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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>13.7.2 Reader-Extension Procedures</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.
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through <span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read%29%29" class="RktValLink" data-pltdoc="x">read</a></span>, <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%29%29" class="RktValLink" data-pltdoc="x">read-syntax</a></span>.
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In addition, a special-read procedure
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can be triggered by calls to <span class="RktSym"><a href="Byte_and_String_Input.html#%28def._%28%28quote._~23~25kernel%29._read-char-or-special%29%29" class="RktValLink" data-pltdoc="x">read-char-or-special</a></span>, or
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by the context of <span class="RktSym"><a href="Byte_and_String_Input.html#%28def._%28%28quote._~23~25kernel%29._read-bytes-avail%21%29%29" class="RktValLink" data-pltdoc="x">read-bytes-avail!</a></span>,
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<span class="RktSym"><a href="Byte_and_String_Input.html#%28def._%28%28quote._~23~25kernel%29._peek-bytes-avail%21%2A%29%29" class="RktValLink" data-pltdoc="x">peek-bytes-avail!*</a></span>, <span class="RktSym"><a href="Byte_and_String_Input.html#%28def._%28%28quote._~23~25kernel%29._read-bytes-avail%21%29%29" class="RktValLink" data-pltdoc="x">read-bytes-avail!</a></span>, and
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<span class="RktSym"><a href="Byte_and_String_Input.html#%28def._%28%28quote._~23~25kernel%29._peek-bytes-avail%21%2A%29%29" class="RktValLink" data-pltdoc="x">peek-bytes-avail!*</a></span>.</p><p>Optional arities for reader-macro and special-result procedures allow
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them to distinguish reads via <span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read%29%29" class="RktValLink" data-pltdoc="x">read</a></span>, etc<span class="Sendabbrev">.</span>, from reads via
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<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>, etc<span class="Sendabbrev">.</span> (where the source value is <span class="RktVal">#f</span> and
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no other location information is available).</p><p>When a reader-extension procedure is called in syntax-reading mode
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(via <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>, etc<span class="Sendabbrev">.</span>), it should generally return a syntax
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object that has no lexical context (e.g., a syntax object created
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using <span class="RktSym"><a href="stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span> with <span class="RktVal">#f</span> as the first
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argument and with the given location information as the third
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argument). Another possible result is a special-comment value (see
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<a href="special-comments.html" data-pltdoc="x">Special Comments</a>). If the procedure’s result is not a
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syntax object and not a special-comment value, it is converted to one
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using <span class="RktSym"><a href="stxops.html#%28def._%28%28quote._~23~25kernel%29._datum-~3esyntax%29%29" class="RktValLink" data-pltdoc="x">datum->syntax</a></span>.</p><p>When a reader-extension procedure is called in non-syntax-reading
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modes, it should generally not return a syntax object. If a syntax
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object is returned, it is converted to a plain value using
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<span class="RktSym"><a href="stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax-~3edatum%29%29" class="RktValLink" data-pltdoc="x">syntax->datum</a></span>.</p><p>In either context, when the result from a reader-extension procedure
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is a special-comment value (see <a href="special-comments.html" data-pltdoc="x">Special Comments</a>), then
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<span class="RktSym"><a href="Reading.html#%28def._%28%28quote._~23~25kernel%29._read%29%29" class="RktValLink" data-pltdoc="x">read</a></span>, <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>, etc<span class="Sendabbrev">.</span> treat the value as a
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delimiting comment and otherwise ignore it.</p><p>Also, in either context, the result may be copied to prevent mutation
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to vectors or boxes before the read result is completed, and to
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support the construction of graphs with cycles. Mutable boxes,
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vectors, and <a href="structures.html#%28tech._prefab%29" class="techoutside" data-pltdoc="x"><span class="techinside">prefab</span></a> structures are copied, along with any
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pairs, boxes, vectors, prefab structures that lead to such mutable
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values, to placeholders produced by a recursive read (see
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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 to references of a shared value. Graph
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structure (including cycles) is preserved in the copy.</p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="readtables.html" title="backward to "13.7.1 Readtables"" data-pltdoc="x">← prev</a> <a href="Reader_Extension.html" title="up to "13.7 Reader Extension"" data-pltdoc="x">up</a> <a href="special-comments.html" title="forward to "13.7.3 Special Comments"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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