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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>3.7 Keywords</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="icons.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" 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</td><td><a href="to-scheme.html" class="tocviewlink" data-pltdoc="x">Racket Essentials</a></td></tr><tr><td align="right">3 </td><td><a href="datatypes.html" class="tocviewselflink" data-pltdoc="x">Built-<wbr></wbr>In Datatypes</a></td></tr><tr><td align="right">4 </td><td><a href="scheme-forms.html" class="tocviewlink" data-pltdoc="x">Expressions and Definitions</a></td></tr><tr><td align="right">5 </td><td><a href="define-struct.html" class="tocviewlink" data-pltdoc="x">Programmer-<wbr></wbr>Defined Datatypes</a></td></tr><tr><td align="right">6 </td><td><a href="modules.html" class="tocviewlink" data-pltdoc="x">Modules</a></td></tr><tr><td align="right">7 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">8 </td><td><a href="i_o.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">9 </td><td><a href="regexp.html" class="tocviewlink" data-pltdoc="x">Regular 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Choice</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>3 </td><td><a href="datatypes.html" class="tocviewlink" data-pltdoc="x">Built-<wbr></wbr>In Datatypes</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">3.1 </td><td><a href="booleans.html" class="tocviewlink" data-pltdoc="x">Booleans</a></td></tr><tr><td align="right">3.2 </td><td><a href="numbers.html" class="tocviewlink" data-pltdoc="x">Numbers</a></td></tr><tr><td align="right">3.3 </td><td><a href="characters.html" class="tocviewlink" data-pltdoc="x">Characters</a></td></tr><tr><td align="right">3.4 </td><td><a href="strings.html" class="tocviewlink" data-pltdoc="x">Strings (Unicode)</a></td></tr><tr><td align="right">3.5 </td><td><a href="bytestrings.html" class="tocviewlink" data-pltdoc="x">Bytes and Byte Strings</a></td></tr><tr><td align="right">3.6 </td><td><a href="symbols.html" class="tocviewlink" data-pltdoc="x">Symbols</a></td></tr><tr><td align="right">3.7 </td><td><a href="keywords.html" class="tocviewselflink" data-pltdoc="x">Keywords</a></td></tr><tr><td align="right">3.8 </td><td><a href="pairs.html" class="tocviewlink" data-pltdoc="x">Pairs and Lists</a></td></tr><tr><td align="right">3.9 </td><td><a href="vectors.html" class="tocviewlink" data-pltdoc="x">Vectors</a></td></tr><tr><td align="right">3.10 </td><td><a href="hash-tables.html" class="tocviewlink" data-pltdoc="x">Hash Tables</a></td></tr><tr><td align="right">3.11 </td><td><a href="boxes.html" class="tocviewlink" data-pltdoc="x">Boxes</a></td></tr><tr><td align="right">3.12 </td><td><a href="void_undefined.html" class="tocviewlink" data-pltdoc="x">Void and Undefined</a></td></tr></table></div></div></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://download.racket-lang.org/releases/8.6/doc/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="symbols.html" title="backward to "3.6 Symbols"" data-pltdoc="x">← prev</a> <a href="datatypes.html" title="up to "3 Built-In Datatypes"" data-pltdoc="x">up</a> <a href="pairs.html" title="forward to "3.8 Pairs and Lists"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""keywords"">3.7<tt> </tt><a name="(part._keywords)"></a>Keywords</h4><p>A <a name="(tech._keyword)"></a><span style="font-style: italic">keyword</span> value is similar to a symbol (see
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<a href="symbols.html" data-pltdoc="x">Symbols</a>), but its printed form is prefixed with
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<span class="RktInBG"><span class="hspace"></span><span class="RktIn">#:</span><span class="hspace"></span></span>.</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="magnify.png" alt="+" width="24" height="24"/></span><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=reader.html%23%2528part._parse-keyword%2529&version=8.6" class="Sq" data-pltdoc="x">Reading Keywords</a> in <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=index.html&version=8.6" class="Sq" data-pltdoc="x">The Racket Reference</a> documents the fine points of the syntax of keywords.</p></blockquote></blockquote></blockquote><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=keywords.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-%7E3ekeyword%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">string->keyword</a></span><span class="hspace"> </span><span class="RktVal">"apple"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'#:apple</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktVal">'</span><span class="RktVal">#:apple</span></td></tr><tr><td><p><span class="RktRes">'#:apple</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eq%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">eq?</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">#:apple</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=keywords.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-%7E3ekeyword%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">string->keyword</a></span><span class="hspace"> </span><span class="RktVal">"apple"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#t</span></p></td></tr></table></blockquote></div></p><p>More precisely, a keyword is analogous to an identifier; in the same
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way that an identifier can be quoted to produce a symbol, a keyword
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can be quoted to produce a value. The same term “keyword” is used in
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both cases, but we sometimes use <span style="font-style: italic">keyword value</span> to refer more
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specifically to the result of a quote-keyword expression or of
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<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=keywords.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-%7E3ekeyword%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">string->keyword</a></span>. An unquoted keyword is not an expression,
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just as an unquoted identifier does not produce a symbol:</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktSym">not-a-symbol-expression</span></td></tr><tr><td><p><span class="RktErr">not-a-symbol-expression: undefined;</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">cannot reference an identifier before its definition</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in module: top-level</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">#:not-a-keyword-expression</span></td></tr><tr><td><p><span class="RktErr">eval:2:0: #%datum: keyword misused as an expression</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: #:not-a-keyword-expression</span></p></td></tr></table></blockquote></div></p><p>Despite their similarities, keywords are used in a different way than
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identifiers or symbols. Keywords are intended for use (unquoted) as
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special markers in argument lists and in certain syntactic forms. For
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run-time flags and enumerations, use symbols instead of keywords. The
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example below illustrates the distinct roles of keywords and symbols.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">dir</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Filesystem.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._find-system-path%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">find-system-path</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">temp-dir</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">not </span><span class="RktVal">'</span><span class="RktVal">#:temp-dir</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="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=file-ports.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._with-output-to-file%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">with-output-to-file</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Manipulating_Paths.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._build-path%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">build-path</a></span><span class="hspace"> </span><span class="RktSym">dir</span><span class="hspace"> </span><span class="RktVal">"stuff.txt"</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="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Writing.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._printf%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"example\n"</span><span class="RktPn">)</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">optional </span><span class="RktPn">#:mode</span><span class="RktCmt"> argument can be </span><span class="RktVal">'</span><span class="RktVal">text</span><span class="RktCmt"> or </span><span class="RktVal">'</span><span class="RktVal">binary</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:mode</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">text</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">optional </span><span class="RktPn">#:exists</span><span class="RktCmt"> argument can be </span><span class="RktVal">'</span><span class="RktVal">replace</span><span class="RktCmt">, </span><span class="RktVal">'</span><span class="RktVal">truncate</span><span class="RktCmt">, ...</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:exists</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">replace</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></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" 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