hspace"> </span><span class="RktSym">bar</span><span class="hspace"> </span><span class="RktSym">baz</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#t</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">bar-baz</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#t</span></p></td></tr></table></blockquote><p>Using <span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28def._%28%28lib._racket%2Fsyntax..rkt%29._format-id%29%29" class="RktValLink" data-pltdoc="x">format-id</a></span> is convenient as it handles the tedium of
|
|
converting from syntax to symbol datum to string ... and all the way
|
|
back.</p><h5>4.1.4<tt> </tt><a name="(part._.Another_example)"></a>Another example</h5><p>Finally, here’s a variation that accepts an arbitary number of name
|
|
parts to be joined with hyphens:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span><span class="hspace"> </span><span class="RktSym">racket/string</span><span class="hspace"> </span><span class="RktSym">racket/syntax</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define*</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">names</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%2A%29%29" class="RktStxLink" data-pltdoc="x">let*</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">names/sym</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28lib._racket%2Fprivate%2Fmap..rkt%29._map%29%29" class="RktValLink" data-pltdoc="x">map</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-e%29%29" class="RktValLink" data-pltdoc="x">syntax-e</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3elist%29%29" class="RktValLink" data-pltdoc="x">syntax->list</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">names</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">names/str</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28lib._racket%2Fprivate%2Fmap..rkt%29._map%29%29" class="RktValLink" data-pltdoc="x">map</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._symbol-~3estring%29%29" class="RktValLink" data-pltdoc="x">symbol->string</a></span><span class="hspace"> </span><span class="RktSym">names/sym</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">name/str</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/strings.html#%28def._%28%28lib._racket%2Fstring..rkt%29._string-join%29%29" class="RktValLink" data-pltdoc="x">string-join</a></span><span class="hspace"> </span><span class="RktSym">names/str</span><span class="hspace"> </span><span class="RktVal">"-"</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">name/sym</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._string-~3esymbol%29%29" class="RktValLink" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktSym">name/str</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktSym">name/sym</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#`</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">name</span><span class="hspace"> </span><span class="RktSym">args</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">body0</span><span class="hspace"> </span><span class="RktSym">body</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></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></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hyphen-define*</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">foo</span><span class="hspace"> </span><span class="RktSym">bar</span><span class="hspace"> </span><span class="RktSym">baz</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">v</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">foo-bar-baz</span><span class="hspace"> </span><span class="RktVal">50</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">100</span></p></td></tr></table></blockquote><p>To review:</p><ul><li><p>You can’t use a pattern variable outside of a template. But
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you can use <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%29%29" class="RktStxLink" data-pltdoc="x">syntax</a></span> or <span class="stt">#</span><span class="stt">’</span><span class="stt"></span> on a pattern variable to make
|
|
an ad hoc, "fun size" template.</p></li><li><p>If you want to munge pattern variables for use in the
|
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template, <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span> is your friend, because it lets you
|
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create new pattern variables.</p></li><li><p>Usually you’ll need to use <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax->datum</a></span> to get the
|
|
interesting value inside.</p></li><li><p><span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28def._%28%28lib._racket%2Fsyntax..rkt%29._format-id%29%29" class="RktValLink" data-pltdoc="x">format-id</a></span> is convenient for formatting identifier
|
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names.</p></li></ul><h4>4.2<tt> </tt><a name="(part._.Making_our_own_struct)"></a>Making our own <span class="RktSym"><a href="http://docs.racket-lang.org/reference/define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span></h4><p>Let’s apply what we just learned to a more-realistic example. We’ll
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pretend that Racket doesn’t already have a <span class="RktSym"><a href="http://docs.racket-lang.org/reference/define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span>
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capability. Fortunately, we can write a macro to provide our own
|
|
system for defining and using structures. To keep things simple, our
|
|
structure will be immutable (read-only) and it won’t support
|
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inheritance.</p><p>Given a structure declaration like:</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym">our-struct</span><span class="hspace"> </span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">field1</span><span class="hspace"> </span><span class="RktSym">field2</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span></p></blockquote><p>We need to define some procedures:</p><ul><li><p>A constructor procedure whose name is the struct name. We’ll
|
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represent structures as a <span class="RktSym"><a href="http://docs.racket-lang.org/reference/vectors.html#%28def._%28%28quote._~23~25kernel%29._vector%29%29" class="RktValLink" data-pltdoc="x">vector</a></span>. The structure name will be
|
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element zero. The fields will be elements one onward.</p></li><li><p>A predicate, whose name is the struct name with <span class="stt">?</span>
|
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appended.</p></li><li><p>For each field, an accessor procedure to get its value. These
|
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will be named struct-field (the name of the struct, a hyphen, and the
|
|
field name).</p></li></ul><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span><span class="hspace"> </span><span class="RktSym">racket/syntax</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">our-struct</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Library_Syntax_Classes_and_Literal_Sets.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._id%29%29" class="RktStxLink" data-pltdoc="x">id</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">fields</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">pred-id</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28def._%28%28lib._racket%2Fsyntax..rkt%29._format-id%29%29" class="RktValLink" data-pltdoc="x">format-id</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktVal">"~a?"</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Library_Syntax_Classes_and_Literal_Sets.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._id%29%29" class="RktStxLink" data-pltdoc="x">id</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#`</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/begin.html#%28form._%28%28quote._~23~25kernel%29._begin%29%29" class="RktStxLink" data-pltdoc="x">begin</a></span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Define a constructor.</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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"><a href="http://docs.racket-lang.org/syntax/Library_Syntax_Classes_and_Literal_Sets.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._id%29%29" class="RktStxLink" data-pltdoc="x">id</a></span><span class="hspace"> </span><span class="RktSym">fields</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/procedures.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._apply%29%29" class="RktValLink" data-pltdoc="x">apply</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/vectors.html#%28def._%28%28quote._~23~25kernel%29._vector%29%29" class="RktValLink" data-pltdoc="x">vector</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">id</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._list%29%29" class="RktValLink" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">fields</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Define a predicate.</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">pred-id</span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._and%29%29" class="RktStxLink" data-pltdoc="x">and</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/vectors.html#%28def._%28%28quote._~23~25kernel%29._vector~3f%29%29" class="RktValLink" data-pltdoc="x">vector?</a></span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/booleans.html#%28def._%28%28quote._~23~25kernel%29._eq~3f%29%29" class="RktValLink" data-pltdoc="x">eq?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/vectors.html#%28def._%28%28quote._~23~25kernel%29._vector-ref%29%29" class="RktValLink" data-pltdoc="x">vector-ref</a></span><span class="hspace"> </span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">id</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Define an accessor for each field.</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#,@</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/for.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for%2Flist%29%29" class="RktStxLink" data-pltdoc="x">for/list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3elist%29%29" class="RktValLink" data-pltdoc="x">syntax->list</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">fields</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">n</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/sequences.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._in-naturals%29%29" class="RktValLink" data-pltdoc="x">in-naturals</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">acc-id</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28def._%28%28lib._racket%2Fsyntax..rkt%29._format-id%29%29" class="RktValLink" data-pltdoc="x">format-id</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktVal">"~a-~a"</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Library_Syntax_Classes_and_Literal_Sets.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._id%29%29" class="RktStxLink" data-pltdoc="x">id</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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">ix</span><span class="hspace"> </span><span class="RktSym">n</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#`</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">acc-id</span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/when_unless.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._unless%29%29" class="RktStxLink" data-pltdoc="x">unless</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">pred-id</span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/exns.html#%28def._%28%28quote._~23~25kernel%29._error%29%29" class="RktValLink" data-pltdoc="x">error</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">acc-id</span><span class="hspace"> </span><span class="RktVal">"~a is not a ~a struct"</span><span class="hspace"> </span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">id</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/vectors.html#%28def._%28%28quote._~23~25kernel%29._vector-ref%29%29" class="RktValLink" data-pltdoc="x">vector-ref</a></span><span class="hspace"> </span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktSym">ix</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><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Test it out</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktSym">rackunit</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">our-struct</span><span class="hspace"> </span><span class="RktSym">foo</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">s</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">foo</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="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">check-true</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">foo?</span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">check-false</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">foo?</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">check-equal?</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">foo-a</span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">check-equal?</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">foo-b</span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">check-exn</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/exns.html#%28def._%28%28quote._~23~25kernel%29._exn~3afail~3f%29%29" class="RktValLink" data-pltdoc="x">exn:fail?</a></span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">foo-a</span><span class="hspace"> </span><span class="RktVal">"furble"</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">The tests passed.</span></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Next, what if someone tries to declare:</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">our-struct</span><span class="hspace"> </span><span class="RktVal">"blah"</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVal">"blah"</span><span class="hspace"> </span><span class="RktVal">"blah"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">format-id: contract violation</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">expected: (or/c string? symbol? identifier? keyword? char?</span></p></td></tr><tr><td><p><span class="RktErr">number?)</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">given: #<syntax:83:0 "blah"></span></p></td></tr></table></blockquote><p>The error message is not very helpful. It’s coming from
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<span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28def._%28%28lib._racket%2Fsyntax..rkt%29._format-id%29%29" class="RktValLink" data-pltdoc="x">format-id</a></span>, which is a private implementation detail of our macro.</p><p>You may know that a <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> clause can take an
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|
optional "guard" or "fender" expression. Instead of</p><blockquote class="SCodeFlow"><p><span class="RktPn">[</span><span class="RktSym"><a href="http://docs.racket-lang.org/syntax/stxparse-specifying.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._pattern%29%29" class="RktStxLink" data-pltdoc="x">pattern</a></span><span class="hspace"> </span><span class="RktSym">template</span><span class="RktPn">]</span></p></blockquote><p>It can be:</p><blockquote class="SCodeFlow"><p><span class="RktPn">[</span><span class="RktSym"><a href="http://docs.racket-lang.org/syntax/stxparse-specifying.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._pattern%29%29" class="RktStxLink" data-pltdoc="x">pattern</a></span><span class="hspace"> </span><span class="RktSym">guard</span><span class="hspace"> </span><span class="RktSym">template</span><span class="RktPn">]</span></p></blockquote><p>Let’s add a guard expression to our clause:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span><span class="hspace"> </span><span class="RktSym">racket/syntax</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">our-struct</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Library_Syntax_Classes_and_Literal_Sets.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._id%29%29" class="RktStxLink" data-pltdoc="x">id</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">fields</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Guard or "fender" expression:</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28lib._racket%2Fprivate%2Fmap..rkt%29._for-each%29%29" class="RktValLink" data-pltdoc="x">for-each</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/when_unless.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._unless%29%29" class="RktStxLink" data-pltdoc="x">unless</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28lib._racket%2Fprivate%2Fstx..rkt%29._identifier~3f%29%29" class="RktValLink" data-pltdoc="x">identifier?</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/exns.html#%28def._%28%28quote._~23~25kernel%29._raise-syntax-error%29%29" class="RktValLink" data-pltdoc="x">raise-syntax-error</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktVal">"not an identifier"</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Library_Syntax_Classes_and_Literal_Sets.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._id%29%29" class="RktStxLink" data-pltdoc="x">id</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3elist%29%29" class="RktValLink" data-pltdoc="x">syntax->list</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">fields</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">pred-id</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28def._%28%28lib._racket%2Fsyntax..rkt%29._format-id%29%29" class="RktValLink" data-pltdoc="x">format-id</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktVal">"~a?"</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Library_Syntax_Classes_and_Literal_Sets.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._id%29%29" class="RktStxLink" data-pltdoc="x">id</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#`</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/begin.html#%28form._%28%28quote._~23~25kernel%29._begin%29%29" class="RktStxLink" data-pltdoc="x">begin</a></span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Define a constructor.</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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"><a href="http://docs.racket-lang.org/syntax/Library_Syntax_Classes_and_Literal_Sets.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._id%29%29" class="RktStxLink" data-pltdoc="x">id</a></span><span class="hspace"> </span><span class="RktSym">fields</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/procedures.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._apply%29%29" class="RktValLink" data-pltdoc="x">apply</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/vectors.html#%28def._%28%28quote._~23~25kernel%29._vector%29%29" class="RktValLink" data-pltdoc="x">vector</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">id</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._list%29%29" class="RktValLink" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">fields</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Define a predicate.</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">pred-id</span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._and%29%29" class="RktStxLink" data-pltdoc="x">and</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/vectors.html#%28def._%28%28quote._~23~25kernel%29._vector~3f%29%29" class="RktValLink" data-pltdoc="x">vector?</a></span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/booleans.html#%28def._%28%28quote._~23~25kernel%29._eq~3f%29%29" class="RktValLink" data-pltdoc="x">eq?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/vectors.html#%28def._%28%28quote._~23~25kernel%29._vector-ref%29%29" class="RktValLink" data-pltdoc="x">vector-ref</a></span><span class="hspace"> </span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">id</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Define an accessor for each field.</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#,@</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/for.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for%2Flist%29%29" class="RktStxLink" data-pltdoc="x">for/list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3elist%29%29" class="RktValLink" data-pltdoc="x">syntax->list</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">fields</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">n</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/sequences.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._in-naturals%29%29" class="RktValLink" data-pltdoc="x">in-naturals</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">acc-id</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/syntax-util.html#%28def._%28%28lib._racket%2Fsyntax..rkt%29._format-id%29%29" class="RktValLink" data-pltdoc="x">format-id</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktVal">"~a-~a"</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Library_Syntax_Classes_and_Literal_Sets.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._id%29%29" class="RktStxLink" data-pltdoc="x">id</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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">ix</span><span class="hspace"> </span><span class="RktSym">n</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#`</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">acc-id</span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/when_unless.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._unless%29%29" class="RktStxLink" data-pltdoc="x">unless</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">pred-id</span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/exns.html#%28def._%28%28quote._~23~25kernel%29._error%29%29" class="RktValLink" data-pltdoc="x">error</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">acc-id</span><span class="hspace"> </span><span class="RktVal">"~a is not a ~a struct"</span><span class="hspace"> </span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">id</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/vectors.html#%28def._%28%28quote._~23~25kernel%29._vector-ref%29%29" class="RktValLink" data-pltdoc="x">vector-ref</a></span><span class="hspace"> </span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktSym">ix</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><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Now the same misuse gives a better error message:</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">our-struct</span><span class="hspace"> </span><span class="RktVal">"blah"</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVal">"blah"</span><span class="hspace"> </span><span class="RktVal">"blah"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">eval:86:0: our-struct: not an identifier</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: "blah"</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: (our-struct "blah" ("blah" "blah"))</span></p></td></tr></table></blockquote><p>Later, we’ll see how <span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Parsing_Syntax.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._syntax-parse%29%29" class="RktStxLink" data-pltdoc="x">syntax-parse</a></span> makes it even easier to
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check usage and provide helpful messages about mistakes.</p><h4>4.3<tt> </tt><a name="(part._hash..refs)"></a>Using dot notation for nested hash lookups</h4><p>The previous two examples used a macro to define functions whose names
|
|
were made by joining identifiers provided to the macro. This example
|
|
does the opposite: The identifier given to the macro is split into
|
|
pieces.</p><p>If you write programs for web services you deal with JSON, which is
|
|
represented in Racket by a <span class="RktSym">jsexpr?</span>. JSON often has
|
|
dictionaries that contain other dictionaries. In a <span class="RktSym">jsexpr?</span>
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these are represented by nested <span class="RktSym"><a href="http://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28quote._~23~25kernel%29._hasheq%29%29" class="RktValLink" data-pltdoc="x">hasheq</a></span> tables:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Nested </span><span class="RktCmt">‘</span><span class="RktCmt">hasheq</span><span class="RktCmt">'</span><span class="RktCmt">s typical of a jsexpr:</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/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">js</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28quote._~23~25kernel%29._hasheq%29%29" class="RktValLink" data-pltdoc="x">hasheq</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">a</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28quote._~23~25kernel%29._hasheq%29%29" class="RktValLink" data-pltdoc="x">hasheq</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">b</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28quote._~23~25kernel%29._hasheq%29%29" class="RktValLink" data-pltdoc="x">hasheq</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">c</span><span class="hspace"> </span><span class="RktVal">"value"</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>In JavaScript you can use dot notation:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="RktSym">foo</span><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._~3d%29%29" class="RktValLink" data-pltdoc="x">=</a></span><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktSym">js.a.b.c</span><span class="RktCmt">;</span><span class="RktMeta"></span></td></tr></table></blockquote><p>In Racket it’s not so convenient:</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28quote._~23~25kernel%29._hash-ref%29%29" class="RktValLink" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28quote._~23~25kernel%29._hash-ref%29%29" class="RktValLink" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28quote._~23~25kernel%29._hash-ref%29%29" class="RktValLink" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktSym">js</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">a</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">b</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">c</span><span class="RktPn">)</span></p></blockquote><p>We can write a helper function to make this a bit cleaner:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">This helper function:</span></td></tr><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fregion..rkt%29._define%2Fcontract%29%29" class="RktStxLink" data-pltdoc="x">define/contract</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hash-refs</span><span class="hspace"> </span><span class="RktSym">h</span><span class="hspace"> </span><span class="RktSym">ks</span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">def</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28quote._~23~25kernel%29._hash~3f%29%29" class="RktValLink" data-pltdoc="x">hash?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._listof%29%29" class="RktValLink" data-pltdoc="x">listof</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%2A%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span>*</a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/exns.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._with-handlers%29%29" class="RktStxLink" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/exns.html#%28def._%28%28quote._~23~25kernel%29._exn~3afail~3f%29%29" class="RktValLink" data-pltdoc="x">exn:fail?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/procedures.html#%28def._%28%28lib._racket%2Ffunction..rkt%29._const%29%29" class="RktValLink" data-pltdoc="x">const</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._cond%29%29" class="RktStxLink" data-pltdoc="x">cond</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/procedures.html#%28def._%28%28quote._~23~25kernel%29._procedure~3f%29%29" class="RktValLink" data-pltdoc="x">procedure?</a></span><span class="hspace"> </span><span class="RktSym">def</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">def</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._else%29%29" class="RktStxLink" data-pltdoc="x">else</a></span><span class="hspace"> </span><span class="RktSym">def</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/for.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for%2Ffold%29%29" class="RktStxLink" data-pltdoc="x">for/fold</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">h</span><span class="hspace"> </span><span class="RktSym">h</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/sequences.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._in-list%29%29" class="RktValLink" data-pltdoc="x">in-list</a></span><span class="hspace"> </span><span class="RktSym">ks</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28quote._~23~25kernel%29._hash-ref%29%29" class="RktValLink" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktSym">h</span><span class="hspace"> </span><span class="RktSym">k</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Lets us say:</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hash-refs</span><span class="hspace"> </span><span class="RktSym">js</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">a</span><span class="hspace"> </span><span class="RktVal">b</span><span class="hspace"> </span><span class="RktVal">c</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"value"</span></p></td></tr></table></blockquote><p>That’s better. Can we go even further and use a dot notation somewhat
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like JavaScript?</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">This macro:</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span><span class="hspace"> </span><span class="RktSym">racket/syntax</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">If the optional </span><span class="RktCmt">‘</span><span class="RktCmt">default</span><span class="RktCmt">'</span><span class="RktCmt"> is missing, assume it</span><span class="RktCmt">'</span><span class="RktCmt">s #f.</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">chain</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="hspace"> </span><span class="RktSym">chain</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">chain</span><span class="hspace"> </span><span class="RktSym">default</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">xs</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28lib._racket%2Fprivate%2Fmap..rkt%29._map%29%29" class="RktValLink" data-pltdoc="x">map</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._string-~3esymbol%29%29" class="RktValLink" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/regexp.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._regexp-split%29%29" class="RktValLink" data-pltdoc="x">regexp-split</a></span><span class="hspace"> </span><span class="RktVal">#rx"\\."</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._symbol-~3estring%29%29" class="RktValLink" data-pltdoc="x">symbol->string</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax->datum</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">chain</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><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">h</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._car%29%29" class="RktValLink" data-pltdoc="x">car</a></span><span class="hspace"> </span><span class="RktSym">xs</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">ks</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._cdr%29%29" class="RktValLink" data-pltdoc="x">cdr</a></span><span class="hspace"> </span><span class="RktSym">xs</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">hash-refs</span><span class="hspace"> </span><span class="RktSym">h</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">ks</span><span class="hspace"> </span><span class="RktSym">default</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></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Gives us "sugar" to say this:</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="hspace"> </span><span class="RktSym">js.a.b.c</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"value"</span></p></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Try finding a key that doesn</span><span class="RktCmt">'</span><span class="RktCmt">t exist:</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="hspace"> </span><span class="RktSym">js.blah</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#f</span></p></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Try finding a key that doesn</span><span class="RktCmt">'</span><span class="RktCmt">t exist, specifying the default:</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="hspace"> </span><span class="RktSym">js.blah</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">did-not-exist</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'did-not-exist</span></p></td></tr></table></blockquote><p>It works!</p><p>We’ve started to appreciate that our macros should give helpful
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messages when used in error. Let’s try to do that here.</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span><span class="hspace"> </span><span class="RktSym">racket/syntax</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" 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="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Check for no args at all</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/exns.html#%28def._%28%28quote._~23~25kernel%29._raise-syntax-error%29%29" class="RktValLink" data-pltdoc="x">raise-syntax-error</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktVal">"Expected (hash.key0[.key1 ...] [default])"</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">chain</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">chain</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="hspace"> </span><span class="RktSym">chain</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxLink" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">chain</span><span class="hspace"> </span><span class="RktSym">default</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Check that chain is a symbol, not e.g. a number or string</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/when_unless.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._unless%29%29" class="RktStxLink" data-pltdoc="x">unless</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._symbol~3f%29%29" class="RktValLink" data-pltdoc="x">symbol?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-e%29%29" class="RktValLink" data-pltdoc="x">syntax-e</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">chain</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/exns.html#%28def._%28%28quote._~23~25kernel%29._raise-syntax-error%29%29" class="RktValLink" data-pltdoc="x">raise-syntax-error</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktVal">"Expected (hash.key0[.key1 ...] [default])"</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">chain</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">xs</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28lib._racket%2Fprivate%2Fmap..rkt%29._map%29%29" class="RktValLink" data-pltdoc="x">map</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._string-~3esymbol%29%29" class="RktValLink" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/regexp.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._regexp-split%29%29" class="RktValLink" data-pltdoc="x">regexp-split</a></span><span class="hspace"> </span><span class="RktVal">#rx"\\."</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/symbols.html#%28def._%28%28quote._~23~25kernel%29._symbol-~3estring%29%29" class="RktValLink" data-pltdoc="x">symbol->string</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax->datum</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">chain</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><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Check that we have at least hash.key</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/when_unless.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._unless%29%29" class="RktStxLink" data-pltdoc="x">unless</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._and%29%29" class="RktStxLink" data-pltdoc="x">and</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._~3e~3d%29%29" class="RktValLink" data-pltdoc="x">>=</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._length%29%29" class="RktValLink" data-pltdoc="x">length</a></span><span class="hspace"> </span><span class="RktSym">xs</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/booleans.html#%28def._%28%28quote._~23~25kernel%29._not%29%29" class="RktValLink" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/booleans.html#%28def._%28%28quote._~23~25kernel%29._eq~3f%29%29" class="RktValLink" data-pltdoc="x">eq?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-e%29%29" class="RktValLink" data-pltdoc="x">syntax-e</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._cadr%29%29" class="RktValLink" data-pltdoc="x">cadr</a></span><span class="hspace"> </span><span class="RktSym">xs</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">||</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/exns.html#%28def._%28%28quote._~23~25kernel%29._raise-syntax-error%29%29" class="RktValLink" data-pltdoc="x">raise-syntax-error</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktVal">"Expected hash.key"</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktSym">chain</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">h</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._car%29%29" class="RktValLink" data-pltdoc="x">car</a></span><span class="hspace"> </span><span class="RktSym">xs</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">ks</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28quote._~23~25kernel%29._cdr%29%29" class="RktValLink" data-pltdoc="x">cdr</a></span><span class="hspace"> </span><span class="RktSym">xs</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPlain"><span class="hspace"> </span></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">hash-refs</span><span class="hspace"> </span><span class="RktSym">h</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">ks</span><span class="hspace"> </span><span class="RktSym">default</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></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">See if we catch each of the misuses</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">eval:96:0: hash.refs: Expected (hash.key0[.key1 ...]</span></p></td></tr><tr><td><p><span class="RktErr">[default])</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: chain</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: (hash.refs)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">eval:98:0: hash.refs: Expected (hash.key0[.key1 ...]</span></p></td></tr><tr><td><p><span class="RktErr">[default])</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: 0</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: (hash.refs 0 #f)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="hspace"> </span><span class="RktSym">js</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">eval:99:0: hash.refs: Expected hash.key</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: js</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: (hash.refs js #f)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="hspace"> </span><span class="RktSym">js.</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">eval:100:0: hash.refs: Expected hash.key</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: js.</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: (hash.refs js. #f)</span></p></td></tr></table></blockquote><p>Not too bad. Of course, the version with error-checking is quite a bit
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longer. Error-checking code generally tends to obscure the logic, and
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does here. Fortuantely we’ll soon see how <span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Parsing_Syntax.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._syntax-parse%29%29" class="RktStxLink" data-pltdoc="x">syntax-parse</a></span> can
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help mitigate that, in much the same way as contracts in normal
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Racket or types in Typed Racket.</p><p>Maybe we’re not convinced that writing <span class="RktPn">(</span><span class="RktSym">hash.refs</span><span class="stt"> </span><span class="RktSym">js.a.b.c</span><span class="RktPn">)</span>
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is really clearer than <span class="RktPn">(</span><span class="RktSym">hash-refs</span><span class="stt"> </span><span class="RktSym">js</span><span class="stt"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">a</span><span class="stt"> </span><span class="RktVal">b</span><span class="stt"> </span><span class="RktVal">c</span><span class="RktVal">)</span><span class="RktPn">)</span>. Maybe we
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won’t actually use this approach. But the Racket macro system makes it
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a possible choice.</p><div class="navsetbottom"><span class="navleft"> </span><span class="navright"> <a href="Transform_.html" title="backward to "3 Transform!"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "Fear of Macros"" data-pltdoc="x">up</a> <a href="Syntax_parameters.html" title="forward to "5 Syntax parameters"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html> |