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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>17.2 Reader Extensions</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="icons.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" 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align="right"></td><td><a href="doc-bibliography.html" class="tocviewlink" data-pltdoc="x">Bibliography</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_1");">▼</a></td><td>17 </td><td><a href="languages.html" class="tocviewlink" data-pltdoc="x">Creating Languages</a></td></tr></table><div class="tocviewsublist" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">17.1 </td><td><a href="module-languages.html" class="tocviewlink" data-pltdoc="x">Module Languages</a></td></tr><tr><td align="right">17.2 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Reader Extensions</a></td></tr><tr><td align="right">17.3 </td><td><a href="hash-languages.html" class="tocviewlink" data-pltdoc="x">Defining new <span class="RktModLink"><span class="RktMod">#lang</span></span> Languages</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_2");">►</a></td><td>17.2 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Reader Extensions</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_2"><table cellspacing="0" cellpadding="0"><tr><td align="right">17.2.1 </td><td><a href="#%28part._.Source_.Locations%29" class="tocviewlink" data-pltdoc="x">Source Locations</a></td></tr><tr><td align="right">17.2.2 </td><td><a href="#%28part._readtable%29" class="tocviewlink" data-pltdoc="x">Readtables</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">17.2.1<tt> </tt></span><a href="#%28part._.Source_.Locations%29" class="tocsubseclink" data-pltdoc="x">Source Locations</a></td></tr><tr><td><span class="tocsublinknumber">17.2.2<tt> </tt></span><a href="#%28part._readtable%29" class="tocsubseclink" data-pltdoc="x">Readtables</a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="module-languages.html" title="backward to "17.1 Module Languages"" data-pltdoc="x">← prev</a> <a href="languages.html" title="up to "17 Creating Languages"" data-pltdoc="x">up</a> <a href="hash-languages.html" title="forward to "17.3 Defining new #lang Languages"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""hash-reader"">17.2<tt> </tt><a name="(part._hash-reader)"></a>Reader Extensions</h4><p>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 Racket language can be extended through
the <span class="RktMeta">#reader</span> form. A reader extension is implemented
as a module that is named after <span class="RktMeta">#reader</span>. The module
exports functions that parse raw characters into a form to be consumed
by the <a href="Pairs__Lists__and_Racket_Syntax.html#%28tech._expander%29" class="techoutside" data-pltdoc="x"><span class="techinside">expander</span></a> layer.</p><p>The syntax of <span class="RktMeta">#reader</span> is</p><blockquote class="SCodeFlow"><p><span class="RktInBG"><span class="hspace"></span><span class="RktIn">#reader</span><span class="hspace"></span></span><span class="hspace"> </span><span class="sroman">‹<span style="font-style: italic">module-path</span>›</span><span class="hspace"> </span><span class="sroman">‹<span style="font-style: italic">reader-specific</span>›</span></p></blockquote><p>where <span class="sroman">‹<span style="font-style: italic">module-path</span>›</span> names a module that provides
<span class="RktSym">read</span> and <span class="RktSym">read-syntax</span> functions. The
<span class="sroman">‹<span style="font-style: italic">reader-specific</span>›</span> part is a sequence of characters that is
parsed as determined by the <span class="RktSym">read</span> and
<span class="RktSym">read-syntax</span> functions from <span class="sroman">‹<span style="font-style: italic">module-path</span>›</span>.</p><p>For example, suppose that file <span class="stt">"five.rkt"</span> contains</p><blockquote class="SCodeFlow"><blockquote class="Rfilebox"><p class="Rfiletitle"><span class="Rfilename"><span class="stt">"five.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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._provide%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">provide</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Byte_and_String_Input.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-string%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-string</a></span><span class="hspace"> </span><span class="RktVal">5</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Byte_and_String_Input.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-string%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-string</a></span><span class="hspace"> </span><span class="RktVal">5</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote></blockquote></blockquote><p>Then, the program</p><blockquote class="SCodeFlow"><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktMeta">#reader</span><span class="RktVal">"five.rkt"</span><span class="stt">23456</span><span class="RktVal">7</span><span class="hspace"> </span><span class="RktVal">8</span><span class="RktVal">)</span></td></tr></table></blockquote><p>is equivalent to</p><blockquote class="SCodeFlow"><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">(</span><span class="RktVal">"23456"</span><span class="RktVal">)</span><span class="hspace"> </span><span class="RktVal">7</span><span class="hspace"> </span><span class="RktVal">8</span><span class="RktVal">)</span></td></tr></table></blockquote><p>because the <span class="RktSym">read</span> and <span class="RktSym">read-syntax</span>
functions of <span class="stt">"five.rkt"</span> both read five characters from the
input stream and put them into a string and then a list. The reader
functions from <span class="stt">"five.rkt"</span> are not obliged to follow Racket
lexical conventions and treat the continuous sequence <span class="RktInBG"><span class="hspace"></span><span class="RktIn">234567</span><span class="hspace"></span></span>
as a single number. Since only the <span class="RktInBG"><span class="hspace"></span><span class="RktIn">23456</span><span class="hspace"></span></span> part is consumed by
<span class="RktSym">read</span> or <span class="RktSym">read-syntax</span>, the <span class="RktInBG"><span class="hspace"></span><span class="RktIn">7</span><span class="hspace"></span></span>
remains to be parsed in the usual Racket way. Similarly, the reader
functions from <span class="stt">"five.rkt"</span> are not obliged to ignore
whitespace, and</p><blockquote class="SCodeFlow"><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktMeta">#reader</span><span class="RktVal">"five.rkt"</span><span class="hspace"> </span><span class="stt">2345</span><span class="RktVal">67</span><span class="hspace"> </span><span class="RktVal">8</span><span class="RktVal">)</span></td></tr></table></blockquote><p>is equivalent to</p><blockquote class="SCodeFlow"><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">(</span><span class="RktVal">" 2345"</span><span class="RktVal">)</span><span class="hspace"> </span><span class="RktVal">67</span><span class="hspace"> </span><span class="RktVal">8</span><span class="RktVal">)</span></td></tr></table></blockquote><p>since the first character immediately after <span class="RktVal">"five.rkt"</span> is a
space.</p><p>A <span class="RktMeta">#reader</span> form can be used in the <a href="intro.html#%28tech._repl%29" class="techoutside" data-pltdoc="x"><span class="techinside">REPL</span></a>, too:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktVal">'</span><span class="RktMeta">#reader</span><span class="RktVal">"five.rkt"</span><span class="stt">abcde</span></td></tr><tr><td><p><span class="RktRes">'("abcde")</span></p></td></tr></table></blockquote><h5 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""Source_Locations"">17.2.1<tt> </tt><a name="(part._.Source_.Locations)"></a>Source Locations</h5><p>The difference between <span class="RktSym">read</span> and
<span class="RktSym">read-syntax</span> is that <span class="RktSym">read</span> is meant to be
used for data while <span class="RktSym">read-syntax</span> is meant to be used to
parse programs. More precisely, the <span class="RktSym">read</span> function will
be used when the enclosing stream is being parsed by the Racket
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span>, and <span class="RktSym">read-syntax</span> is used when the
enclosing stream is being parsed by the Racket <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span>
function. Nothing requires <span class="RktSym">read</span> and
<span class="RktSym">read-syntax</span> to parse input in the same way, but making
them different would confuse programmers and tools.</p><p>The <span class="RktSym">read-syntax</span> function can return the same kind of
value as <span class="RktSym">read</span>, but it should normally return a
<a href="stx-obj.html#%28tech._syntax._object%29" class="techoutside" data-pltdoc="x"><span class="techinside">syntax object</span></a> that connects the parsed expression with source
locations. Unlike the <span class="stt">"five.rkt"</span> example, the
<span class="RktSym">read-syntax</span> function is typically implemented directly
to produce <a href="stx-obj.html#%28tech._syntax._object%29" class="techoutside" data-pltdoc="x"><span class="techinside">syntax objects</span></a>, and then <span class="RktSym">read</span> can use
<span class="RktSym">read-syntax</span> and strip away <a href="stx-obj.html#%28tech._syntax._object%29" class="techoutside" data-pltdoc="x"><span class="techinside">syntax object</span></a>
wrappers to produce a raw result.</p><p>The following <span class="stt">"arith.rkt"</span> module implements a reader to
parse simple infix arithmetic expressions into Racket forms. For
example, <span class="RktInBG"><span class="hspace"></span><span class="RktIn">1*2+3</span><span class="hspace"></span></span> parses into the Racket form <span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252A%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">*</a></span><span class="stt"> </span><span class="RktVal">1</span><span class="stt"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="stt"> </span><span class="RktVal">3</span><span class="RktPn">)</span>. The supported operators are <span class="RktInBG"><span class="hspace"></span><span class="RktIn">+</span><span class="hspace"></span></span>, <span class="RktInBG"><span class="hspace"></span><span class="RktIn">-</span><span class="hspace"></span></span>,
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">*</span><span class="hspace"></span></span>, and <span class="RktInBG"><span class="hspace"></span><span class="RktIn">/</span><span class="hspace"></span></span>, while operands can be unsigned integers
or single-letter variables. The implementation uses
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=linecol.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._port-next-location%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">port-next-location</a></span> to obtain the current source location, and
it uses <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=stxops.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._datum-%7E3esyntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">datum->syntax</a></span> to turn raw values into <a href="stx-obj.html#%28tech._syntax._object%29" class="techoutside" data-pltdoc="x"><span class="techinside">syntax
objects</span></a>.</p><blockquote class="SCodeFlow"><blockquote class="Rfilebox"><p class="Rfiletitle"><span class="Rfilename"><span class="stt">"arith.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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" 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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktSym">syntax/readerr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._provide%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">provide</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=stxops.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._syntax-%7E3edatum%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">syntax->datum</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">skip-whitespace</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">read-arith</span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">skip-whitespace</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#px"^\\s*"</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">read-arith</span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._define-values%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-values</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">line</span><span class="hspace"> </span><span class="RktSym">col</span><span class="hspace"> </span><span class="RktSym">pos</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=linecol.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._port-next-location%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">port-next-location</a></span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">expr-match</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Match an operand followed by any number of </span></td></tr><tr><td><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">operator</span><span class="RktCmt">–</span><span class="RktCmt">operand sequences, and prohibit an</span></td></tr><tr><td><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">additional operator from following immediately:</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">#px"^([a-z]|[0-9]+)(?:[-+*/]([a-z]|[0-9]+))*(?![-+*/])"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">to-syntax</span><span class="hspace"> </span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktSym">delta</span><span class="hspace"> </span><span class="RktSym">span-str</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=stxops.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._datum-%7E3esyntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">datum->syntax</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._make-srcloc%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-srcloc</a></span><span class="hspace"> </span><span class="RktSym">delta</span><span class="hspace"> </span><span class="RktSym">span-str</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._make-srcloc%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-srcloc</a></span><span class="hspace"> </span><span class="RktSym">delta</span><span class="hspace"> </span><span class="RktSym">span-str</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._and%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">and</a></span><span class="hspace"> </span><span class="RktSym">line</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=vectors.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._vector%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">vector</a></span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">line</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">col</span><span class="hspace"> </span><span class="RktSym">delta</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">pos</span><span class="hspace"> </span><span class="RktSym">delta</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-length%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">string-length</a></span><span class="hspace"> </span><span class="RktSym">span-str</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">parse-expr</span><span class="hspace"> </span><span class="RktSym">s</span><span class="hspace"> </span><span class="RktSym">delta</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=match.html%23%2528form._%2528%2528lib._racket%252Fmatch..rkt%2529._match%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._or%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">or</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"^(.*?)([+-])(.*)$"</span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"^(.*?)([*/])(.*)$"</span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=stx-patterns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fstxcase-scheme..rkt%2529.__%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">_</a></span><span class="hspace"> </span><span class="RktSym">a-str</span><span class="hspace"> </span><span class="RktSym">op-str</span><span class="hspace"> </span><span class="RktSym">b-str</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">a-len</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-length%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">string-length</a></span><span class="hspace"> </span><span class="RktSym">a-str</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">parse-expr</span><span class="hspace"> </span><span class="RktSym">a-str</span><span class="hspace"> </span><span class="RktSym">delta</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">parse-expr</span><span class="hspace"> </span><span class="RktSym">b-str</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">delta</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktSym">a-len</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">op</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">to-syntax</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=symbols.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-%7E3esymbol%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktSym">op-str</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktSym">delta</span><span class="hspace"> </span><span class="RktSym">a-len</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">op-str</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">to-syntax</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">op</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="hspace"> </span><span class="RktSym">delta</span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._else%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">else</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">to-syntax</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._or%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">or</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-%7E3enumber%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">string->number</a></span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=symbols.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-%7E3esymbol%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">string->symbol</a></span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">delta</span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=when_unless.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._unless%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">unless</a></span><span class="hspace"> </span><span class="RktSym">expr-match</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=syntax&rel=reader-helpers.html%23%2528def._%2528%2528lib._syntax%252Freaderr..rkt%2529._raise-read-error%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">raise-read-error</a></span><span class="hspace"> </span><span class="RktVal">"bad arithmetic syntax"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">line</span><span class="hspace"> </span><span class="RktSym">col</span><span class="hspace"> </span><span class="RktSym">pos</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._and%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">and</a></span><span class="hspace"> </span><span class="RktSym">pos</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._-%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x"><span class="nobreak">-</span></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=port-buffers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._file-position%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">file-position</a></span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">pos</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">parse-expr</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=bytestrings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._bytes-%7E3estring%252Futf-8%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">bytes->string/utf-8</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">car</a></span><span class="hspace"> </span><span class="RktSym">expr-match</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote></blockquote></blockquote><p>If the <span class="stt">"arith.rkt"</span> reader is used in an expression position,
then its parse result will be treated as a Racket expression. If it is
used in a quoted form, however, then it just produces a number or a
list:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktMeta">#reader</span><span class="RktVal">"arith.rkt"</span><span class="hspace"> </span><span class="stt">1*2+3</span></td></tr><tr><td><p><span class="RktRes">5</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktVal">'</span><span class="RktMeta">#reader</span><span class="RktVal">"arith.rkt"</span><span class="hspace"> </span><span class="stt">1*2+3</span></td></tr><tr><td><p><span class="RktRes">'(+ (* 1 2) 3)</span></p></td></tr></table></blockquote><p>The <span class="stt">"arith.rkt"</span> reader could also be used in positions that
make no sense. Since the <span class="RktSym">read-syntax</span> implementation
tracks source locations, syntax errors can at least refer to parts of
the input in terms of their original locations (at the beginning of
the error message):</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktMeta">#reader</span><span class="RktVal">"arith.rkt"</span><span class="hspace"> </span><span class="stt">1*2+3</span><span class="hspace"> </span><span class="RktVal">8</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">repl:1:27: let: bad syntax (not an identifier and expression</span></p></td></tr><tr><td><p><span class="RktErr">for a binding)</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: +</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: (let (+ (* 1 2) 3) 8)</span></p></td></tr></table></blockquote><h5 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""readtable"">17.2.2<tt> </tt><a name="(part._readtable)"></a>Readtables</h5><p>A reader extension’s ability to parse input characters in an arbitrary
way can be powerful, but many cases of lexical extension call for a
less general but more composable approach. In much the same way that
the <a href="Pairs__Lists__and_Racket_Syntax.html#%28tech._expander%29" class="techoutside" data-pltdoc="x"><span class="techinside">expander</span></a> level of Racket syntax can be extended through
<a href="macros.html#%28tech._macro%29" class="techoutside" data-pltdoc="x"><span class="techinside">macros</span></a>, the <a href="Pairs__Lists__and_Racket_Syntax.html#%28tech._reader%29" class="techoutside" data-pltdoc="x"><span class="techinside">reader</span></a> level of Racket syntax can be
composably extended through a <a name="(tech._readtable)"></a><span style="font-style: italic">readtable</span>.</p><p>The Racket reader is a recursive-descent parser, and the
<a href="#%28tech._readtable%29" class="techoutside" data-pltdoc="x"><span class="techinside">readtable</span></a> maps characters to parsing handlers. For example, the
default readtable maps <span class="RktInBG"><span class="hspace"></span><span class="RktIn">(</span><span class="hspace"></span></span> to a handler that recursively
parses subforms until it finds a <span class="RktInBG"><span class="hspace"></span><span class="RktIn">)</span><span class="hspace"></span></span>. The
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-readtable%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-readtable</a></span> <a href="parameterize.html#%28tech._parameter%29" class="techoutside" data-pltdoc="x"><span class="techinside">parameter</span></a> determines the
<a href="#%28tech._readtable%29" class="techoutside" data-pltdoc="x"><span class="techinside">readtable</span></a> that is used by <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span> or
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span>. Rather than parsing raw characters directly, a
reader extension can install an extended <a href="#%28tech._readtable%29" class="techoutside" data-pltdoc="x"><span class="techinside">readtable</span></a> and then
chain to <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span> or <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span>.</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="finger.png" alt="+" width="24" height="24"/></span>See <a href="parameterize.html" data-pltdoc="x">Dynamic Binding: <span class="RktSym"><span class="RktStxLink">parameterize</span></span></a> for an introduction to
<a href="parameterize.html#%28tech._parameter%29" class="techoutside" data-pltdoc="x"><span class="techinside">parameters</span></a>.</p></blockquote></blockquote></blockquote><p>The <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=readtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-readtable%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-readtable</a></span> function constructs a new <a href="#%28tech._readtable%29" class="techoutside" data-pltdoc="x"><span class="techinside">readtable</span></a>
as an extension of an existing one. It accepts a sequence of
specifications in terms of a character, a type of mapping for the
character, and (for certain types of mappings) a parsing
procedure. For example, to extend the readtable so that <span class="RktInBG"><span class="hspace"></span><span class="RktIn">$</span><span class="hspace"></span></span>
can be used to start and end infix expressions, implement a
<span class="RktSym">read-dollar</span> function and use:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=readtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-readtable%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-readtable</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-readtable%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-readtable</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">#\$</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">terminating-macro</span><span class="hspace"> </span><span class="RktSym">read-dollar</span><span class="RktPn">)</span></td></tr></table></blockquote><p>The protocol for <span class="RktSym">read-dollar</span> requires the function to accept
different numbers of arguments depending on whether it is being used
in <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span> or <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span> mode. In <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span> mode,
the parser function is given two arguments: the character that
triggered the parser function and the input port that is being
read. In <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span> mode, the function must accept four
additional arguments that provide the source location of the
character.</p><p>The following <span class="stt">"dollar.rkt"</span> module defines a
<span class="RktSym">read-dollar</span> function in terms of the <span class="RktSym">read</span> and
<span class="RktSym">read-syntax</span> functions provided by <span class="stt">"arith.rkt"</span>,
and it puts <span class="RktSym">read-dollar</span> together with new <span class="RktSym">read</span> and
<span class="RktSym">read-syntax</span> functions that install the readtable and
chain to Racket’s <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span> or <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span>:</p><blockquote class="SCodeFlow"><blockquote class="Rfilebox"><p class="Rfiletitle"><span class="Rfilename"><span class="stt">"dollar.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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" 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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktSym">syntax/readerr</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._prefix-in%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">prefix-in</a></span><span class="hspace"> </span><span class="RktSym">arith:</span><span class="hspace"> </span><span class="RktVal">"arith.rkt"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._provide%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">provide</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._rename-out%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">rename-out</a></span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">$-read</span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">$-read-syntax</span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">$-read</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-readtable%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-readtable</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-$-readtable</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read</a></span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">$-read-syntax</span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-readtable%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-readtable</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-$-readtable</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-syntax%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-syntax</a></span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-$-readtable</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=readtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-readtable%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-readtable</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Reading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-readtable%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-readtable</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">#\$</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">terminating-macro</span><span class="hspace"> </span><span class="RktSym">read-dollar</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">read-dollar</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._case-lambda%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">case-lambda</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">ch</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">check-$-after</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">arith:read</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">in</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=inspectors.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._object-name%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">object-name</a></span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">ch</span><span class="hspace"> </span><span class="RktSym">in</span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">line</span><span class="hspace"> </span><span class="RktSym">col</span><span class="hspace"> </span><span class="RktSym">pos</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">check-$-after</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">arith:read-syntax</span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">in</span><span class="hspace"> </span><span class="RktSym">src</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">check-$-after</span><span class="hspace"> </span><span class="RktSym">val</span><span class="hspace"> </span><span class="RktSym">in</span><span class="hspace"> </span><span class="RktSym">src</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#px"^\\s*"</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">skip whitespace</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">ch</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Byte_and_String_Input.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._peek-char%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">peek-char</a></span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=when_unless.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._unless%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">unless</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">ch</span><span class="hspace"> </span><span class="RktVal">#\$</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">bad-ending</span><span class="hspace"> </span><span class="RktSym">ch</span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Byte_and_String_Input.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._read-char%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">read-char</a></span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">val</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">bad-ending</span><span class="hspace"> </span><span class="RktSym">ch</span><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._let-values%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">let-values</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">line</span><span class="hspace"> </span><span class="RktSym">col</span><span class="hspace"> </span><span class="RktSym">pos</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=linecol.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._port-next-location%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">port-next-location</a></span><span class="hspace"> </span><span class="RktSym">in</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._if%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">if</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=port-ops.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eof-object%7E3f%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">eof-object?</a></span><span class="hspace"> </span><span class="RktSym">ch</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=syntax&rel=reader-helpers.html%23%2528def._%2528%2528lib._syntax%252Freaderr..rkt%2529._raise-read-error%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">raise-read-error</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=syntax&rel=reader-helpers.html%23%2528def._%2528%2528lib._syntax%252Freaderr..rkt%2529._raise-read-eof-error%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">raise-read-eof-error</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">"expected a closing `$'"</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">src</span><span class="hspace"> </span><span class="RktSym">line</span><span class="hspace"> </span><span class="RktSym">col</span><span class="hspace"> </span><span class="RktSym">pos</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._if%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">if</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=port-ops.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eof-object%7E3f%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">eof-object?</a></span><span class="hspace"> </span><span class="RktSym">ch</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">0</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote></blockquote></blockquote><p>With this reader extension, a single <span class="RktMeta">#reader</span> can be
used at the beginning of an expression to enable multiple uses of
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">$</span><span class="hspace"></span></span> that switch to infix arithmetic:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktMeta">#reader</span><span class="RktVal">"dollar.rkt"</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">a</span><span class="stt"> </span><span class="stt">$1*2+3$</span><span class="RktPn">]</span><span class="stt"> </span><span class="RktPn">[</span><span class="RktSym">b</span><span class="stt"> </span><span class="stt">$5/6$</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="stt"> </span><span class="RktSym">$a+b$</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">35/6</span></p></td></tr></table></blockquote><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="module-languages.html" title="backward to "17.1 Module Languages"" data-pltdoc="x">← prev</a> <a href="languages.html" title="up to "17 Creating Languages"" data-pltdoc="x">up</a> <a href="hash-languages.html" title="forward to "17.3 Defining new #lang Languages"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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