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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>13.7.1 Readtables</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.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 Reference</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="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2 </td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3 </td><td><a href="syntax.html" class="tocviewlink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4 </td><td><a href="data.html" class="tocviewlink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6 </td><td><a href="mzlib_class.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">7 </td><td><a href="mzlib_unit.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">8 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">9 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewselflink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18 </td><td><a href="running.html" class="tocv
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corresponds to the default <a name="(tech._readtable)"></a><span style="font-style: italic">readtable</span>. By creating a new
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readtable and installing it via the <span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._current-readtable%29%29" class="RktValLink" data-pltdoc="x">current-readtable</a></span>
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parameter, the reader’s behavior can be extended.</p><p>A readtable is consulted at specific times by the reader:</p><ul><li><p>when looking for the start of a datum;</p></li><li><p>when determining how to parse a datum that starts with
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<span class="RktInBG"><span class="hspace"></span><span class="RktIn">#</span><span class="hspace"></span></span>;</p></li><li><p>when looking for a delimiter to terminate a symbol or number;</p></li><li><p>when looking for an opener (such as <span class="RktInBG"><span class="hspace"></span><span class="RktIn">(</span><span class="hspace"></span></span>), closer (such
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as <span class="RktInBG"><span class="hspace"></span><span class="RktIn">)</span><span class="hspace"></span></span>), or <span class="RktInBG"><span class="hspace"></span><span class="RktIn">.</span><span class="hspace"></span></span> after the first character parsed as
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a sequence for a pair, list, vector, or hash table; or</p></li><li><p>when looking for an opener after <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#</span><span class="hspace"></span></span><span class="sroman">‹<span style="font-style: italic">n</span>›</span> in a
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vector of specified length <span class="sroman">‹<span style="font-style: italic">n</span>›</span>.</p></li></ul><p>The readtable is ignored at other times. In particular, after parsing
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a character that is mapped to the default behavior of <span class="RktInBG"><span class="hspace"></span><span class="RktIn">;</span><span class="hspace"></span></span>, the
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readtable is ignored until the comment’s terminating newline is
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discovered. Similarly, the readtable does not affect string parsing
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until a closing double-quote is found. Meanwhile, if a character is
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mapped to the default behavior of <span class="RktInBG"><span class="hspace"></span><span class="RktIn">(</span><span class="hspace"></span></span>, then it starts sequence
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that is closed by any character that is mapped to a closing parenthesis
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<span class="RktInBG"><span class="hspace"></span><span class="RktIn">)</span><span class="hspace"></span></span>. An apparent exception is that the default parsing of
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<span class="RktInBG"><span class="hspace"></span><span class="RktIn">|</span><span class="hspace"></span></span> quotes a symbol until a matching character is found, but
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the parser is simply using the character that started the quote; it
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does not consult the readtable.</p><p>For many contexts, <span class="RktVal">#f</span> identifies the default readtable. In
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particular, <span class="RktVal">#f</span> is the initial value for the
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<span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._current-readtable%29%29" class="RktValLink" data-pltdoc="x">current-readtable</a></span> parameter, which causes the reader to
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behave as described in <a href="reader.html" data-pltdoc="x">The Reader</a>.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._readtable~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="readtables.html#%28def._%28%28quote._~23~25kernel%29._readtable~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">readtable?</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">v</span> is a readtable, <span class="RktVal">#f</span>
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otherwise.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._make-readtable))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="readtables.html#%28def._%28%28quote._~23~25kernel%29._make-readtable%29%29" class="RktValDef RktValLink" data-pltdoc="x">make-readtable</a></span></span><span class="hspace"> </span><span class="RktVar">readtable</span><span class="hspace"> </span><span class="RktVar">key</span><span class="hspace"> </span><span class="RktVar">mode</span><span class="hspace"> </span><span class="RktVar">action</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="readtables.html#%28def._%28%28quote._~23~25kernel%29._readtable~3f%29%29" class="RktValLink" data-pltdoc="x">readtable?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">readtable</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._or%2Fc%29%29" class="RktValLink" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktSym"><a href="readtables.html#%28def._%28%28quote._~23~25kernel%29._readtable~3f%29%29" class="RktValLink" data-pltdoc="x">readtable?</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">key</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._or%2Fc%29%29" class="RktValLink" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktSym"><a href="characters.html#%28def._%28%28quote._~23~25kernel%29._char~3f%29%29" class="RktValLink" data-pltdoc="x">char?</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">mode</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">:</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._or%2Fc%29%29" class="RktValLink" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">terminating-macro</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">non-terminating-macro</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">dispatch-macro</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym"><a href="characters.html#%28def._%28%28quote._~23~25kernel%29._char~3f%29%29" class="RktValLink" data-pltdoc="x">char?</a></span><span class="RktPn">)</span></td></tr></table></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">action</span></td><td valign="top"
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<span class="RktVal">#f</span> to indicate the default readtable),
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except that the reader’s behavior is modified for each
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<span class="RktVar">key</span> according to the given <span class="RktVar">mode</span> and
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<span class="RktVar">action</span>. The <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span> for <span class="RktSym"><a href="readtables.html#%28def._%28%28quote._~23~25kernel%29._make-readtable%29%29" class="RktValLink" data-pltdoc="x">make-readtable</a></span> applies
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to all three of <span class="RktVar">key</span>, <span class="RktVar">mode</span>, and <span class="RktVar">action</span>; in
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other words, the total number of arguments to <span class="RktSym"><a href="readtables.html#%28def._%28%28quote._~23~25kernel%29._make-readtable%29%29" class="RktValLink" data-pltdoc="x">make-readtable</a></span>
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must be <span style="font-style: italic"></span>1<span style="font-style: italic"></span> modulo <span style="font-style: italic"></span>3<span style="font-style: italic"></span>.</div></p><p>The possible combinations for <span class="RktVar">key</span>, <span class="RktVar">mode</span>, and
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<span class="RktVar">action</span> are as follows:</p><ul><li><p><span class="RktVar">char</span><span class="stt"> </span><a name="(idx._(gentag._269._(lib._scribblings/reference/reference..scrbl)))"></a><span class="RktVal">'</span><span class="RktVal">terminating-macro</span><span class="stt"> </span><span class="RktVar">proc</span> —<wbr></wbr> causes
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<span class="RktVar">char</span> to be parsed as a delimiter, and an
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unquoted/uncommented <span class="RktVar">char</span> in the input string triggers a
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call to the <a name="(tech._reader._macro)"></a><span style="font-style: italic">reader macro</span> <span class="RktVar">proc</span>; the activity of
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<span class="RktVar">proc</span> is described further below. Conceptually, characters
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like <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> are mapped to
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terminating reader macros in the default readtable.</p></li><li><p><span class="RktVar">char</span><span class="stt"> </span><a name="(idx._(gentag._270._(lib._scribblings/reference/reference..scrbl)))"></a><span class="RktVal">'</span><span class="RktVal">non-terminating-macro</span><span class="stt"> </span><span class="RktVar">proc</span> —<wbr></wbr> like
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the <span class="RktVal">'</span><span class="RktVal">terminating-macro</span> variant, but <span class="RktVar">char</span> is not
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treated as a delimiter, so it can be used in the middle of an
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identifier or number. Conceptually, <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#</span><span class="hspace"></span></span> is mapped to a
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non-terminating macro in the default readtable.</p></li><li><p><span class="RktVar">char</span><span class="stt"> </span><a name="(idx._(gentag._271._(lib._scribblings/reference/reference..scrbl)))"></a><span class="RktVal">'</span><span class="RktVal">dispatch-macro</span><span class="stt"> </span><span class="RktVar">proc</span> —<wbr></wbr> like the
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<span class="RktVal">'</span><span class="RktVal">non-terminating-macro</span> variant, but for <span class="RktVar">char</span> only
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when it follows a <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#</span><span class="hspace"></span></span> (or, more precisely, when the character
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follows one that has been mapped to the behavior of <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#</span><span class="hspace"></span></span>
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in the default readtable).</p></li><li><p><span class="RktVar">char</span><span class="stt"> </span><span class="RktVar">like-char</span><span class="stt"> </span><span class="RktVar">readtable</span> —<wbr></wbr> causes
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<span class="RktVar">char</span> to be parsed in the same way that <span class="RktVar">like-char</span>
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is parsed in <span class="RktVar">readtable</span>, where <span class="RktVar">readtable</span> can be
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<span class="RktVal">#f</span> to indicate the default readtable. (The mapping of
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<span class="RktVar">char</span> does not apply after <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#</span><span class="hspace"></span></span>, which is configured
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separately via <span class="RktVal">'</span><span class="RktVal">dispatch-macro</span>.) Mapping a character to
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the same actions as <span class="RktInBG"><span class="hspace"></span><span class="RktIn">|</span><span class="hspace"></span></span> in the default reader means that the
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character starts quoting for symbols, and the same character
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terminates the quote; in contrast, mapping a character to the same
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action as a <span class="RktInBG"><span class="hspace"></span><span class="RktIn">"</span><span class="hspace"></span></span> means that the character starts a string, but
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the string is still terminated with a closing <span class="RktInBG"><span class="hspace"></span><span class="RktIn">"</span><span class="hspace"></span></span>. Finally,
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mapping a character to an action in the default readtable means that
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the character’s behavior is sensitive to parameters that affect the
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original character; for example, mapping a character to the same
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action as a curly brace <span class="RktInBG"><span class="hspace"></span><span class="RktIn">{</span><span class="hspace"></span></span> in the default readtable means
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that the character is disallowed when the
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<span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read-curly-brace-as-paren%29%29" class="RktValLink" data-pltdoc="x">read-curly-brace-as-paren</a></span> parameter is set to <span class="RktVal">#f</span>.</p></li><li><p><span class="RktVal">#f</span><span class="stt"> </span><a name="(idx._(gentag._272._(lib._scribblings/reference/reference..scrbl)))"></a><span class="RktVal">'</span><span class="RktVal">non-terminating-macro</span><span class="stt"> </span><span class="RktVar">proc</span> —<wbr></wbr>
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replaces the macro used to parse characters with no specific mapping:
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i.e., the characters (other than <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#</span><span class="hspace"></span></span> or <span class="RktInBG"><span class="hspace"></span><span class="RktIn">|</span><span class="hspace"></span></span>) that can
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start a symbol or number with the default readtable.</p></li></ul><p>If multiple <span class="RktVal">'</span><span class="RktVal">dispatch-macro</span> mappings are provided for a
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single <span class="RktVar">char</span>, all but the last one are ignored. Similarly, if
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multiple non-<span class="RktVal">'</span><span class="RktVal">dispatch-macro</span> mappings are provided for a
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single <span class="RktVar">char</span>, all but the last one are ignored.</p><p>A reader macro <span class="RktVar">proc</span> must accept six arguments, and it can
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optionally accept two arguments. The first two arguments are always
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the character that triggered the reader macro and the input port for
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reading. When the reader macro is triggered by <span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read-syntax%29%29" class="RktValLink" data-pltdoc="x">read-syntax</a></span>
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(or <span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read-syntax%2Frecursive%29%29" class="RktValLink" data-pltdoc="x">read-syntax/recursive</a></span>), the procedure is passed four
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additional arguments that represent a source location for
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already-consumed character(s): the source name, a line number or
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<span class="RktVal">#f</span>, a column number or <span class="RktVal">#f</span>, and a position or
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<span class="RktVal">#f</span>. When the reader macro is triggered by <span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read%29%29" class="RktValLink" data-pltdoc="x">read</a></span> (or
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<span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read%2Frecursive%29%29" class="RktValLink" data-pltdoc="x">read/recursive</a></span>), the procedure is passed only two arguments
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if it accepts two arguments, otherwise it is passed six arguments
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where the third is always <span class="RktVal">#f</span>. See <a href="reader-procs.html" data-pltdoc="x">Reader-Extension Procedures</a>
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for information on the procedure’s results.</p><p>A reader macro normally reads characters from the given input port to
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produce a value to be used as the “reader macro-expansion” of the
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consumed characters. The reader macro might produce a special-comment
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value (see <a href="special-comments.html" data-pltdoc="x">Special Comments</a>) to cause the consumed
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character to be treated as whitespace, and it might use
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<span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read%2Frecursive%29%29" class="RktValLink" data-pltdoc="x">read/recursive</a></span> or <span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read-syntax%2Frecursive%29%29" class="RktValLink" data-pltdoc="x">read-syntax/recursive</a></span>.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._readtable-mapping))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="readtables.html#%28def._%28%28quote._~23~25kernel%29._readtable-mapping%29%29" class="RktValDef RktValLink" data-pltdoc="x">readtable-mapping</a></span></span><span class="hspace"> </span><span class="RktVar">readtable</span><span class="hspace"> </span><span class="RktVar">char</span><span class="RktPn">)</span></p></blockquote></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="prototype"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top">→</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><table cellspacing="0" cellpadding="0"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._or%2Fc%29%29" class="RktValLink" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktSym"><a href="characters.html#%28def._%28%28quote._~23~25kernel%29._char~3f%29%29" class="RktValLink" data-pltdoc="x">char?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">terminating-macro</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">non-terminating-macro</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._or%2Fc%29%29" class="RktValLink" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktSym"><a href="procedures.html#%28def._%28%28quote._~23~25kernel%29._procedure~3f%29%29" class="RktValLink" data-pltdoc="x">procedure?</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._or%2Fc%29%29" class="RktValLink" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="hspace"> </span><span class="RktSym"><a href="procedures.html#%28def._%28%28quote._~23~25kernel%29._procedure~3f%29%29" class="RktValLink" data-pltdoc="x">procedure?</a></span><span class="RktPn">)</span></td></tr></table></td></tr></table></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">readtable</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="readtables.html#%28def._%28%28quote._~23~25kernel%29._readtable~3f%29%29" class="RktValLink" data-pltdoc="x">readtable?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">char</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="characters.html#%28def._%28%28quote._~23~25kernel%29._char~3f%29%29" class="RktValLink" data-pltdoc="x">char?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Produces information about the mappings in <span class="RktVar">readtable</span> for
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<span class="RktVar">char</span>. The result is three values:</div></p><ul><li><p>either a character (mapping to the same behavior as the
|
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character in the default readtable), <span class="RktVal">'</span><span class="RktVal">terminating-macro</span>, or
|
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<span class="RktVal">'</span><span class="RktVal">non-terminating-macro</span>; this result reports the main (i.e.,
|
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non-<span class="RktVal">'</span><span class="RktVal">dispatch-macro</span>) mapping for <span class="RktVar">char</span>. When the result
|
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is a character, then <span class="RktVar">char</span> is mapped to the same behavior as the
|
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returned character in the default readtable.</p></li><li><p>either <span class="RktVal">#f</span> or a reader-macro procedure; the result is a
|
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procedure when the first result is <span class="RktVal">'</span><span class="RktVal">terminating-macro</span> or
|
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<span class="RktVal">'</span><span class="RktVal">non-terminating-macro</span>.</p></li><li><p>either <span class="RktVal">#f</span> or a reader-macro procedure; the result is a
|
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procedure when the character has a <span class="RktVal">'</span><span class="RktVal">dispatch-macro</span> mapping in
|
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<span class="RktVar">readtable</span> to override the default dispatch behavior.</p></li></ul><p>Note that reader-macro procedures for the default readtable are not
|
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directly accessible. To invoke default behaviors, use
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<span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read%2Frecursive%29%29" class="RktValLink" data-pltdoc="x">read/recursive</a></span> or <span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read-syntax%2Frecursive%29%29" class="RktValLink" data-pltdoc="x">read-syntax/recursive</a></span> with a
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character and the <span class="RktVal">#f</span> readtable.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Provides </span><span class="RktSym">raise-read-error</span><span class="RktCmt"> and </span><span class="RktSym">raise-read-eof-error</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktSym">syntax/readerr</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">skip-whitespace</span><span class="hspace"> </span><span class="RktSym">port</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Skips whitespace characters, sensitive to the current</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">readtable</span><span class="RktCmt">'</span><span class="RktCmt">s definition of whitespace</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">ch</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="Byte_and_String_Input.html#%28def._%28%28quote._~23~25kernel%29._peek-char%29%29" class="RktValLink" data-pltdoc="x">peek-char</a></span><span class="hspace"> </span><span class="RktSym">port</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="when_unless.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._unless%29%29" class="RktStxLink" data-pltdoc="x">unless</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="port-ops.html#%28def._%28%28quote._~23~25kernel%29._eof-object~3f%29%29" class="RktValLink" 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="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">Consult current readtable:</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="let.html#%28form._%28%28quote._~23~25kernel%29._let-values%29%29" class="RktStxLink" 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">like-ch/sym</span><span class="hspace"> </span><span class="RktSym">proc</span><span class="hspace"> </span><span class="RktSym">dispatch-proc</span><span class="RktPn">)</span></td
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