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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>12.1 Pattern-Based Syntax Matching</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="tocviewselflink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" 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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produced by <span class="RktVar">stx-expr</span>, and for which the corresponding
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<span class="RktVar">fender-expr</span> (if any) produces a true value; the result is
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from the corresponding <span class="RktVar">result-expr</span>, which is in tail position
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for the <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> form. If no <span class="RktVar">clause</span> matches, then
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the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3asyntax%29%29" class="RktValLink" data-pltdoc="x">exn:fail:syntax</a></span> exception is raised; the exception is generated by calling
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<span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._raise-syntax-error%29%29" class="RktValLink" data-pltdoc="x">raise-syntax-error</a></span> with <span class="RktVal">#f</span> as the “name” argument,
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a string with a generic error message, and the result of <span class="RktVar">stx-expr</span>.</div></p><p>A syntax object matches a <span class="RktVar">pattern</span> as follows:</p><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktVar">_</span></td></tr></table></blockquote><p>A <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> pattern (i.e., an identifier with the same binding as
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<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> and not among the <span class="RktVar">literal-id</span>s) matches any syntax object.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktVar">id</span></td></tr></table></blockquote><p>An <span class="RktVar">id</span> matches any syntax object when it is not bound to
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<span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span> or <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> and does not have the same binding as
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any <span class="RktVar">literal-id</span>. The <span class="RktVar">id</span> is further bound as
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<a name="(tech._pattern._variable)"></a><span style="font-style: italic">pattern variable</span> for the corresponding <span class="RktVar">fender-expr</span>
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(if any) and <span class="RktVar">result-expr</span>. A pattern-variable binding is a
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transformer binding; the pattern variable can be referenced only
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through forms like <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%29%29" class="RktStxLink" data-pltdoc="x">syntax</a></span>. The binding’s value is the syntax
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object that matched the pattern with a <a name="(tech._depth._marker)"></a><span style="font-style: italic">depth marker</span> of
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<span style="font-style: italic"></span>0<span style="font-style: italic"></span>.</p><p>With a <span class="RktVar">stat-pattern</span>, <span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span> is not treated specially.
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It either matches a <span class="RktVar">literal-id</span> or is bound as a
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pattern variable.</p><p>An <span class="RktVar">id</span> that has the same binding as a <span class="RktVar">literal-id</span>
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matches a syntax object that is an identifier with the same binding
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in the sense of <span class="RktSym"><a href="stxcmp.html#%28def._%28%28quote._~23~25kernel%29._free-identifier~3d~3f%29%29" class="RktValLink" data-pltdoc="x">free-identifier=?</a></span>. The match does not
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introduce any <a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variables</span></a>.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote><p>A <span class="RktPn">(</span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktMeta">...</span><span class="RktPn">)</span> pattern matches a syntax object whose datum
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form (i.e., without lexical information) is a list with as many
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elements as sub-<span class="RktVar">pattern</span>s in the pattern, and where each
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syntax object that corresponds to an element of the list matches
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the corresponding sub-<span class="RktVar">pattern</span>.</p><p>Any <a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variables</span></a> bound by the sub-<span class="RktVar">pattern</span>s are
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bound by the complete pattern; the bindings must all be distinct.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVar">np-pattern</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Like the previous kind of pattern, but matches syntax objects that
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are not necessarily lists; for <span style="font-style: italic">n</span> sub-<span class="RktVar">pattern</span>s before
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the final <span class="RktVar">np-pattern</span>, the syntax object’s datum must be a
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pair such that <span style="font-style: italic">n-</span>1<span style="font-style: italic"></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>s produce pairs. The final
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<span class="RktVar">np-pattern</span> is matched against the syntax object
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corresponding to the <span style="font-style: italic">n</span>th <span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._cdr%29%29" class="RktValLink" data-pltdoc="x">cdr</a></span> (or the
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<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> coercion of the datum using the nearest
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enclosing syntax object’s lexical context and source location).</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="hspace"> </span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktVar">ellipsis</span><span class="hspace"> </span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Like the <span class="RktPn">(</span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktMeta">...</span><span class="RktPn">)</span> kind of pattern, but matching a
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syntax object with any number (zero or more) elements that match the
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sub-<span class="RktVar">pattern</span> followed by <span class="RktVar">ellipsis</span> in the
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corresponding position relative to other sub-<span class="RktVar">pattern</span>s.</p><p>For each pattern variable bound by the sub-<span class="RktVar">pattern</span> followed
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by <span class="RktVar">ellipsis</span>, the larger pattern binds the same pattern
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variable to a list of values, one for each element of the syntax
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object matched to the sub-<span class="RktVar">pattern</span>, with an incremented
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<a href="stx-patterns.html#%28tech._depth._marker%29" class="techoutside" data-pltdoc="x"><span class="techinside">depth marker</span></a>. (The sub-<span class="RktVar">pattern</span> itself may contain
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<span class="RktVar">ellipsis</span>, leading to a pattern variables bound to lists of
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lists of syntax objects with a <a href="stx-patterns.html#%28tech._depth._marker%29" class="techoutside" data-pltdoc="x"><span class="techinside">depth marker</span></a> of <span style="font-style: italic"></span>2<span style="font-style: italic"></span>, and
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so on.)</p><p>All patterns forms with <span class="RktVar">ellipsis</span> apply only when
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<span class="RktVar">ellipsis</span> is not among the <span class="RktVar">literal-id</span>s.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="hspace"> </span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktVar">ellipsis</span><span class="hspace"> </span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVar">np-pattern</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Like the previous kind of pattern, but with a final
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<span class="RktVar">np-pattern</span> as for <span class="RktPn">(</span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktMeta"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=stxparse-patterns.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._......%252B%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">...+</a></span><span class="stt"> </span><span class="RktPn">. </span><span class="RktVar">np-pattern</span><span class="RktPn">)</span>. The
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final <span class="RktVar">np-pattern</span> never matches a syntax object whose datum is a
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pair.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="stt">#</span><span class="RktPn">(</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Like a <span class="RktPn">(</span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktMeta">...</span><span class="RktPn">)</span> pattern, but matching a vector syntax object
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whose elements match the corresponding sub-<span class="RktVar">pattern</span>s.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="stt">#</span><span class="RktPn">(</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="hspace"> </span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktVar">ellipsis</span><span class="hspace"> </span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Like a <span class="RktPn">(</span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="stt"> </span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktVar">ellipsis</span><span class="stt"> </span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span> pattern,
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but matching a vector syntax object whose elements match the
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corresponding sub-<span class="RktVar">pattern</span>s.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="stt">#&</span><span class="RktVar">pattern</span></td></tr></table></blockquote><p>Matches a box syntax object whose content matches the
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<span class="RktVar">pattern</span>.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="stt">#s</span><span class="RktPn">(</span><span class="RktVar">key-datum</span><span class="hspace"> </span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Like a <span class="RktPn">(</span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktMeta">...</span><span class="RktPn">)</span> pattern, but matching a <a href="structures.html#%28tech._prefab%29" class="techoutside" data-pltdoc="x"><span class="techinside">prefab</span></a>
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structure syntax object whose fields match the corresponding
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sub-<span class="RktVar">pattern</span>s. The <span class="RktVar">key-datum</span> must correspond to a
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valid first argument to <span class="RktSym"><a href="structutils.html#%28def._%28%28quote._~23~25kernel%29._make-prefab-struct%29%29" class="RktValLink" data-pltdoc="x">make-prefab-struct</a></span>.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="stt">#s</span><span class="RktPn">(</span><span class="RktVar">key-datum</span><span class="hspace"> </span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="hspace"> </span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktVar">ellipsis</span><span class="hspace"> </span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Like a <span class="RktPn">(</span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktMeta">...</span><span class="stt"> </span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktVar">ellipsis</span><span class="stt"> </span><span class="RktVar">pattern</span><span class="stt"> </span><span class="RktMeta">...</span><span class="RktPn">)</span> pattern,
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but matching a <a href="structures.html#%28tech._prefab%29" class="techoutside" data-pltdoc="x"><span class="techinside">prefab</span></a> structure syntax object whose elements
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match the corresponding sub-<span class="RktVar">pattern</span>s.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktVar">ellipsis</span><span class="hspace"> </span><span class="RktVar">stat-pattern</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Matches the same as <span class="RktVar">stat-pattern</span>, which is like a <span class="RktVar">pattern</span>,
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but identifiers with the binding <span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span> are treated the same as
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other <span class="RktVar">id</span>s.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktVar">const</span></td></tr></table></blockquote><p>A <span class="RktVar">const</span> is any datum that does not match one of the
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preceding forms; a syntax object matches a <span class="RktVar">const</span> pattern
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when its datum is <span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span> to the <span class="RktSym"><a href="quote.html#%28form._%28%28quote._~23~25kernel%29._quote%29%29" class="RktStxLink" data-pltdoc="x">quote</a></span>d
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<span class="RktVar">const</span>.</p></blockquote><p>If <span class="RktVar">stx-expr</span> produces a non-<a href="syntax-model.html#%28tech._syntax._object%29" class="techoutside" data-pltdoc="x"><span class="techinside">syntax object</span></a>, then its
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result is converted to a syntax object 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>
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and the lexical context and source location of the <span class="RktVar">stx-expr</span>.</p><p>If <span class="RktVar">stx-expr</span> produces a syntax object that is <a href="stxcerts.html#%28tech._tainted%29" class="techoutside" data-pltdoc="x"><span class="techinside">tainted</span></a>,
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then any syntax object bound by a <span class="RktVar">pattern</span>
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are <a href="stxcerts.html#%28tech._tainted%29" class="techoutside" data-pltdoc="x"><span class="techinside">tainted</span></a>.</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="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="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span><span class="hspace"> </span><span class="RktSym">racket/base</span><span class="RktPn">)</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-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">swap</span><span class="hspace"> </span><span class="RktSym">stx</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="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </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="RktPn">(</span><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><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktRdr">#'</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="RktSym">a</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="set_.html#%28form._%28%28quote._~23~25kernel%29._set%21%29%29" class="RktStxLink" data-pltdoc="x">set!</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</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="set_.html#%28form._%28%28quote._~23~25kernel%29._set%21%29%29" class="RktStxLink" data-pltdoc="x">set!</a></span><span class="hspace"> </span><span class="RktSym">b</span><span class="hspace"> </span><span class="RktSym">t</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><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span clas
|
||
|
|
procedure that accepts two arguments. A <span class="RktVar">literal-id</span> in a
|
||
|
|
<span class="RktVar">pattern</span> matches an identifier for which the procedure
|
||
|
|
returns true when given the identifier to match (as the first argument)
|
||
|
|
and the identifier in the <span class="RktVar">pattern</span> (as the second argument).</div></p><p>In other words, <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> is like <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%2A%29%29" class="RktStxLink" data-pltdoc="x">syntax-case*</a></span> with
|
||
|
|
an <span class="RktVar">id-compare-expr</span> that produces <span class="RktSym"><a href="stxcmp.html#%28def._%28%28quote._~23~25kernel%29._free-identifier~3d~3f%29%29" class="RktValLink" data-pltdoc="x">free-identifier=?</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>syntax</p></div></div><table cellspacing="0" cellpadding="0" class="RktBlk RForeground"><tr><td><span class="RktPn">(</span><a name="(form._((lib._racket/private/stxcase-scheme..rkt)._with-syntax))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">with-syntax</a></span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktVar">stx-expr</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></td></tr></table></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Similar to <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span>, in that it matches a <span class="RktVar">pattern</span>
|
||
|
|
to a syntax object. Unlike <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span>, all <span class="RktVar">pattern</span>s
|
||
|
|
are matched, each to the result of a corresponding <span class="RktVar">stx-expr</span>,
|
||
|
|
and the pattern variables from all matches (which must be distinct)
|
||
|
|
are bound with a single <span class="RktVar">body</span> sequence. The result of the
|
||
|
|
<span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span> form is the result of the last <span class="RktVar">body</span>,
|
||
|
|
which is in tail position with respect to the <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span>
|
||
|
|
form.</div></p><p>If any <span class="RktVar">pattern</span> fails to match the corresponding
|
||
|
|
<span class="RktVar">stx-expr</span>, the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3asyntax%29%29" class="RktValLink" data-pltdoc="x">exn:fail:syntax</a></span> exception is raised.</p><p>A <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span> form is roughly equivalent to the following
|
||
|
|
<span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> form:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></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="RktVar">stx-expr</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</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="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=stxparse-patterns.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._......%252B%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">...+</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></blockquote><p>However, if any individual <span class="RktVar">stx-expr</span> produces a
|
||
|
|
non-<a href="syntax-model.html#%28tech._syntax._object%29" class="techoutside" data-pltdoc="x"><span class="techinside">syntax object</span></a>, then it is converted to one 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> and the lexical context and source location of
|
||
|
|
the individual <span class="RktVar">stx-expr</span>.</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="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hello</span><span class="hspace"> </span><span class="RktSym">stx</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="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </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="RktPn">(</span><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><span class="hspace"> </span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktSym"><a href="places.html#%28form._%28%28lib._racket%2Fplace..rkt%29._place%29%29" class="RktStxLink" data-pltdoc="x">place</a></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="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">print-name</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._printf%29%29" class="RktValLink" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a\n"</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">name</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">print-place</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._printf%29%29" class="RktValLink" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"~a\n"</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">place</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="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="begin.html#%28form._%28%28quote._~23~25kernel%29._begin%29%29" class="RktStxLink" data-pltdoc="x">begin</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </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-pltdo
|
||
|
|
<span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~3f%29%29" class="RktStxLink" data-pltdoc="x">~?</a></span>, and <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~40%29%29" class="RktStxLink" data-pltdoc="x">~@</a></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"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td></tr><tr><td align="right" valign="baseline"><span class="RktVar">ellipsis</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="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Constructs a syntax object based on a <span class="RktVar">template</span>, which can
|
||
|
|
include <a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variables</span></a> bound by <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> or
|
||
|
|
<span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span>.</div></p><p>A <span class="RktVar">template</span> produces a single syntax object. A
|
||
|
|
<span class="RktVar">head-template</span> produces a sequence of zero or more syntax
|
||
|
|
objects. A <span class="RktVar">stat-template</span> is like a <span class="RktVar">template</span>, except that
|
||
|
|
<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>, <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~3f%29%29" class="RktStxLink" data-pltdoc="x">~?</a></span>, and <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~40%29%29" class="RktStxLink" data-pltdoc="x">~@</a></span> are interpreted as
|
||
|
|
constants instead of template forms.</p><p>A <span class="RktVar">template</span> produces a syntax object as follows:</p><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktVar">id</span></td></tr></table></blockquote><p>If <span class="RktVar">id</span> is bound as a <a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variable</span></a>, then
|
||
|
|
<span class="RktVar">id</span> as a template produces the <a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variable</span></a>’s
|
||
|
|
match result. Unless the <span class="RktVar">id</span> is a sub-<span class="RktVar">template</span> that is
|
||
|
|
replicated by <span class="RktVar">ellipsis</span> in a larger <span class="RktVar">template</span>, the
|
||
|
|
<a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variable</span></a>’s value must be a syntax object with a
|
||
|
|
<a href="stx-patterns.html#%28tech._depth._marker%29" class="techoutside" data-pltdoc="x"><span class="techinside">depth marker</span></a> of <span style="font-style: italic"></span>0<span style="font-style: italic"></span> (as opposed to a list of
|
||
|
|
matches).</p><p>More generally, if the <a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variable</span></a>’s value has a depth
|
||
|
|
marker <span style="font-style: italic">n</span>, then it can only appear within a template where it
|
||
|
|
is replicated by at least <span style="font-style: italic">n</span> <span class="RktVar">ellipsis</span>es. In that case,
|
||
|
|
the template will be replicated enough times to use each match result
|
||
|
|
at least once.</p><p>If <span class="RktVar">id</span> is not bound as a pattern variable, then <span class="RktVar">id</span>
|
||
|
|
as a template produces <span class="RktPn">(</span><span class="RktSym"><a href="Syntax_Quoting__quote-syntax.html#%28form._%28%28quote._~23~25kernel%29._quote-syntax%29%29" class="RktStxLink" data-pltdoc="x">quote-syntax</a></span><span class="stt"> </span><span class="RktVar">id</span><span class="RktPn">)</span>.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktVar">head-template</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Produces a syntax object whose datum is a list, and where the
|
||
|
|
elements of the list correspond to syntax objects produced by the
|
||
|
|
<span class="RktVar">head-template</span>s.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktVar">head-template</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVar">template</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Like the previous form, but the result is not necessarily a list;
|
||
|
|
instead, the place of the empty list in the resulting syntax object’s
|
||
|
|
datum is taken by the syntax object produced by <span class="RktVar">template</span>.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="stt">#</span><span class="RktPn">(</span><span class="RktVar">head-template</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Like the <span class="RktPn">(</span><span class="RktVar">head-template</span><span class="stt"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span> form, but producing a syntax
|
||
|
|
object whose datum is a vector instead of a list.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="stt">#&</span><span class="RktVar">template</span></td></tr></table></blockquote><p>Produces a syntax object whose datum is a box holding the
|
||
|
|
syntax object produced by <span class="RktVar">template</span>.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="stt">#s</span><span class="RktPn">(</span><span class="RktVar">key-datum</span><span class="hspace"> </span><span class="RktVar">head-template</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Like the <span class="RktPn">(</span><span class="RktVar">head-template</span><span class="stt"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span> form, but producing a syntax
|
||
|
|
object whose datum is a <a href="structures.html#%28tech._prefab%29" class="techoutside" data-pltdoc="x"><span class="techinside">prefab</span></a> structure instead of a list.
|
||
|
|
The <span class="RktVar">key-datum</span> must correspond to a valid first argument of
|
||
|
|
<span class="RktSym"><a href="structutils.html#%28def._%28%28quote._~23~25kernel%29._make-prefab-struct%29%29" class="RktValLink" data-pltdoc="x">make-prefab-struct</a></span>.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~3f%29%29" class="RktStxLink" data-pltdoc="x">~?</a></span><span class="hspace"> </span><span class="RktVar">template1</span><span class="hspace"> </span><span class="RktVar">template2</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Produces the result of <span class="RktVar">template1</span> if <span class="RktVar">template1</span> has no
|
||
|
|
pattern variables with “missing values”; otherwise, produces the result of
|
||
|
|
<span class="RktVar">template2</span>.</p><p>A pattern variable bound by <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> never has a missing value, but
|
||
|
|
pattern variables bound by <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=Parsing_Syntax.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._syntax-parse%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">syntax-parse</a></span> (for example, <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=stxparse-patterns.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._%7E7eor%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">~or</a></span> or
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=stxparse-patterns.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._%7E7eoptional%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">~optional</a></span> patterns) can.</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="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=syntax&rel=Parsing_Syntax.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._syntax-parse%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">syntax-parse</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym">m</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">3</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><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><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=syntax&rel=stxparse-patterns.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._%7E7eoptional%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">~optional</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=syntax&rel=stxparse-patterns.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._%7E7eseq%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">~seq</a></span><span class="hspace"> </span><span class="RktPn">#:op</span><span class="hspace"> </span><span class="RktSym">op:expr</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">arg:expr</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~3f%29%29" class="RktStxLink" data-pltdoc="x">~?</a></span><span class="hspace"> </span><span class="RktSym">op</span><span class="hspace"> </span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">arg</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">#<syntax:eval:3:0 (+ 1 2 3)></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="https://download.racket-lang.org/releases/8.6/doc/
|
||
|
|
<span class="RktVar">template</span>, but <span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span>, <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~3f%29%29" class="RktStxLink" data-pltdoc="x">~?</a></span>, and <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~40%29%29" class="RktStxLink" data-pltdoc="x">~@</a></span>
|
||
|
|
are treated like an <span class="RktVar">id</span> (with no pattern binding).</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktVar">const</span></td></tr></table></blockquote><p>A <span class="RktVar">const</span> template is any form that does not match the
|
||
|
|
preceding cases, and it produces the result <span class="RktPn">(</span><span class="RktSym"><a href="Syntax_Quoting__quote-syntax.html#%28form._%28%28quote._~23~25kernel%29._quote-syntax%29%29" class="RktStxLink" data-pltdoc="x">quote-syntax</a></span><span class="stt"> </span><span class="RktVar">const</span><span class="RktPn">)</span>.</p></blockquote><p>A <span class="RktVar">head-template</span> produces a sequence of syntax objects; that sequence is
|
||
|
|
“inlined” into the result of the enclosing <span class="RktVar">template</span>. The result of a
|
||
|
|
<span class="RktVar">head-template</span> is defined as follows:</p><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktVar">template</span></td></tr></table></blockquote><p>Produces one syntax object, according to the rules for <span class="RktVar">template</span>
|
||
|
|
above.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktVar">head-template</span><span class="hspace"> </span><span class="RktVar">ellipsis</span><span class="hspace"> </span><span class="RktMeta">...+</span></td></tr></table></blockquote><p>Generates a sequence of syntax objects by “<span class="RktSym"><a href="pairs.html#%28def._%28%28lib._racket%2Fprivate%2Fmap..rkt%29._map%29%29" class="RktValLink" data-pltdoc="x">map</a></span>ping” the
|
||
|
|
<span class="RktVar">head-template</span> over the values of its pattern variables. The number of
|
||
|
|
iterations depends on the values of the <a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variables</span></a> referenced
|
||
|
|
within the sub-template.</p><p>To be more precise: Let <span class="RktVar">outer</span> be <span class="RktVar">inner</span> followed by one
|
||
|
|
ellipsis. A <a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variable</span></a> is an <a name="(tech._iteration._pattern._variable)"></a><span style="font-style: italic">iteration pattern variable</span>
|
||
|
|
for <span class="RktVar">outer</span> if occurs at a depth equal to its <a href="stx-patterns.html#%28tech._depth._marker%29" class="techoutside" data-pltdoc="x"><span class="techinside">depth
|
||
|
|
marker</span></a>. There must be at least one; otherwise, an error is raised. If there
|
||
|
|
are multiple iteration variables, then all of their values must be lists of
|
||
|
|
the same length. The result for <span class="RktVar">outer</span> is produced by
|
||
|
|
<span class="RktSym"><a href="pairs.html#%28def._%28%28lib._racket%2Fprivate%2Fmap..rkt%29._map%29%29" class="RktValLink" data-pltdoc="x">map</a></span>ping the <span class="RktVar">inner</span> template over the <a href="stx-patterns.html#%28tech._iteration._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">iteration pattern
|
||
|
|
variable</span></a> values and decreasing their effective <a href="stx-patterns.html#%28tech._depth._marker%29" class="techoutside" data-pltdoc="x"><span class="techinside">depth markers</span></a> by 1
|
||
|
|
within <span class="RktVar">inner</span>. The <span class="RktVar">outer</span> result is formed by appending the
|
||
|
|
<span class="RktVar">inner</span> results.</p><p>Consequently, if a <a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variable</span></a> occurs at a depth greater than its
|
||
|
|
<a href="stx-patterns.html#%28tech._depth._marker%29" class="techoutside" data-pltdoc="x"><span class="techinside">depth marker</span></a>, it is used as an <a href="stx-patterns.html#%28tech._iteration._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">iteration pattern variable</span></a> for
|
||
|
|
the innermost ellipses but not the outermost. A <a href="stx-patterns.html#%28tech._pattern._variable%29" class="techoutside" data-pltdoc="x"><span class="techinside">pattern variable</span></a> must
|
||
|
|
not occur at a depth less than its <a href="stx-patterns.html#%28tech._depth._marker%29" class="techoutside" data-pltdoc="x"><span class="techinside">depth marker</span></a>; otherwise, an error is
|
||
|
|
raised.</p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~40%29%29" class="RktStxLink" data-pltdoc="x">~@</a></span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVar">template</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Produces the sequence of elements in the syntax list produced by
|
||
|
|
<span class="RktVar">template</span>. If <span class="RktVar">template</span> does not produce a proper syntax list,
|
||
|
|
an exception is raised.</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="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">key</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktVal">'</span><span class="RktVal">a</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">b</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">c</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="RktPn">(</span><span class="RktSym">val</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">3</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="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="hashtables.html#%28def._%28%28quote._~23~25kernel%29._hash%29%29" class="RktValLink" data-pltdoc="x">hash</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~40%29%29" class="RktStxLink" data-pltdoc="x">~@</a></span><span class="hspace"> </span><span class="RktSym">key</span><span class="hspace"> </span><span class="RktSym">val</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">#<syntax:eval:2:0 (hash (quote a) 1 (quote b) 2 (quote c) 3)></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="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">xs</span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktVal">4</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="RktRdr">#'</span>
|
||
|
|
variables have “missing values”; otherwise produces the result of
|
||
|
|
<span class="RktVar">head-template2</span>. </p></blockquote><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~3f%29%29" class="RktStxLink" data-pltdoc="x">~?</a></span><span class="hspace"> </span><span class="RktVar">head-template</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Produces the result of <span class="RktVar">head-template</span> if none of its pattern
|
||
|
|
variables have “missing values”; otherwise produces nothing.</p><p>Equivalent to <span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~3f%29%29" class="RktStxLink" data-pltdoc="x">~?</a></span><span class="stt"> </span><span class="RktVar">head-template</span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~40%29%29" class="RktStxLink" data-pltdoc="x">~@</a></span><span class="RktPn">)</span><span class="RktPn">)</span>. </p></blockquote><p>A <span class="RktPn">(</span><span class="RktKw">syntax</span><span class="stt"> </span><span class="RktVar">template</span><span class="RktPn">)</span> form is normally
|
||
|
|
abbreviated as <span class="RktRdr">#'</span><span class="RktVar">template</span>; see also
|
||
|
|
<a href="reader.html#%28part._parse-quote%29" data-pltdoc="x">Reading Quotes</a>. If <span class="RktVar">template</span> contains no pattern
|
||
|
|
variables, then <span class="RktRdr">#'</span><span class="RktVar">template</span> is equivalent to
|
||
|
|
<span class="RktPn">(</span><span class="RktSym"><a href="Syntax_Quoting__quote-syntax.html#%28form._%28%28quote._~23~25kernel%29._quote-syntax%29%29" class="RktStxLink" data-pltdoc="x">quote-syntax</a></span><span class="stt"> </span><span class="RktVar">template</span><span class="RktPn">)</span>.</p><p class="SHistory">Changed in version 6.90.0.25 of package <span class="stt">base</span>: Added <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~40%29%29" class="RktStxLink" data-pltdoc="x">~@</a></span> and <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~3f%29%29" class="RktStxLink" data-pltdoc="x">~?</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>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/private/qqstx..rkt)._quasisyntax))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._quasisyntax%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">quasisyntax</a></span></span><span class="hspace"> </span><span class="RktVar">template</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%29%29" class="RktStxLink" data-pltdoc="x">syntax</a></span>, but <span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._unsyntax%29%29" class="RktStxLink" data-pltdoc="x">unsyntax</a></span><span class="stt"> </span><span class="RktVar">expr</span><span class="RktPn">)</span> and <span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._unsyntax-splicing%29%29" class="RktStxLink" data-pltdoc="x">unsyntax-splicing</a></span><span class="stt"> </span><span class="RktVar">expr</span><span class="RktPn">)</span>
|
||
|
|
escape to an expression within the <span class="RktVar">template</span>.</div></p><p>The <span class="RktVar">expr</span> must produce a syntax object (or syntax list) to be
|
||
|
|
substituted in place of the <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._unsyntax%29%29" class="RktStxLink" data-pltdoc="x">unsyntax</a></span> or
|
||
|
|
<span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._unsyntax-splicing%29%29" class="RktStxLink" data-pltdoc="x">unsyntax-splicing</a></span> form within the quasiquoting template, just
|
||
|
|
like <span class="RktSym"><a href="quasiquote.html#%28form._%28%28quote._~23~25kernel%29._unquote%29%29" class="RktStxLink" data-pltdoc="x">unquote</a></span> and <span class="RktSym"><a href="quasiquote.html#%28form._%28%28quote._~23~25kernel%29._unquote-splicing%29%29" class="RktStxLink" data-pltdoc="x">unquote-splicing</a></span> within
|
||
|
|
<span class="RktSym"><a href="quasiquote.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._quasiquote%29%29" class="RktStxLink" data-pltdoc="x">quasiquote</a></span>, except that a hash table value position is not
|
||
|
|
an escape position for <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._quasisyntax%29%29" class="RktStxLink" data-pltdoc="x">quasisyntax</a></span>. (If the escaped expression does not generate a
|
||
|
|
syntax object, it is converted to one in the same way as for the
|
||
|
|
right-hand side of <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._with-syntax%29%29" class="RktStxLink" data-pltdoc="x">with-syntax</a></span>.) Nested
|
||
|
|
<span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._quasisyntax%29%29" class="RktStxLink" data-pltdoc="x">quasisyntax</a></span>es introduce quasiquoting layers in the same way
|
||
|
|
as nested <span class="RktSym"><a href="quasiquote.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._quasiquote%29%29" class="RktStxLink" data-pltdoc="x">quasiquote</a></span>s.</p><p>Also analogous to <span class="RktSym"><a href="quasiquote.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._quasiquote%29%29" class="RktStxLink" data-pltdoc="x">quasiquote</a></span>, the reader converts <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#`</span><span class="hspace"></span></span>
|
||
|
|
to <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._quasisyntax%29%29" class="RktStxLink" data-pltdoc="x">quasisyntax</a></span>, <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#,</span><span class="hspace"></span></span> to <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._unsyntax%29%29" class="RktStxLink" data-pltdoc="x">unsyntax</a></span>, and
|
||
|
|
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">#,@</span><span class="hspace"></span></span> to <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._unsyntax-splicing%29%29" class="RktStxLink" data-pltdoc="x">unsyntax-splicing</a></span>. See also
|
||
|
|
<a href="reader.html#%28part._parse-quote%29" data-pltdoc="x">Reading Quotes</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>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/private/qqstx..rkt)._unsyntax))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._unsyntax%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">unsyntax</a></span></span><span class="hspace"> </span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=syntax&rel=Library_Syntax_Classes_and_Literal_Sets.html%23%2528form._%2528%2528lib._syntax%252Fparse..rkt%2529._expr%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">expr</a></span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Illegal as an expression form. The <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._unsyntax%29%29" class="RktStxLink" data-pltdoc="x">unsyntax</a></span> form is for use
|
||
|
|
only with a <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._quasisyntax%29%29" class="RktStxLink" data-pltdoc="x">quasisyntax</a></span> template.</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>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/private/qqstx..rkt)._unsyntax-splicing))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._unsyntax-splicing%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">unsyntax-splicing</a></span></span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Illegal as an expression form. The <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._unsyntax-splicing%29%29" class="RktStxLink" data-pltdoc="x">unsyntax-splicing</a></span> form is
|
||
|
|
for use only with a <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._quasisyntax%29%29" class="RktStxLink" data-pltdoc="x">quasisyntax</a></span> template.</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>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/private/stxcase-scheme..rkt)._syntax/loc))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%2Floc%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">syntax/loc</a></span></span><span class="hspace"> </span><span class="RktVar">loc-expr</span><span class="hspace"> </span><span class="RktVar">template</span><span class="RktPn">)</span></p></blockquote></td></tr><tr><td><span class="stt"> </span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td><span class="hspace"> </span></td><td><span class="RktVar">loc-expr</span></td><td><span class="hspace"> </span></td><td>:</td><td><span class="hspace"> </span></td><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="RktVal">#f</span><span class="hspace"> </span><span class="RktSym"><a href="exns.html#%28def._%28%28quote._~23~25kernel%29._srcloc~3f%29%29" class="RktValLink" data-pltdoc="x">srcloc?</a></span><span class="hspace"> </span><span class="RktSym"><a href="stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax~3f%29%29" class="RktValLink" data-pltdoc="x">syntax?</a></span></td></tr><tr><td><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._list%2Fc%29%29" class="RktValLink" data-pltdoc="x">list/c</a></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><tr><td><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="number-types.html#%28def._%28%28quote._~23~25kernel%29._exact-positive-integer~3f%29%29" class="RktValLink" data-pltdoc="x">exact-positive-integer?</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="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="number-types.html#%28def._%28%28quote._~23~25kernel%29._exact-nonnegative-integer~3f%29%29" class="RktValLink" data-pltdoc="x">exact-nonnegative-integer?</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="RktPn">(</span><span class="RktSym"><a href="data-structu
|
||
|
|
object takes its source-location information from the result of
|
||
|
|
<span class="RktVar">loc-expr</span>.</div></p><p>Only the source location of the immediate result—<wbr></wbr>the “outermost”
|
||
|
|
syntax object—<wbr></wbr>is adjusted. The source location is <span class="emph">not</span>
|
||
|
|
adjusted if both the source and position of <span class="RktVar">loc-expr</span> are
|
||
|
|
<span class="RktVal">#f</span>. The source location is adjusted only if the resulting
|
||
|
|
syntax object comes from the template itself rather than the value of
|
||
|
|
a syntax pattern variable. For example, if <span class="RktVar">x</span> is a syntax
|
||
|
|
pattern variable, then <span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%2Floc%29%29" class="RktStxLink" data-pltdoc="x">syntax/loc</a></span><span class="stt"> </span><span class="RktVar">loc-expr</span><span class="stt"> </span><span class="RktVar">x</span><span class="RktPn">)</span> does not use
|
||
|
|
the location of <span class="RktVar">loc-expr</span>.</p><p class="SHistory">Changed in version 6.90.0.25 of package <span class="stt">base</span>: Previously, <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%2Floc%29%29" class="RktStxLink" data-pltdoc="x">syntax/loc</a></span>
|
||
|
|
did not enforce the contract on <span class="RktVar">loc-expr</span> if <span class="RktVar">template</span>
|
||
|
|
was just a pattern variable.<br/>Changed in version 8.2.0.6: Allows <span class="RktVar">loc-expr</span> to be any
|
||
|
|
source location value that <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> accepts.</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>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/private/qqstx..rkt)._quasisyntax/loc))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fqqstx..rkt%29._quasisyntax%2Floc%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">quasisyntax/loc</a></span></span><span class="hspace"> </span><span class="RktVar">loc-expr</span><span class="hspace"> </span><span class="RktVar">template</span><span class="RktPn">)</span></p></blockquote></td></tr><tr><td><span class="stt"> </span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td><span class="hspace"> </span></td><td><span class="RktVar">loc-expr</span></td><td><span class="hspace"> </span></td><td>:</td><td><span class="hspace"> </span></td><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="RktVal">#f</span><span class="hspace"> </span><span class="RktSym"><a href="exns.html#%28def._%28%28quote._~23~25kernel%29._srcloc~3f%29%29" class="RktValLink" data-pltdoc="x">srcloc?</a></span><span class="hspace"> </span><span class="RktSym"><a href="stxops.html#%28def._%28%28quote._~23~25kernel%29._syntax~3f%29%29" class="RktValLink" data-pltdoc="x">syntax?</a></span></td></tr><tr><td><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._list%2Fc%29%29" class="RktValLink" data-pltdoc="x">list/c</a></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><tr><td><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="number-types.html#%28def._%28%28quote._~23~25kernel%29._exact-positive-integer~3f%29%29" class="RktValLink" data-pltdoc="x">exact-positive-integer?</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="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="number-types.html#%28def._%28%28quote._~23~25kernel%29._exact-nonnegative-integer~3f%29%29" class="RktValLink" data-pltdoc="x">exact-nonnegative-integer?</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="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.htm
|
||
|
|
<span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%2Floc%29%29" class="RktStxLink" data-pltdoc="x">syntax/loc</a></span>.</div></p><p class="SHistory">Changed in version 8.2.0.6 of package <span class="stt">base</span>: Allows <span class="RktVar">loc-expr</span> to be any
|
||
|
|
source location value that <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> accepts.</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>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/private/stxcase-scheme..rkt)._quote-syntax/prune))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._quote-syntax%2Fprune%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">quote-syntax/prune</a></span></span><span class="hspace"> </span><span class="RktVar">id</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="Syntax_Quoting__quote-syntax.html#%28form._%28%28quote._~23~25kernel%29._quote-syntax%29%29" class="RktStxLink" data-pltdoc="x">quote-syntax</a></span>, but the lexical context of <span class="RktVar">id</span> is
|
||
|
|
pruned via <span class="RktSym"><a href="stxops.html#%28def._%28%28quote._~23~25kernel%29._identifier-prune-lexical-context%29%29" class="RktValLink" data-pltdoc="x">identifier-prune-lexical-context</a></span> to including
|
||
|
|
binding only for the symbolic name of <span class="RktVar">id</span> and for
|
||
|
|
<span class="RktVal">'</span><span class="RktVal">#%top</span>. Use this form to quote an identifier when its lexical
|
||
|
|
information will not be transferred to other syntax objects (except
|
||
|
|
maybe to <span class="RktVal">'</span><span class="RktVal">#%top</span> for a top-level binding).</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>syntax</p></div></div><table cellspacing="0" cellpadding="0" class="RktBlk RForeground"><tr><td><span class="RktPn">(</span><a name="(form._((lib._racket/private/stxcase-scheme..rkt)._syntax-rules))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-rules%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">syntax-rules</a></span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">literal-id</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVar">pattern</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">template</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Equivalent to</div></p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><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">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">literal-id</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktVar">generated-id</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVar">pattern</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="stxcerts.html#%28def._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-protect%29%29" class="RktValLink" data-pltdoc="x">syntax-protect</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktVar">template</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>where each <span class="RktVar">generated-id</span> binds no identifier in the
|
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corresponding <span class="RktVar">template</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>syntax</p></div></div><table cellspacing="0" cellpadding="0" class="RktBlk RForeground"><tr><td><span class="RktPn">(</span><a name="(form._((lib._racket/private/stxcase-scheme..rkt)._syntax-id-rules))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-id-rules%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">syntax-id-rules</a></span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">literal-id</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktVar">template</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Equivalent to</div></p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="stxtrans.html#%28def._%28%28quote._~23~25kernel%29._make-set%21-transformer%29%29" class="RktValLink" data-pltdoc="x">make-set!-transformer</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">stx</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span><span class="hspace"> </span><span class="RktSym">stx</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">literal-id</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktVar">pattern</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="stxcerts.html#%28def._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-protect%29%29" class="RktValLink" data-pltdoc="x">syntax-protect</a></span><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktVar">template</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><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>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/private/misc..rkt)._define-syntax-rule))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-pa
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||
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<span class="RktVar">pattern</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>syntax</p></div></div><p class="RForeground"><a name="(form._((lib._racket/private/stxcase-scheme..rkt)._......))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">...</a></span></span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">The <span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span> transformer binding prohibits <span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span> from
|
||
|
|
being used as an expression. This binding is useful only in syntax
|
||
|
|
patterns and templates (or other unrelated expression forms
|
||
|
|
that treat it specially like <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>), where it indicates repetitions
|
||
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|
of a pattern or template. See <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> and <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%29%29" class="RktStxLink" data-pltdoc="x">syntax</a></span>.</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>syntax</p></div></div><p class="RForeground"><a name="(form._((lib._racket/private/stxcase-scheme..rkt).__))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">_</a></span></span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">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> transformer binding prohibits <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> from being
|
||
|
|
used as an expression. This binding is useful only in syntax patterns,
|
||
|
|
where it indicates a pattern that matches any syntax object. See
|
||
|
|
<span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></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>syntax</p></div></div><p class="RForeground"><a name="(form._((lib._racket/private/stxcase-scheme..rkt)._~7e~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~3f%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">~?</a></span></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>syntax</p></div></div><p class="RForeground"><a name="(form._((lib._racket/private/stxcase-scheme..rkt)._~7e~40))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~40%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">~@</a></span></span></p></blockquote></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">The <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~3f%29%29" class="RktStxLink" data-pltdoc="x">~?</a></span> and <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._~7e~40%29%29" class="RktStxLink" data-pltdoc="x">~@</a></span> transformer bindings prohibit these forms from
|
||
|
|
being used as an expression. The bindings are useful only in syntax templates.
|
||
|
|
See <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%29%29" class="RktStxLink" data-pltdoc="x">syntax</a></span>.</div></p><p class="SHistory">Added in version 6.90.0.25 of package <span class="stt">base</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._((lib._racket/private/stxcase-scheme..rkt)._syntax-pattern-variable~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="stx-patterns.html#%28def._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-pattern-variable~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">syntax-pattern-variable?</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 value that, as a
|
||
|
|
transformer-binding value, makes the bound variable as pattern
|
||
|
|
variable in <span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax%29%29" class="RktStxLink" data-pltdoc="x">syntax</a></span> and other forms. To check whether an
|
||
|
|
identifier is a pattern variable, use <span class="RktSym"><a href="stxtrans.html#%28def._%28%28quote._~23~25kernel%29._syntax-local-value%29%29" class="RktValLink" data-pltdoc="x">syntax-local-value</a></span> to
|
||
|
|
get the identifier’s transformer value, and then test the value with
|
||
|
|
<span class="RktSym"><a href="stx-patterns.html#%28def._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-pattern-variable~3f%29%29" class="RktValLink" data-pltdoc="x">syntax-pattern-variable?</a></span>.</div></p><p>The <span class="RktSym"><a href="stx-patterns.html#%28def._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-pattern-variable~3f%29%29" class="RktValLink" data-pltdoc="x">syntax-pattern-variable?</a></span> procedure is provided
|
||
|
|
<span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-syntax%29%29" class="RktStxLink" data-pltdoc="x">for-syntax</a></span> by <a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/base</span></a>.</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="Macros.html" title="backward to "12 Macros"" data-pltdoc="x">← prev</a> <a href="Macros.html" title="up to "12 Macros"" data-pltdoc="x">up</a> <a href="stxops.html" title="forward to "12.2 Syntax Object Content"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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