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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>13.8 Printer Extension</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="tocviewlink" data-pltdoc="x">Running Racket</a></td></tr><tr><td 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>13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">13.1 </td><td><a href="ports.html" class="tocviewlink" data-pltdoc="x">Ports</a></td></tr><tr><td align="right">13.2 </td><td><a href="Byte_and_String_Input.html" class="tocviewlink" data-pltdoc="x">Byte and String Input</a></td></tr><tr><td align="right">13.3 </td><td><a href="Byte_and_String_Output.html" class="tocviewlink" data-pltdoc="x">Byte and String Output</a></td></tr><tr><td align="right">13.4 </td><td><a href="Reading.html" class="tocviewlink" data-pltdoc="x">Reading</a></td></tr><tr><td align="right">13.5 </td><td><a href="Writing.html" class="tocviewlink" data-pltdoc="x">Writing</a></td></tr><tr><td align="right">13.6 </td><td><a href="pretty-print.html" class="tocviewlink" data-pltdoc="x">Pretty Printing</a></td></tr><tr><td align="right">13.7 </td><td><a href="Reader_Extension.html" class="tocviewlink" data-pltdoc="x">Reader Extension</a></td></tr><tr><td align="right">13.8 </td><td><a href="Printer_Extension.html" class="tocviewselflink" data-pltdoc="x">Printer Extension</a></td></tr><tr><td align="right">13.9 </td><td><a href="serialization.html" class="tocviewlink" data-pltdoc="x">Serialization</a></td></tr><tr><td align="right">13.10 </td><td><a href="fasl.html" class="tocviewlink" data-pltdoc="x">Fast-<wbr></wbr>Load Serialization</a></td></tr><tr><td align="right">13.11 </td><td><a href="sha.html" class="tocviewlink" data-pltdoc="x">Cryptographic Hashing</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="Printer_Extension.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._gen~3acustom-write%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">gen:<span class="mywbr"> </span>custom-<wbr></wbr>write</span></span></a></td></tr><tr><td><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._prop~3acustom-write%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">prop:<span class="mywbr"> </span>custom-<wbr></wbr>write</span></span></a></td></tr><tr><td><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._custom-write~3f%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">custom-<wbr></wbr>write?</span></span></a></td></tr><tr><td><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._custom-write-accessor%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">custom-<wbr></wbr>write-<wbr></wbr>accessor</span></span></a></td></tr><tr><td><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._prop~3acustom-print-quotable%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">prop:<span class="mywbr"> </span>custom-<wbr></wbr>print-<wbr></wbr>quotable</span></span></a></td></tr><tr><td><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._custom-print-quotable~3f%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">custom-<wbr></wbr>print-<wbr></wbr>quotable?</span></span></a></td></tr><tr><td><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._custom-print-quotable-accessor%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">custom-<wbr></wbr>print-<wbr></wbr>quotable-<wbr></wbr>accessor</span></span></a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><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="special-comments.html" title="backward to "13.7.3 Special Comments"" data-pltdoc="x">← prev</a> <a href="input-and-output.html" title="up to "13 Input and Output"" data-pltdoc="x">up</a> <a href="serialization.html" title="forward to "13.9 Serialization"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Printer_Extension"">13.8<tt> </tt><a name="(part._.Printer_.Extension)"></a>Printer Extension</h4><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>value</p></div></div><p class="RForeground"><a name="(def._((lib._racket/private/base..rkt)._gen~3acustom-write))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._gen~3acustom-write%29%29" class="RktValDef RktValLink" data-pltdoc="x">gen:custom-write</a></span></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></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">A <a href="struct-generics.html#%28tech._generic._interface%29" class="techoutside" data-pltdoc="x"><span class="techinside">generic interface</span></a> (see <a href="struct-generics.html" data-pltdoc="x">Generic Interfaces</a>) that
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supplies a method, <span class="RktSym">write-proc</span> used by the default printer to
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<span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span>, <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._write%29%29" class="RktValLink" data-pltdoc="x">write</a></span>, or <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._print%29%29" class="RktValLink" data-pltdoc="x">print</a></span> instances of the
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structure type.</div></p><p>A <span class="RktSym">write-proc</span> method takes three
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arguments: the structure to be printed, the target port, and an
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argument that is <span class="RktVal">#t</span> for <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._write%29%29" class="RktValLink" data-pltdoc="x">write</a></span> mode, <span class="RktVal">#f</span> for
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<span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span> mode, or <span class="RktVal">0</span> or <span class="RktVal">1</span> indicating
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the current <a href="printing.html#%28tech._quoting._depth%29" class="techoutside" data-pltdoc="x"><span class="techinside">quoting depth</span></a> for <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._print%29%29" class="RktValLink" data-pltdoc="x">print</a></span> mode. The
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procedure should print the value to the given port using
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<span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._write%29%29" class="RktValLink" data-pltdoc="x">write</a></span>, <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span>, <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._print%29%29" class="RktValLink" data-pltdoc="x">print</a></span>, <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._fprintf%29%29" class="RktValLink" data-pltdoc="x">fprintf</a></span>,
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<span class="RktSym"><a href="Byte_and_String_Output.html#%28def._%28%28quote._~23~25kernel%29._write-special%29%29" class="RktValLink" data-pltdoc="x">write-special</a></span>, etc.</p><p>The <a href="Writing.html#%28tech._port._write._handler%29" class="techoutside" data-pltdoc="x"><span class="techinside">port write handler</span></a>, <a href="Writing.html#%28tech._port._display._handler%29" class="techoutside" data-pltdoc="x"><span class="techinside">port display handler</span></a>,
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and <a href="eval.html#%28tech._print._handler%29" class="techoutside" data-pltdoc="x"><span class="techinside">print handler</span></a> are specially
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configured for a port given to a custom-write procedure. Printing to
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the port through <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span>, <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._write%29%29" class="RktValLink" data-pltdoc="x">write</a></span>, or <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._print%29%29" class="RktValLink" data-pltdoc="x">print</a></span>
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prints a value recursively with sharing annotations. To avoid a
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recursive print (i.e., to print without regard to sharing with a value
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currently being printed), print instead to a string or pipe and
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transfer the result to the target port using <span class="RktSym"><a href="Byte_and_String_Output.html#%28def._%28%28quote._~23~25kernel%29._write-string%29%29" class="RktValLink" data-pltdoc="x">write-string</a></span> or
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<span class="RktSym"><a href="Byte_and_String_Output.html#%28def._%28%28quote._~23~25kernel%29._write-special%29%29" class="RktValLink" data-pltdoc="x">write-special</a></span>. To print recursively to a port other than
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the one given to the custom-write procedure, copy the given port’s
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write handler, display handler, and print handler to the other port.</p><p>The port given to <span class="RktSym">write-proc</span> is not necessarily the actual
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target port. In particular, to detect cycles, sharing, and quoting
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modes (in the case of <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._print%29%29" class="RktValLink" data-pltdoc="x">print</a></span>), the printer invokes a
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custom-write procedure with a port that records information about
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recursive prints, and does not retain any other output. This
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information-gathering phase needs the same objects (in the
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<span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._eq~3f%29%29" class="RktValLink" data-pltdoc="x">eq?</a></span> sense) to be printed as later, so that the recorded
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information can be correlated with printed values.</p><p>Recursive print operations may trigger an escape from a call to
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<span class="RktSym">write-proc</span>. For example, printing may escape during
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pretty-printing where a tentative print attempt overflows the line, or
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it may escape while printing error output that is constrained to a
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limited width.</p><p>The following example definition of a <span class="RktSym">tuple</span> type includes
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a <span class="RktSym">write-proc</span> procedure that prints the tuple’s list content using
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angle brackets in <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._write%29%29" class="RktValLink" data-pltdoc="x">write</a></span> and <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._print%29%29" class="RktValLink" data-pltdoc="x">print</a></span> mode and no brackets in
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<span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span> mode. Elements of the tuple are printed recursively,
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so that graph and cycle structure can be represented.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><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">tuple-print</span><span class="hspace"> </span><span class="RktSym">tuple</span><span class="hspace"> </span><span class="RktSym">port</span><span class="hspace"> </span><span class="RktSym">mode</span><span class="RktPn">)</span></td></tr><tr><td><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._when%29%29" class="RktStxLink" data-pltdoc="x">when</a></span><span class="hspace"> </span><span class="RktSym">mode</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="Byte_and_String_Output.html#%28def._%28%28quote._~23~25kernel%29._write-string%29%29" class="RktValLink" data-pltdoc="x">write-string</a></span><span class="hspace"> </span><span class="RktVal">"<"</span><span class="hspace"> </span><span class="RktSym">port</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="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">l</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">tuple-ref</span><span class="hspace"> </span><span class="RktSym">tuple</span><span class="RktPn">)</span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">recur</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="case.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._case%29%29" class="RktStxLink" data-pltdoc="x">case</a></span><span class="hspace"> </span><span class="RktSym">mode</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktVal">#t</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._write%29%29" class="RktValLink" data-pltdoc="x">write</a></span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktVal">#f</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._else%29%29" class="RktStxLink" data-pltdoc="x">else</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">p</span><span class="hspace"> </span><span class="RktSym">port</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._print%29%29" class="RktValLink" data-pltdoc="x">print</a></span><span class="hspace"> </span><span class="RktSym">p</span><span class="hspace"> </span><span class="RktSym">port</span><span class="hspace"> </span><span class="RktSym">mode</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="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="number-types.html#%28def._%28%28quote._~23~25kernel%29._zero~3f%29%29" class="RktValLink" data-pltdoc="x">zero?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="vectors.html#%28def._%28%28quote._~23~25kernel%29._vector-length%29%29" class="RktValLink" data-pltdoc="x">vector-length</a></span><span class="hspace"> </span><span class="RktSym">l</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">recur</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="vectors.html#%28def._%28%28quote._~23~25kernel%29._vector-ref%29%29" class="RktValLink" data-pltdoc="x">vector-ref</a></span><span class="hspace"> </span><span class="RktSym">l</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">port</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28lib._racket%2Fprivate%2Fmap..rkt%29._for-each%29%29" class="RktValLink" data-pltdoc="x">for-each</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">e</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="Byte_and_String_Output.html#%28def._%28%28quote._~23~25kernel%29._write-string%29%29" class="RktValLink" data-pltdoc="x">write-string</a></span><span class="hspace"> </span><span class="RktVal">", "</span><span class="hspace"> </span><span class="RktSym">port</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">recur</span><span class="hspace"> </span><span class="RktSym">e</span><span class="hspace"> </span><span class="RktSym">port</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="pairs.html#%28def._%28%28quote._~23~25kernel%29._cdr%29%29" class="RktValLink" data-pltdoc="x">cdr</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="vectors.html#%28def._%28%28quote._~23~25kernel%29._vector-~3elist%29%29" class="RktValLink" data-pltdoc="x">vector->list</a></span><span class="hspace"> </span><span class="RktSym">l</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="when_unless.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._when%29%29" class="RktStxLink" data-pltdoc="x">when</a></span><span class="hspace"> </span><span class="RktSym">mode</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="Byte_and_String_Output.html#%28def._%28%28quote._~23~25kernel%29._write-string%29%29" class="RktValLink" data-pltdoc="x">write-string</a></span><span class="hspace"> </span><span class="RktVal">">"</span><span class="hspace"> </span><span class="RktSym">port</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p> </p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">tuple</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">ref</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">#:methods</span><span class="hspace"> </span><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._gen~3acustom-write%29%29" class="RktValLink" data-pltdoc="x">gen:custom-write</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</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="RktSym">write-proc</span><span class="hspace"> </span><span class="RktSym">tuple-print</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p> </p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">tuple</span><span class="hspace"> </span><span class="RktVal">#</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">"a"</span><span class="RktVal">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktOut">1, 2, a</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._print%29%29" class="RktValLink" data-pltdoc="x">print</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">tuple</span><span class="hspace"> </span><span class="RktVal">#</span><span class="RktVal">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">"a"</span><span class="RktVal">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktOut"><1, 2, "a"></span></p></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="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">t</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">tuple</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="vectors.html#%28def._%28%28quote._~23~25kernel%29._vector%29%29" class="RktValLink" data-pltdoc="x">vector</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">"a"</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="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="vectors.html#%28def._%28%28quote._~23~25kernel%29._vector-set%21%29%29" class="RktValLink" data-pltdoc="x">vector-set!</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">tuple-ref</span><span class="hspace"> </span><span class="RktSym">t</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">0</span><span class="hspace"> </span><span class="RktSym">t</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="Writing.html#%28def._%28%28quote._~23~25kernel%29._write%29%29" class="RktValLink" data-pltdoc="x">write</a></span><span class="hspace"> </span><span class="RktSym">t</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktOut">#0=<#0#, 2, "a"></span></p></td></tr></table></blockquote></div></p><p>The <span class="RktSym"><a href="structutils.html#%28def._%28%28lib._racket%2Fstruct..rkt%29._make-constructor-style-printer%29%29" class="RktValLink" data-pltdoc="x">make-constructor-style-printer</a></span> function can help in the
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implementation of a <span class="RktSym">write-proc</span>, as in this example:</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><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">racket/struct</span><span class="RktPn">)</span></td></tr><tr><td><p> </p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">point</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">#:methods</span><span class="hspace"> </span><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._gen~3acustom-write%29%29" class="RktValLink" data-pltdoc="x">gen:custom-write</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</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="RktSym">write-proc</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="structutils.html#%28def._%28%28lib._racket%2Fstruct..rkt%29._make-constructor-style-printer%29%29" class="RktValLink" data-pltdoc="x">make-constructor-style-printer</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">obj</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">point</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">obj</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._list%29%29" class="RktValLink" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">point-x</span><span class="hspace"> </span><span class="RktSym">obj</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">point-y</span><span class="hspace"> </span><span class="RktSym">obj</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p> </p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._print%29%29" class="RktValLink" data-pltdoc="x">print</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">point</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktOut">(point 1 2)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._write%29%29" class="RktValLink" data-pltdoc="x">write</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">point</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktOut">#<point: 1 2></span></p></td></tr></table></blockquote></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>value</p></div></div><p class="RForeground"><a name="(def._((quote._~23~25kernel)._prop~3acustom-write))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._prop~3acustom-write%29%29" class="RktValDef RktValLink" data-pltdoc="x">prop:custom-write</a></span></span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="structprops.html#%28def._%28%28quote._~23~25kernel%29._struct-type-property~3f%29%29" class="RktValLink" data-pltdoc="x">struct-type-property?</a></span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">A <a href="structprops.html#%28tech._structure._type._property%29" class="techoutside" data-pltdoc="x"><span class="techinside">structure type property</span></a> (see <a href="structprops.html" data-pltdoc="x">Structure Type Properties</a>)
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that supplies a procedure that corresponds to <span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._gen~3acustom-write%29%29" class="RktValLink" data-pltdoc="x">gen:custom-write</a></span>’s
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<span class="RktSym">write-proc</span>. Using the <span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._prop~3acustom-write%29%29" class="RktValLink" data-pltdoc="x">prop:custom-write</a></span> property is
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discouraged; use the <span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._gen~3acustom-write%29%29" class="RktValLink" data-pltdoc="x">gen:custom-write</a></span> <a href="struct-generics.html#%28tech._generic._interface%29" class="techoutside" data-pltdoc="x"><span class="techinside">generic interface</span></a>
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instead.</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)._custom-write~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._custom-write~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">custom-write?</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> has the <span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._prop~3acustom-write%29%29" class="RktValLink" data-pltdoc="x">prop:custom-write</a></span>
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property, <span class="RktVal">#f</span> 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)._custom-write-accessor))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._custom-write-accessor%29%29" class="RktValDef RktValLink" data-pltdoc="x">custom-write-accessor</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span></p></blockquote></td></tr><tr><td><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._custom-write~3f%29%29" class="RktValLink" data-pltdoc="x">custom-write?</a></span><span class="hspace"> </span><span class="RktSym"><a href="port-ops.html#%28def._%28%28quote._~23~25kernel%29._output-port~3f%29%29" class="RktValLink" data-pltdoc="x">output-port?</a></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="RktVal">#t</span><span class="hspace"> </span><span class="RktVal">#f</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="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._custom-write~3f%29%29" class="RktValLink" data-pltdoc="x">custom-write?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns the custom-write procedure associated with <span class="RktVar">v</span>.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>value</p></div></div><p class="RForeground"><a name="(def._((quote._~23~25kernel)._prop~3acustom-print-quotable))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._prop~3acustom-print-quotable%29%29" class="RktValDef RktValLink" data-pltdoc="x">prop:custom-print-quotable</a></span></span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="structprops.html#%28def._%28%28quote._~23~25kernel%29._struct-type-property~3f%29%29" class="RktValLink" data-pltdoc="x">struct-type-property?</a></span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>value</p></div></div><p class="RForeground"><a name="(def._((quote._~23~25kernel)._custom-print-quotable~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._custom-print-quotable~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">custom-print-quotable?</a></span></span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="structprops.html#%28def._%28%28quote._~23~25kernel%29._struct-type-property~3f%29%29" class="RktValLink" data-pltdoc="x">struct-type-property?</a></span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>value</p></div></div><p class="RForeground"><a name="(def._((quote._~23~25kernel)._custom-print-quotable-accessor))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._custom-print-quotable-accessor%29%29" class="RktValDef RktValLink" data-pltdoc="x">custom-print-quotable-accessor</a></span></span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="structprops.html#%28def._%28%28quote._~23~25kernel%29._struct-type-property~3f%29%29" class="RktValLink" data-pltdoc="x">struct-type-property?</a></span></p></blockquote></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">A property and associated predicate and accessor. The property value
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is one of <span class="RktVal">'</span><span class="RktVal">self</span>, <span class="RktVal">'</span><span class="RktVal">never</span>, <span class="RktVal">'</span><span class="RktVal">maybe</span>, or
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<span class="RktVal">'</span><span class="RktVal">always</span>. When a structure has this property in addition to a
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<span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._prop~3acustom-write%29%29" class="RktValLink" data-pltdoc="x">prop:custom-write</a></span> property value, then the property value
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affects printing in <span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._print%29%29" class="RktValLink" data-pltdoc="x">print</a></span> mode; see <a href="printing.html" data-pltdoc="x">The Printer</a>. When
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a value does not have the <span class="RktSym"><a href="Printer_Extension.html#%28def._%28%28quote._~23~25kernel%29._prop~3acustom-print-quotable%29%29" class="RktValLink" data-pltdoc="x">prop:custom-print-quotable</a></span>, it is
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equivalent to having the <span class="RktVal">'</span><span class="RktVal">self</span> property value, which is
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suitable both for self-quoting forms and printed forms that are
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unreadable.</div></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="special-comments.html" title="backward to "13.7.3 Special Comments"" data-pltdoc="x">← prev</a> <a href="input-and-output.html" title="up to "13 Input and Output"" data-pltdoc="x">up</a> <a href="serialization.html" title="forward to "13.9 Serialization"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html> |