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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>4.1 Notation</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="../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 Guide</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="intro.html" class="tocviewlink" data-pltdoc="x">Welcome to Racket</a></td></tr><tr><td align="right">2 </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="tocviewlink" data-pltdoc="x">Built-<wbr></wbr>In Datatypes</a></td></tr><tr><td align="right">4 </td><td><a href="scheme-forms.html" class="tocviewselflink" 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 Expressions</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Exceptions and Control</a></td></tr><tr><td align="right">11 </td><td><a href="for.html" class="tocviewlink" data-pltdoc="x">Iterations and Comprehensions</a></td></tr><tr><td align="right">12 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">13 </td><td><a href="classes.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">14 </td><td><a href="units.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">15 </td><td><a href="reflection.html" class="tocviewlink" data-pltdoc="x">Reflection and Dynamic Evaluation</a></td></tr><tr><td align="right">16 </td><td><a href="macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">17 </td><td><a href="languages.html" class="tocviewlink" data-pltdoc="x">Creating Languages</a></td></tr><tr><td align="right">18 </td><td><a href="concurrency.html" class="tocviewlink" data
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different notation than the character-based grammars of the
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<a href="to-scheme.html" data-pltdoc="x">Racket Essentials</a> chapter. The grammar for a use of a syntactic
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form <span class="RktKw">something</span> is shown like this:</p><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktKw">something</span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktVar">an-expr</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote></blockquote><p>The italicized meta-variables in this specification, such as
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<span class="RktVar">id</span> and <span class="RktVar">an-expr</span>, use the syntax of Racket
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identifiers, so <span class="RktVar">an-expr</span> is one meta-variable. A naming
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convention implicitly defines the meaning of many meta-variables:</p><ul><li><p>A meta-variable that ends in <span class="RktVar">id</span> stands for an
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identifier, such as <span class="RktSym">x</span> or
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<span class="RktSym">my-favorite-martian</span>.</p></li><li><p>A meta-identifier that ends in <span class="RktVar">keyword</span> stands
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for a keyword, such as <span class="RktPn">#:tag</span>.</p></li><li><p>A meta-identifier that ends with <span class="RktVar">expr</span> stands for any
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sub-form, and it will be parsed as an expression.</p></li><li><p>A meta-identifier that ends with <span class="RktVar">body</span> stands for any
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sub-form; it will be parsed as either a local definition or an
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expression. The last <span class="RktVar">body</span> must be an expression;
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see also <a href="define.html#%28part._intdefs%29" data-pltdoc="x">Internal Definitions</a>.</p></li></ul><p>Square brackets in the grammar indicate a parenthesized sequence of
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forms, where square brackets are normally used (by convention). That
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is, square brackets <span style="font-style: italic">do not</span> mean optional parts of the
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syntactic form.</p><p>A <span class="RktMeta">...</span> indicates zero or more repetitions of the
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preceding form, and <span class="RktMeta">...+</span> indicates one or more
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repetitions of the preceding datum. Otherwise, non-italicized
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identifiers stand for themselves.</p><p>Based on the above grammar, then, here are a few conforming uses of
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<span class="RktKw">something</span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktKw">something</span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">x</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktKw">something</span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">x</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=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">+</a></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><span class="RktPn">(</span><span class="RktKw">something</span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">my-favorite-martian</span><span class="hspace"> </span><span class="RktSym">x</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=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">+</a></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="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Some syntactic-form specifications refer to meta-variables that are
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not implicitly defined and not previously defined. Such meta-variables
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are defined after the main form, using a BNF-like format for
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alternatives:</p><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktKw">something-else</span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVar">thing</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktVar">an-expr</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt"> </span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="specgrammar"><tr><td align="right" valign="baseline"><span class="RktVar">thing</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktVar">thing-id</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktVar">thing-keyword</span></td></tr></table></td></tr></table></blockquote></blockquote><p>The above example says that, within a <span class="RktKw">something-else</span>
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form, a <span class="RktVar">thing</span> is either an identifier or a keyword.</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://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="scheme-forms.html" title="backward to "4 Expressions and Definitions"" data-pltdoc="x">← prev</a> <a href="scheme-forms.html" title="up to "4 Expressions and Definitions"" data-pltdoc="x">up</a> <a href="binding.html" title="forward to "4.2 Identifiers and Binding"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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