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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>17.3.3 Using #lang s-exp syntax/module-reader</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="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">►</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">The Racket Guide</a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="intro.html" class="tocviewlink" data-pltdoc="x">Welcome to Racket</a></td></tr><tr><td align="right">2 </td><td><a href="to-scheme.html" class="tocviewlink" data-pltdoc="x">Racket Essentials</a></td></tr><tr><td align="right">3 </td><td><a href="datatypes.html" class="tocviewlink" data-pltdoc="x">Built-<wbr></wbr>In Datatypes</a></td></tr><tr><td align="right">4 </td><td><a href="scheme-forms.html" class="tocviewlink" data-pltdoc="x">Expressions and Definitions</a></td></tr><tr><td align="right">5 </td><td><a href="define-struct.html" class="tocviewlink" data-pltdoc="x">Programmer-<wbr></wbr>Defined Datatypes</a></td></tr><tr><td align="right">6 </td><td><a href="modules.html" class="tocviewlink" data-pltdoc="x">Modules</a></td></tr><tr><td align="right">7 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">8 </td><td><a href="i_o.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">9 </td><td><a href="regexp.html" class="tocviewlink" data-pltdoc="x">Regular Expressions</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Exceptions and Control</a></td></tr><tr><td align="right">11 </td><td><a href="for.html" class="tocviewlink" data-pltdoc="x">Iterations and Comprehensions</a></td></tr><tr><td align="right">12 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">13 </td><td><a href="classes.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">14 </td><td><a href="units.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">15 </td><td><a href="reflection.html" class="tocviewlink" data-pltdoc="x">Reflection and Dynamic Evaluation</a></td></tr><tr><td align="right">16 </td><td><a href="macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">17 </td><td><a href="languages.html" class="tocviewselflink" data-pltdoc="x">Creating Languages</a></td></tr><tr><td align="right">18 </td><td><a href="concurren
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<span class="stt">"literal.rkt"</span>. A more typical module parser must iterate to
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parse multiple forms for a module body. A language is also more likely
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to extend Racket syntax—<wbr></wbr>perhaps through a <a href="hash-reader.html#%28tech._readtable%29" class="techoutside" data-pltdoc="x"><span class="techinside">readtable</span></a>—<wbr></wbr>instead
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of replacing Racket syntax completely.</p><p>The <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=reader-helpers.html%23%2528mod-path._syntax%252Fmodule-reader%2529&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">syntax/module-reader</span></a> <a href="module-languages.html#%28tech._module._language%29" class="techoutside" data-pltdoc="x"><span class="techinside">module language</span></a>
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abstracts over common parts of a language implementation to simplify
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the creation of new languages. In its most basic form, a language
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implemented with <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=reader-helpers.html%23%2528mod-path._syntax%252Fmodule-reader%2529&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">syntax/module-reader</span></a> simply specifies
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the <a href="module-languages.html#%28tech._module._language%29" class="techoutside" data-pltdoc="x"><span class="techinside">module language</span></a> to be used for the language, in which case
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the <a href="Pairs__Lists__and_Racket_Syntax.html#%28tech._reader%29" class="techoutside" data-pltdoc="x"><span class="techinside">reader</span></a> layer of the language is the same as Racket. For
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example, with</p><blockquote class="SCodeFlow"><blockquote class="Rfilebox"><p class="Rfiletitle"><span class="Rfilename"><span class="stt">"raquet-mlang.rkt"</span></span></p><blockquote class="Rfilecontent"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><a href="Module_Syntax.html#%28part._hash-lang%29" class="RktModLink" data-pltdoc="x"><span class="RktMod">#lang</span></a><span class="hspace"> </span><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=index.html&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket</span></a></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._provide%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">provide</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._except-out%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">except-out</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._all-from-out%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">all-from-out</a></span><span class="hspace"> </span><span class="RktSym">racket</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._rename-out%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">rename-out</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktSym">function</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote></blockquote></blockquote><p>and</p><blockquote class="SCodeFlow"><blockquote class="Rfilebox"><p class="Rfiletitle"><span class="Rfilename"><span class="stt">"raquet.rkt"</span></span></p><blockquote class="Rfilecontent"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><a href="Module_Syntax.html#%28part._hash-lang%29" class="RktModLink" data-pltdoc="x"><span class="RktMod">#lang</span></a><span class="hspace"> </span><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=reader.html%23%2528mod-path._s-exp%2529&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">s-exp</span></a><span class="hspace"> </span><span class="RktSym">syntax/module-reader</span></td></tr><tr><td><span class="RktVal">"raquet-mlang.rkt"</span></td></tr></table></blockquote></blockquote></blockquote><p>then</p><blockquote class="SCodeFlow"><table cellspacing=
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<span class="stt">"raquet-mlang.rkt"</span> exports <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span> as
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<span class="RktSym">function</span>.</p><p>The <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=reader-helpers.html%23%2528mod-path._syntax%252Fmodule-reader%2529&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">syntax/module-reader</span></a> language accepts many optional
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specifications to adjust other features of the language. For example,
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an alternate <span class="RktSym">read</span> and <span class="RktSym">read-syntax</span> for
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parsing the language can be specified with <span class="RktPn">#:read</span> and
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<span class="RktPn">#:read-syntax</span>, respectively. The following
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<span class="stt">"dollar-racket.rkt"</span> language uses <span class="stt">"dollar.rkt"</span> (see
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<a href="hash-reader.html#%28part._readtable%29" data-pltdoc="x">Readtables</a>) to build a language that is like
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<a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=index.html&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket</span></a> but with a <span class="RktInBG"><span class="hspace"></span><span class="RktIn">$</span><span class="hspace"></span></span> escape to simple infix
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arithmetic:</p><blockquote class="SCodeFlow"><blockquote class="Rfilebox"><p class="Rfiletitle"><span class="Rfilename"><span class="stt">"dollar-racket.rkt"</span></span></p><blockquote class="Rfilecontent"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><a href="Module_Syntax.html#%28part._hash-lang%29" class="RktModLink" data-pltdoc="x"><span class="RktMod">#lang</span></a><span class="hspace"> </span><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=reader.html%23%2528mod-path._s-exp%2529&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">s-exp</span></a><span class="hspace"> </span><span class="RktSym">syntax/module-reader</span></td></tr><tr><td><span class="RktSym">racket</span></td></tr><tr><td><span class="RktPn">#:read</span><span class="hspace"> </span><span class="RktSym">$-read</span></td></tr><tr><td><span class="RktPn">#:read-syntax</span><span class="hspace"> </span><span class="RktSym">$-read-syntax</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._prefix-in%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">prefix-in</a></span><span class="hspace"> </span><span class="RktSym">$-</span><span class="hspace"> </span><span class="RktVal">"dollar.rkt"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote></blockquote></blockquote><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">require</a></span> form appears at the end of the module,
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because all of the keyword-tagged optional specifications for
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<a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=reader-helpers.html%23%2528mod-path._syntax%252Fmodule-reader%2529&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">syntax/module-reader</span></a> must appear before any helper
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imports or definitions.</p><p>The following module uses <span class="stt">"dollar-racket.rkt"</span> to implement a
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<span class="RktSym">cost</span> function using a <span class="RktInBG"><span class="hspace"></span><span class="RktIn">$</span><span class="hspace"></span></span> escape:</p><blockquote class="SCodeFlow"><blockquote class="Rfilebox"><p class="Rfiletitle"><span class="Rfilename"><span class="stt">"store.rkt"</span></span></p><blockquote class="Rfilecontent"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><a href="Module_Syntax.html#%28part._hash-lang%29" class="RktModLink" data-pltdoc="x"><span class="RktMod">#lang</span></a><span class="hspace"> </span><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=reader.html%23%2528mod-path._reader%2529&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">reader</span></a><span class="hspace"> </span><span class="RktVal">"dollar-racket.rkt"</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._provide%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">provide</a></span><span class="hspace"> </span><span class="RktSym">cost</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Cost of </span><span class="RktCmt">‘</span><span class="RktCmt">n</span><span class="RktCmt">'</span><span class="RktCmt"> $1 rackets with 7% sales</span></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">tax and shipping-and-handling fee </span><span class="RktCmt">‘</span><span class="RktCmt">h</span><span class="RktCmt">'</span><span class="RktCmt">:</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">cost</span><span class="hspace"> </span><span class="RktSym">n</span><span class="hspace"> </span><span class="RktSym">h</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">$n*107/100+h$</span><span class="RktPn">)</span></td></tr></table></blockquote></blockquote></blockquote><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="hash-lang_reader.html" title="backward to "17.3.2 Using #lang reader"" data-pltdoc="x">← prev</a> <a href="hash-languages.html" title="up to "17.3 Defining new #lang Languages"" data-pltdoc="x">up</a> <a href="language-collection.html" title="forward to "17.3.4 Installing a Language"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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