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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>5.1 Defining Structure Types: struct</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="icons.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" 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"><span style="font-weight: bold">The Racket Reference</span></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="tocviewselflink" 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 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align="right">18 </td><td><a href="running.html" class="tocviewlink" data-pltdoc="x">Running Racket</a></td></tr><tr><td align="right"></td><td><a href="doc-bibliography.html" class="tocviewlink" data-pltdoc="x">Bibliography</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_1");">▼</a></td><td>5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">5.1 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Defining Structure Types:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">struct</span></span></a></td></tr><tr><td align="right">5.2 </td><td><a href="creatingmorestructs.html" class="tocviewlink" data-pltdoc="x">Creating Structure Types</a></td></tr><tr><td align="right">5.3 </td><td><a href="structprops.html" class="tocviewlink" data-pltdoc="x">Structure Type Properties</a></td></tr><tr><td align="right">5.4 </td><td><a href="struct-generics.html" class="tocviewlink" data-pltdoc="x">Generic Interfaces</a></td></tr><tr><td align="right">5.5 </td><td><a href="struct-copy.html" class="tocviewlink" data-pltdoc="x">Copying and Updating Structures</a></td></tr><tr><td align="right">5.6 </td><td><a href="structutils.html" class="tocviewlink" data-pltdoc="x">Structure Utilities</a></td></tr><tr><td align="right">5.7 </td><td><a href="structinfo.html" class="tocviewlink" data-pltdoc="x">Structure Type Transformer Binding</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">struct</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct-field-index%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">struct-<wbr></wbr>field-<wbr></wbr>index</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">define-<wbr></wbr>struct</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%2Fderived%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">define-<wbr></wbr>struct/<span class="mywbr"> </span>derived</span></span></a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="structures.html" title="backward to "5 Structures"" data-pltdoc="x">← prev</a> <a href="structures.html" title="up to "5 Structures"" data-pltdoc="x">up</a> <a href="creatingmorestructs.html" title="forward to "5.2 Creating Structure Types"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""define-struct"">5.1<tt> </tt><a name="(part._define-struct)"></a>Defining Structure Types: <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span></h4><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="finger.png" alt="+" width="24" height="24"/></span><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=guide&rel=define-struct.html&version=6.7" class="Sq" data-pltdoc="x">Programmer-Defined Datatypes</a> in <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=guide&rel=index.html&version=6.7" class="Sq" data-pltdoc="x"><span style="font-weight: bold">The Racket Guide</span></a> introduces <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span>.</p></blockquote></blockquote></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><table cellspacing="0" cellpadding="0" class="RktBlk RForeground"><tr><td><span class="RktPn">(</span><a name="(form._((lib._racket/private/base..rkt)._struct))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">struct</a></span></span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">maybe-super</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">field</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">struct-option</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote></td></tr><tr><td><span class="stt"> </span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="specgrammar"><tr><td align="right" valign="baseline"><span class="RktVar">maybe-super</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"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td></td></tr></table></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktVar">super-id</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline"><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">field</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktVar">field-id</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">[</span><span class="RktVar">field-id</span><span class="hspace"> </span><span class="RktVar">field-option</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">]</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">struct-option</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="RktPn">#:mutable</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:super</span><span class="hspace"> </span><span class="RktVar">super-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:inspector</span><span class="hspace"> </span><span class="RktVar">inspector-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:auto-value</span><span class="hspace"> </span><span class="RktVar">auto-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:guard</span><span class="hspace"> </span><span class="RktVar">guard-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:property</span><span class="hspace"> </span><span class="RktVar">prop-expr</span><span class="hspace"> </span><span class="RktVar">val-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:transparent</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:prefab</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:name</span><span class="hspace"> </span><span class="RktVar">name-id</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:extra-name</span><span class="hspace"> </span><span class="RktVar">name-id</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:constructor-name</span><span class="hspace"> </span><span class="RktVar">constructor-id</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:extra-constructor-name</span><span class="hspace"> </span><span class="RktVar">constructor-id</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:reflection-name</span><span class="hspace"> </span><span class="RktVar">symbol-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:methods</span><span class="hspace"> </span><span class="RktVar">gen:name</span><span class="hspace"> </span><span class="RktVar">method-defs</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:omit-define-syntaxes</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:omit-define-values</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">field-option</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="RktPn">#:mutable</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:auto</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">method-defs</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="RktPn">(</span><span class="RktVar">definition</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></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">gen:name</span></td><td><span class="hspace"> </span></td><td>:</td><td><span class="hspace"> </span></td><td><span class="RktSym"><a href="stxops.html#%28def._%28%28lib._racket%2Fprivate%2Fstx..rkt%29._identifier~3f%29%29" class="RktValLink" data-pltdoc="x">identifier?</a></span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Creates a new <a href="structures.html#%28tech._structure._type%29" data-pltdoc="x">structure type</a> (or uses a pre-existing
structure type if <span class="RktPn">#:prefab</span> is specified), and binds
transformers and variables related to the <a href="structures.html#%28tech._structure._type%29" class="techoutside" data-pltdoc="x"><span class="techinside">structure type</span></a>.</div></p><p>A <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span> form with <span style="font-style: italic">n</span> <span class="RktVar">field</span>s defines up
to <span style="font-style: italic"></span>4<span style="font-style: italic">+</span>2<span style="font-style: italic">n</span> names:</p><ul><li><p><span class="RktSym">struct:</span><span class="RktVar">id</span>, a <a name="(tech._structure._type._descriptor)"></a><span style="font-style: italic">structure type
descriptor</span> value that represents the <a href="structures.html#%28tech._structure._type%29" class="techoutside" data-pltdoc="x"><span class="techinside">structure type</span></a>.</p></li><li><p><span class="RktVar">constructor-id</span> (which defaults to <span class="RktVar">id</span>), a
<a name="(tech._constructor)"></a><span style="font-style: italic">constructor</span> procedure that takes <span style="font-style: italic">m</span> arguments
and returns a new instance of the <a href="structures.html#%28tech._structure._type%29" class="techoutside" data-pltdoc="x"><span class="techinside">structure type</span></a>, where
<span style="font-style: italic">m</span> is the number of <span class="RktVar">field</span>s that do not include
an <span class="RktPn">#:auto</span> option.</p></li><li><p><span class="RktVar">name-id</span> (which defaults to <span class="RktVar">id</span>),
a <a href="syntax-model.html#%28tech._transformer%29" class="techoutside" data-pltdoc="x"><span class="techinside">transformer</span></a> binding that encapsulates
information about the structure type declaration. This binding
is used to define subtypes, and it also works with the
<span class="RktSym"><a href="shared.html#%28form._%28%28lib._racket%2Fshared..rkt%29._shared%29%29" class="RktStxLink" data-pltdoc="x">shared</a></span> and <span class="RktSym"><a href="match.html#%28form._%28%28lib._racket%2Fmatch..rkt%29._match%29%29" class="RktStxLink" data-pltdoc="x">match</a></span> forms. For detailed
information about the binding of <span class="RktVar">name-id</span>, see
<a href="structinfo.html" data-pltdoc="x">Structure Type Transformer Binding</a>.</p><p>The <span class="RktVar">constructor-id</span> and <span class="RktVar">name-id</span> can be the same, in
which case <span class="RktVar">name-id</span> performs both roles. In that case, the
expansion of <span class="RktVar">name-id</span> as an expression produces an otherwise
inaccessible identifier that is bound to the constructor
procedure; the expanded identifier has a
<span class="RktVal">'</span><span class="RktVal">constructor-for</span> property whose value is an identifier
that is <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> to <span class="RktVar">name-id</span> as well as
a syntax property accessible via
<span class="RktSym">syntax-procedure-alias-property</span> with an identifier
that is <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> to <span class="RktVar">name-id</span>.</p></li><li><p><span class="RktVar">id</span><span class="RktSym">?</span>, a <a name="(tech._predicate)"></a><span style="font-style: italic">predicate</span> procedure
that returns <span class="RktVal">#t</span> for instances of the <a href="structures.html#%28tech._structure._type%29" class="techoutside" data-pltdoc="x"><span class="techinside">structure
type</span></a> (constructed by <span class="RktVar">constructor-id</span> or the
<a href="#%28tech._constructor%29" class="techoutside" data-pltdoc="x"><span class="techinside">constructor</span></a> for a subtype) and <span class="RktVal">#f</span> for any other
value.</p></li><li><p><span class="RktVar">id</span><span class="RktSym">-</span><span class="RktVar">field-id</span>, for each
<span class="RktVar">field</span>; an <a name="(tech._accessor)"></a><span style="font-style: italic">accessor</span> procedure that takes an
instance of the <a href="structures.html#%28tech._structure._type%29" class="techoutside" data-pltdoc="x"><span class="techinside">structure type</span></a> and extracts the value
for the corresponding field.</p></li><li><p><span class="RktSym">set-</span><span class="RktVar">id</span><span class="RktSym">-</span><span class="RktVar">field-id</span><span class="RktSym">!</span>,
for each <span class="RktVar">field</span> that includes a
<span class="RktPn">#:mutable</span> option, or when the
<span class="RktPn">#:mutable</span> option is specified as a
<span class="RktVar">struct-option</span>; a <a name="(tech._mutator)"></a><span style="font-style: italic">mutator</span> procedure that
takes an instance of the <a href="structures.html#%28tech._structure._type%29" class="techoutside" data-pltdoc="x"><span class="techinside">structure type</span></a> and a new field
value. The structure is destructively updated with the new
value, and <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=guide&rel=void_undefined.html&version=6.7" class="RktModLink Sq" data-pltdoc="x"><span class="nobreak"><span class="RktRes">#<void></span></span></a> is returned.</p></li></ul><p>If <span class="RktVar">super-id</span> is provided, it must have a transformer binding
of the same sort bound to <span class="RktVar">name-id</span> (see <a href="structinfo.html" data-pltdoc="x">Structure Type Transformer Binding</a>),
and it specifies a supertype for the structure type. Alternately,
the <span class="RktPn">#:super</span> option can be used to specify an expression that
must produce a <a href="#%28tech._structure._type._descriptor%29" class="techoutside" data-pltdoc="x"><span class="techinside">structure type descriptor</span></a>. See
<a href="structures.html" data-pltdoc="x">Structures</a> for more information on structure subtypes
and supertypes. If both <span class="RktVar">super-id</span> and <span class="RktPn">#:super</span> are
provided, a syntax error is reported.</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">document</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">author</span><span class="hspace"> </span><span class="RktSym">title</span><span class="hspace"> </span><span class="RktSym">content</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">book</span><span class="hspace"> </span><span class="RktSym">document</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">publisher</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">paper</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">journal</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:super</span><span class="hspace"> </span><span class="RktSym">struct:document</span><span class="RktPn">)</span></td></tr></table></blockquote></div></p><p>If the <span class="RktPn">#:mutable</span> option is specified for an individual
field, then the field can be mutated in instances of the structure
type, and a <a href="#%28tech._mutator%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutator</span></a> procedure is bound. Supplying
<span class="RktPn">#:mutable</span> as a <span class="RktVar">struct-option</span> is the same as
supplying it for all <span class="RktVar">field</span>s. If <span class="RktPn">#:mutable</span> is
specified as both a <span class="RktVar">field-option</span> and <span class="RktVar">struct-option</span>,
a syntax error is reported.</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">cell</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">content</span><span class="hspace"> </span><span class="RktPn">#:mutable</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:transparent</span><span class="RktPn">)</span></td></tr><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%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">a-cell</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">cell</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">set-cell-content!</span><span class="hspace"> </span><span class="RktSym">a-cell</span><span class="hspace"> </span><span class="RktVal">1</span><span class="RktPn">)</span></td></tr></table></blockquote></div></p><p>The <span class="RktPn">#:inspector</span>, <span class="RktPn">#:auto-value</span>, and <span class="RktPn">#:guard</span>
options specify an inspector, value for automatic fields, and guard
procedure, respectively. See <span class="RktSym"><a href="creatingmorestructs.html#%28def._%28%28quote._~23~25kernel%29._make-struct-type%29%29" class="RktValLink" data-pltdoc="x">make-struct-type</a></span> for more
information on these attributes of a structure type. The
<span class="RktPn">#:property</span> option, which is the only one that can be supplied
multiple times, attaches a property value to the structure type; see
<a href="structprops.html" data-pltdoc="x">Structure Type Properties</a> for more information on properties. The
<span class="RktPn">#:transparent</span> option is a shorthand for <span class="RktPn">#:inspector</span><span class="stt"> </span><span class="RktVal">#f</span>.</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">point</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:inspector</span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">point</span><span class="hspace"> </span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktVal">5</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">(point 3 5)</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">celsius</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">temp</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:guard</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._~ce~bb%29%29" class="RktStxLink" data-pltdoc="x">λ</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">temp</span><span class="hspace"> </span><span class="RktSym">name</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="when_unless.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._unless%29%29" class="RktStxLink" data-pltdoc="x">unless</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._and%29%29" class="RktStxLink" data-pltdoc="x">and</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="number-types.html#%28def._%28%28quote._~23~25kernel%29._real~3f%29%29" class="RktValLink" data-pltdoc="x">real?</a></span><span class="hspace"> </span><span class="RktSym">temp</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._~3e~3d%29%29" class="RktValLink" data-pltdoc="x">>=</a></span><span class="hspace"> </span><span class="RktSym">temp</span><span class="hspace"> </span><span class="RktVal"><span class="nobreak">-2</span>73.15</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="exns.html#%28def._%28%28quote._~23~25kernel%29._error%29%29" class="RktValLink" data-pltdoc="x">error</a></span><span class="hspace"> </span><span class="RktVal">"not a valid temperature"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym">temp</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">celsius</span><span class="hspace"> </span><span class="RktVal"><span class="nobreak">-2</span>75</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">not a valid temperature</span></p></td></tr></table></blockquote></div></p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>Use the <span class="RktSym"><a href="procedures.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._prop~3aprocedure%29%29" class="RktValLink" data-pltdoc="x">prop:procedure</a></span> property to implement an
<a name="(idx._(gentag._169._(lib._scribblings/reference/reference..scrbl)))"></a>applicable structure, use <span class="RktSym"><a href="sync.html#%28def._%28%28quote._~23~25kernel%29._prop~3aevt%29%29" class="RktValLink" data-pltdoc="x">prop:evt</a></span> to create a
structure type whose instances are <a href="sync.html#%28tech._synchronizable._event%29" class="techoutside" data-pltdoc="x"><span class="techinside">synchronizable events</span></a>, and
so on. By convention, property names start with <span class="RktSym">prop:</span>.</p></blockquote></blockquote></blockquote><p>The <span class="RktPn">#:prefab</span> option obtains a <a href="structures.html#%28tech._prefab%29" data-pltdoc="x">prefab</a> (pre-defined,
globally shared) structure type, as opposed to creating a new
structure type. Such a structure type is inherently transparent and
cannot have a guard or properties, so using <span class="RktPn">#:prefab</span> with
<span class="RktPn">#:transparent</span>, <span class="RktPn">#:inspector</span>, <span class="RktPn">#:guard</span>, or
<span class="RktPn">#:property</span> is a syntax error. If a supertype is specified, it
must also be a <a href="structures.html#%28tech._prefab%29" class="techoutside" data-pltdoc="x"><span class="techinside">prefab</span></a> structure type.</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">prefab-point</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:prefab</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">prefab-point</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'#s(prefab-point 1 2)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">prefab-point?</span><span class="hspace"> </span><span class="RktVal">#s</span><span class="RktVal">(</span><span class="RktVal">prefab-point</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#t</span></p></td></tr></table></blockquote></div></p><p>If <span class="RktVar">name-id</span> is supplied via <span class="RktPn">#:extra-name</span> and it is
not <span class="RktVar">id</span>, then both <span class="RktVar">name-id</span> and <span class="RktVar">id</span> are bound
to information about the structure type. Only one of
<span class="RktPn">#:extra-name</span> and <span class="RktPn">#:name</span> can be provided within a
<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span> form.</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">ghost</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">color</span><span class="hspace"> </span><span class="RktSym">name</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:prefab</span><span class="hspace"> </span><span class="RktPn">#:extra-name</span><span class="hspace"> </span><span class="RktSym">GHOST</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="match.html#%28form._%28%28lib._racket%2Fmatch..rkt%29._match%29%29" class="RktStxLink" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">ghost</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">red</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">blinky</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">GHOST</span><span class="hspace"> </span><span class="RktSym">c</span><span class="hspace"> </span><span class="RktSym">n</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">c</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'red</span></p></td></tr></table></blockquote></div></p><p>If <span class="RktVar">constructor-id</span> is supplied, then the <a href="syntax-model.html#%28tech._transformer%29" class="techoutside" data-pltdoc="x"><span class="techinside">transformer</span></a>
binding of <span class="RktVar">name-id</span> records <span class="RktVar">constructor-id</span> as the
constructor binding; as a result, for example, <span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct-out%29%29" class="RktStxLink" data-pltdoc="x">struct-out</a></span>
includes <span class="RktVar">constructor-id</span> as an export. If
<span class="RktVar">constructor-id</span> is supplied via
<span class="RktPn">#:extra-constructor-name</span> and it is not <span class="RktVar">id</span>, applying
<span class="RktSym"><a href="inspectors.html#%28def._%28%28quote._~23~25kernel%29._object-name%29%29" class="RktValLink" data-pltdoc="x">object-name</a></span> on the constructor produces the symbolic form of
<span class="RktVar">id</span> rather than <span class="RktVar">constructor-id</span>. If
<span class="RktVar">constructor-id</span> is supplied via <span class="RktPn">#:constructor-name</span>
and it is not the same as <span class="RktVar">name-id</span>, then <span class="RktVar">name-id</span> does not serve
as a constructor, and <span class="RktSym"><a href="inspectors.html#%28def._%28%28quote._~23~25kernel%29._object-name%29%29" class="RktValLink" data-pltdoc="x">object-name</a></span> on the constructor produces
the symbolic form of <span class="RktVar">constructor-id</span>. Only one of
<span class="RktPn">#:extra-constructor-name</span> and <span class="RktPn">#:constructor-name</span>
can be provided within a <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span> form.</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">color</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">r</span><span class="hspace"> </span><span class="RktSym">g</span><span class="hspace"> </span><span class="RktSym">b</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:constructor-name</span><span class="hspace"> </span><span class="RktSym"><span class="nobreak">-c</span>olor</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">rectangle</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">w</span><span class="hspace"> </span><span class="RktSym">h</span><span class="hspace"> </span><span class="RktSym">color</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:extra-constructor-name</span><span class="hspace"> </span><span class="RktSym">rect</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">rectangle</span><span class="hspace"> </span><span class="RktVal">13</span><span class="hspace"> </span><span class="RktVal">50</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><span class="nobreak">-c</span>olor</span><span class="hspace"> </span><span class="RktVal">192</span><span class="hspace"> </span><span class="RktVal">157</span><span class="hspace"> </span><span class="RktVal">235</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#<rectangle></span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">rect</span><span class="hspace"> </span><span class="RktVal">50</span><span class="hspace"> </span><span class="RktVal">37</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><span class="nobreak">-c</span>olor</span><span class="hspace"> </span><span class="RktVal">35</span><span class="hspace"> </span><span class="RktVal">183</span><span class="hspace"> </span><span class="RktVal">252</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#<rectangle></span></p></td></tr></table></blockquote></div></p><p>If <span class="RktPn">#:reflection-name</span><span class="stt"> </span><span class="RktVar">symbol-expr</span> is provided, then
<span class="RktVar">symbol-expr</span> must produce a symbol that is used to identify
the structure type in reflective operations such as
<span class="RktSym"><a href="inspectors.html#%28def._%28%28quote._~23~25kernel%29._struct-type-info%29%29" class="RktValLink" data-pltdoc="x">struct-type-info</a></span>. It corresponds to the first argument of
<span class="RktSym"><a href="creatingmorestructs.html#%28def._%28%28quote._~23~25kernel%29._make-struct-type%29%29" class="RktValLink" data-pltdoc="x">make-struct-type</a></span>. Structure printing uses the reflective
name, as do the various procedures that are bound by <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span>.</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">circle</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">radius</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:reflection-name</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal"><circle></span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">circle</span><span class="hspace"> </span><span class="RktVal">15</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#<|<circle>|></span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">circle-radius</span><span class="hspace"> </span><span class="RktVal">"bad"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr"><circle>-radius: contract violation</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">expected: <circle>?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">given: "bad"</span></p></td></tr></table></blockquote></div></p><p>If <span class="RktPn">#:methods</span><span class="stt"> </span><span class="RktVar">gen:name</span><span class="stt"> </span><span class="RktVar">method-defs</span> is provided, then
<span class="RktVar">gen:name</span> must be a transformer binding for the static
information about a generic interface produced by <span class="RktSym"><a href="struct-generics.html#%28form._%28%28lib._racket%2Fgeneric..rkt%29._define-generics%29%29" class="RktStxLink" data-pltdoc="x">define-generics</a></span>.
The <span class="RktVar">method-defs</span> define the methods of the <span class="RktVar">gen:name</span>
interface. A <span class="RktSym"><a href="struct-generics.html#%28form._%28%28lib._racket%2Fgeneric..rkt%29._define%2Fgeneric%29%29" class="RktStxLink" data-pltdoc="x">define/generic</a></span> form or auxiliary definitions
and expressions may also appear in <span class="RktVar">method-defs</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">constant-stream</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">val</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">#:methods</span><span class="hspace"> </span><span class="RktSym"><a href="streams.html#%28def._%28%28lib._racket%2Fstream..rkt%29._gen~3astream%29%29" class="RktValLink" data-pltdoc="x">gen:stream</a></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="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="streams.html#%28def._%28%28lib._racket%2Fstream..rkt%29._stream-empty~3f%29%29" class="RktValLink" data-pltdoc="x">stream-empty?</a></span><span class="hspace"> </span><span class="RktSym"><a href="streams.html#%28form._%28%28lib._racket%2Fstream..rkt%29._stream%29%29" class="RktStxLink" data-pltdoc="x">stream</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">#f</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="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="streams.html#%28def._%28%28lib._racket%2Fstream..rkt%29._stream-first%29%29" class="RktValLink" data-pltdoc="x">stream-first</a></span><span class="hspace"> </span><span class="RktSym"><a href="streams.html#%28form._%28%28lib._racket%2Fstream..rkt%29._stream%29%29" class="RktStxLink" data-pltdoc="x">stream</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">constant-stream-val</span><span class="hspace"> </span><span class="RktSym"><a href="streams.html#%28form._%28%28lib._racket%2Fstream..rkt%29._stream%29%29" class="RktStxLink" data-pltdoc="x">stream</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="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="streams.html#%28def._%28%28lib._racket%2Fstream..rkt%29._stream-rest%29%29" class="RktValLink" data-pltdoc="x">stream-rest</a></span><span class="hspace"> </span><span class="RktSym"><a href="streams.html#%28form._%28%28lib._racket%2Fstream..rkt%29._stream%29%29" class="RktStxLink" data-pltdoc="x">stream</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="streams.html#%28form._%28%28lib._racket%2Fstream..rkt%29._stream%29%29" class="RktStxLink" data-pltdoc="x">stream</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="streams.html#%28def._%28%28lib._racket%2Fstream..rkt%29._stream-ref%29%29" class="RktValLink" data-pltdoc="x">stream-ref</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">constant-stream</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">forever</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'forever</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="streams.html#%28def._%28%28lib._racket%2Fstream..rkt%29._stream-ref%29%29" class="RktValLink" data-pltdoc="x">stream-ref</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">constant-stream</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">forever</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">50</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'forever</span></p></td></tr></table></blockquote></div></p><p>If the <span class="RktPn">#:omit-define-syntaxes</span> option is supplied, then
<span class="RktVar">name-id</span> (and <span class="RktVar">id</span>, if <span class="RktPn">#:extra-name</span> is specified])
is not bound as a transformer. If the
<span class="RktPn">#:omit-define-values</span> option is supplied, then none of the
usual variables are bound, but <span class="RktVar">id</span> is bound. If both are
supplied, then the <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span> form is equivalent to
<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><span class="RktPn">)</span>.</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">square</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">side</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:omit-define-syntaxes</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="match.html#%28form._%28%28lib._racket%2Fmatch..rkt%29._match%29%29" class="RktStxLink" data-pltdoc="x">match</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">square</span><span class="hspace"> </span><span class="RktVal">5</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">fails to match because syntax is omitted</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="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">square</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">x</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktErr">eval:28:0: match: square does not refer to a structure</span></p></td></tr><tr><td><p><span class="RktErr">definition</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: square</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: (struct square x)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">ellipse</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">width</span><span class="hspace"> </span><span class="RktSym">height</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:omit-define-values</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktSym">ellipse-width</span></td></tr><tr><td><p><span class="RktErr">ellipse-width: undefined;</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">cannot reference undefined identifier</span></p></td></tr></table></blockquote></div></p><p><div class="SIntrapara"><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>Expressions supplied to <span class="RktPn">#:auto-value</span> are evaluated once and shared
between every instance of the structure type. In particular, updates to
a mutable <span class="RktPn">#:auto-value</span> affect all current and future instances.</p></blockquote></blockquote></blockquote></div><div class="SIntrapara">If <span class="RktPn">#:auto</span> is supplied as a <span class="RktVar">field-option</span>, then the
<a href="#%28tech._constructor%29" class="techoutside" data-pltdoc="x"><span class="techinside">constructor</span></a> procedure for the structure type does not accept an
argument corresponding to the field. Instead, the structure type’s
automatic value is used for the field, as specified by the
<span class="RktPn">#:auto-value</span> option, or as defaults to <span class="RktVal">#f</span> when
<span class="RktPn">#:auto-value</span> is not supplied. The field is mutable (e.g.,
through reflective operations), but a mutator procedure is bound only
if <span class="RktPn">#:mutable</span> is specified.</div></p><p>If a <span class="RktVar">field</span> includes the <span class="RktPn">#:auto</span> option, then all
fields after it must also include <span class="RktPn">#:auto</span>, otherwise a syntax
error is reported. If any <span class="RktVar">field-option</span> or
<span class="RktVar">struct-option</span> keyword is repeated, other than
<span class="RktPn">#:property</span>, a syntax error is reported.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">posn</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">z</span><span class="hspace"> </span><span class="RktPn">#:auto</span><span class="hspace"> </span><span class="RktPn">#:mutable</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">#:auto-value</span><span class="hspace"> </span><span class="RktVal">0</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">#:transparent</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p> </p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">posn</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">(posn 1 2 0)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">posn?</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">posn</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#t</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">posn-y</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">posn</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">2</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">posn-z</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">posn</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">0</span></p></td></tr><tr><td><p> </p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">color-posn</span><span class="hspace"> </span><span class="RktSym">posn</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">hue</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:mutable</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">cp</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">color-posn</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">"blue"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p> </p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">color-posn-hue</span><span class="hspace"> </span><span class="RktSym">cp</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"blue"</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktSym">cp</span></td></tr><tr><td><p><span class="RktRes">(color-posn 1 2 0 ...)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">set-posn-z!</span><span class="hspace"> </span><span class="RktSym">cp</span><span class="hspace"> </span><span class="RktVal">3</span><span class="RktPn">)</span></td></tr></table></blockquote></div></p><p>For serialization, see <span class="RktSym"><a href="serialization.html#%28form._%28%28lib._racket%2Fserialize..rkt%29._define-serializable-struct%29%29" class="RktStxLink" data-pltdoc="x">define-serializable-struct</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/base..rkt)._struct-field-index))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct-field-index%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">struct-field-index</a></span></span><span class="hspace"> </span><span class="RktVar">field-id</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">This form can only appear as an expression within a
<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span> form; normally, it is used with
<span class="RktPn">#:property</span>, especially for a property like
<span class="RktSym"><a href="procedures.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._prop~3aprocedure%29%29" class="RktValLink" data-pltdoc="x">prop:procedure</a></span>. The result of a <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct-field-index%29%29" class="RktStxLink" data-pltdoc="x">struct-field-index</a></span>
expression is an exact, non-negative integer that corresponds to the
position within the structure declaration of the field named by
<span class="RktVar">field-id</span>.</div></p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">mood-procedure</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">base</span><span class="hspace"> </span><span class="RktSym">rating</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">#:property</span><span class="hspace"> </span><span class="RktSym"><a href="procedures.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._prop~3aprocedure%29%29" class="RktValLink" data-pltdoc="x">prop:procedure</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct-field-index%29%29" class="RktStxLink" data-pltdoc="x">struct-field-index</a></span><span class="hspace"> </span><span class="RktSym">base</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29" class="RktStxLink" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">happy+</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">mood-procedure</span><span class="hspace"> </span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._add1%29%29" class="RktValLink" data-pltdoc="x">add1</a></span><span class="hspace"> </span><span class="RktVal">10</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p> </p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">happy+</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">3</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">mood-procedure-rating</span><span class="hspace"> </span><span class="RktSym">happy+</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">10</span></p></td></tr></table></blockquote></div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><table cellspacing="0" cellpadding="0" class="RktBlk RForeground"><tr><td><span class="RktPn">(</span><a name="(form._((lib._racket/private/base..rkt)._define-struct))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">define-struct</a></span></span><span class="hspace"> </span><span class="RktVar">id-maybe-super</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">field</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">struct-option</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr></table></blockquote></td></tr><tr><td><span class="stt"> </span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="specgrammar"><tr><td align="right" valign="baseline"><span class="RktVar">id-maybe-super</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktVar">id</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">(</span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">super-id</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span>, except that the syntax for supplying a
<span class="RktVar">super-id</span> is different, and a <span class="RktVar">constructor-id</span> that
has a <span class="RktSym">make-</span> prefix on <span class="RktVar">id</span> is implicitly
supplied via <span class="RktPn">#:extra-constructor-name</span> if neither
<span class="RktPn">#:extra-constructor-name</span> nor <span class="RktPn">#:constructor-name</span> is
provided.</div></p><p>This form is provided for backwards compatibility; <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span> is
preferred.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%29%29" class="RktStxLink" data-pltdoc="x">define-struct</a></span><span class="hspace"> </span><span class="RktSym">posn</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">z</span><span class="hspace"> </span><span class="RktPn">#:auto</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">#:auto-value</span><span class="hspace"> </span><span class="RktVal">0</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">#:transparent</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p> </p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">make-posn</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">(posn 1 2 0)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">posn?</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-posn</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#t</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">posn-y</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-posn</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">2</span></p></td></tr></table></blockquote></div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><table cellspacing="0" cellpadding="0" class="RktBlk RForeground"><tr><td><span class="RktPn">(</span><a name="(form._((lib._racket/private/base..rkt)._define-struct/derived))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%2Fderived%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">define-struct/derived</a></span></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktVar">rest-form</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">id-maybe-super</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">field</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">struct-option</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">The same as <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%29%29" class="RktStxLink" data-pltdoc="x">define-struct</a></span>, but with an extra <span class="RktPn">(</span><span class="RktVar">id</span><span class="stt"> </span><span class="RktPn">. </span><span class="RktVar">rest-form</span><span class="RktPn">)</span> sub-form that is treated as the overall form for
syntax-error reporting and otherwise ignored. The only constraint on
the sub-form for error reporting is that it starts with <span class="RktVar">id</span>.
The <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%2Fderived%29%29" class="RktStxLink" data-pltdoc="x">define-struct/derived</a></span> form is intended for use by macros
that expand to <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%29%29" class="RktStxLink" data-pltdoc="x">define-struct</a></span>.</div></p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-syntax%29%29" class="RktStxLink" data-pltdoc="x">define-syntax</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">define-xy-struct</span><span class="hspace"> </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="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym">ds</span><span class="hspace"> </span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="pairs.html#%28def._%28%28lib._racket%2Flist..rkt%29._rest%29%29" class="RktValLink" data-pltdoc="x">rest</a></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._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">orig</span><span class="hspace"> </span><span class="RktSym">stx</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktRdr">#'</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define-struct%2Fderived%29%29" class="RktStxLink" data-pltdoc="x">define-struct/derived</a></span><span class="hspace"> </span><span class="RktSym">orig</span><span class="hspace"> </span><span class="RktSym">name</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktSym">y</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="pairs.html#%28def._%28%28lib._racket%2Flist..rkt%29._rest%29%29" class="RktValLink" data-pltdoc="x">rest</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p> </p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">define-xy-struct</span><span class="hspace"> </span><span class="RktSym">posn</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">posn-x</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-posn</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">1</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">define-xy-struct</span><span class="hspace"> </span><span class="RktSym">posn</span><span class="hspace"> </span><span class="RktPn">#:mutable</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">set-posn-x!</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">make-posn</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">this next line will cause an error due to a bad keyword</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">define-xy-struct</span><span class="hspace"> </span><span class="RktSym">posn</span><span class="hspace"> </span><span class="RktPn">#:bad-option</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">eval:54:0: define-xy-struct: unrecognized</span></p></td></tr><tr><td><p><span class="RktErr">struct-specification keyword</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">at: #:bad-option</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">in: (define-xy-struct posn #:bad-option)</span></p></td></tr></table></blockquote></div></p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; 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