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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>5 Structures</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="icons.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"><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="" 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 align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="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.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="" class="tocviewselflink" 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="define-struct.html" class="tocviewlink" 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><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, "6.1", "../");" 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("6.1");">top</a></span><span class="navright"> <a href="undefined.html" title="backward to "4.19 Undefined"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "The Racket Reference"" data-pltdoc="x">up</a> <a href="define-struct.html" title="forward to "5.1 Defining Structure Types: struct"" data-pltdoc="x">next →</a></span> </div><h3 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-part-tag=""structures"">5<tt> </tt><a name="(part._structures)"></a>Structures</h3><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=guide&rel=define-struct.html&version=6.1" class="Sq" data-pltdoc="x">Programmer-Defined Datatypes</a> in <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=guide&rel=index.html&version=6.1" class="Sq" data-pltdoc="x"><span style="font-weight: bold">The Racket Guide</span></a> introduces structure types via <span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span>.</p></blockquote></blockquote></blockquote><p>A <a name="(tech._structure._type)"></a><span style="font-style: italic">structure type</span> is a record datatype composing a number of
<span style="font-style: italic">fields</span>. A <a name="(tech._structure)"></a><span style="font-style: italic">structure</span>, an instance of a structure
type, is a first-class value that contains a value for each field of
the structure type. A structure instance is created with a
type-specific <a href="define-struct.html#%28tech._constructor%29" class="techoutside" data-pltdoc="x"><span class="techinside">constructor</span></a> procedure, and its field values are
accessed and changed with type-specific <a href="define-struct.html#%28tech._accessor%29" class="techoutside" data-pltdoc="x"><span class="techinside">accessor</span></a> and
<a href="define-struct.html#%28tech._mutator%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutator</span></a> procedures. In addition, each structure type has a
<a href="define-struct.html#%28tech._predicate%29" class="techoutside" data-pltdoc="x"><span class="techinside">predicate</span></a> procedure that answers <span class="RktVal">#t</span> for instances of
the structure type and <span class="RktVal">#f</span> for any other value.</p><p>A structure type’s fields are essentially unnamed, though names are
supported for error-reporting purposes. The constructor procedure
takes one value for each field of the structure type, except that some
of the fields of a structure type can be <a name="(tech._automatic._field)"></a><span style="font-style: italic">automatic fields</span>;
the <a href="#%28tech._automatic._field%29" class="techoutside" data-pltdoc="x"><span class="techinside">automatic fields</span></a> are initialized to a constant that is
associated with the structure type, and the corresponding arguments
are omitted from the constructor procedure. All automatic fields in a
structure type follow the non-automatic fields.</p><p>A structure type can be created as a <a name="(idx._(gentag._166._(lib._scribblings/reference/reference..scrbl)))"></a><span style="font-style: italic">structure subtype</span> of
an existing base structure type. An instance of a structure subtype
can always be used as an instance of the base structure type, but the
subtype gets its own predicate procedure, and it may have its own
fields in addition to the fields of the base type.</p><p>A structure subtype “inherits” the fields of its base type. If the
base type has <span style="font-style: italic">m</span> fields, and if <span style="font-style: italic">n</span> fields are specified
for the new structure subtype, then the resulting structure type has
<span style="font-style: italic">m+n</span> fields. The value for automatic fields can be different in
a subtype than in its base type.</p><p>If <span style="font-style: italic">m</span>′<span style="font-style: italic"></span> of the original <span style="font-style: italic">m</span> fields are non-automatic (where
<span style="font-style: italic">m</span>′<span style="font-style: italic"><m</span>), and <span style="font-style: italic">n</span>′<span style="font-style: italic"></span> of the new fields are non-automatic (where
<span style="font-style: italic">n</span>′<span style="font-style: italic"><n</span>), then <span style="font-style: italic">m</span>′<span style="font-style: italic">+n</span>′<span style="font-style: italic"></span> field values must be provided to the
subtype’s constructor procedure. Values for the first <span style="font-style: italic">m</span> fields
of a subtype instance are accessed with selector procedures for the
original base type (or its supertypes), and the last <span style="font-style: italic">n</span> are
accessed with subtype-specific selectors. Subtype-specific
<a href="define-struct.html#%28tech._accessor%29" class="techoutside" data-pltdoc="x"><span class="techinside">accessors</span></a> and <a href="define-struct.html#%28tech._mutator%29" class="techoutside" data-pltdoc="x"><span class="techinside">mutators</span></a> for the first <span style="font-style: italic">m</span> fields do
not exist.</p><p>The <span class="RktSym"><a href="define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span> form and <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>
procedure typically create a new structure type, but they can also
access <a name="(tech._prefab)"></a><span style="font-style: italic">prefab</span> (i.e., previously fabricated) structure types
that are globally shared, and whose instances can be parsed and
written by the default reader (see <a href="reader.html" data-pltdoc="x">The Reader</a>) and printer (see
<a href="printing.html" data-pltdoc="x">The Printer</a>). Prefab structure types can inherit only from
other prefab structure types, and they cannot have guards (see
<a href="creatingmorestructs.html" data-pltdoc="x">Creating Structure Types</a>) or properties (see
<a href="structprops.html" data-pltdoc="x">Structure Type Properties</a>). Exactly one prefab structure type exists for
each combination of name, supertype, field count, automatic field
count, automatic field value (when there is at least one automatic
field), and field mutability.</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="magnify.png" alt="+" width="24" height="24"/></span><a href="serialization.html" data-pltdoc="x">Serialization</a> also provides information on reading and writing structures.</p></blockquote></blockquote></blockquote><p><a name="(idx._(gentag._167._(lib._scribblings/reference/reference..scrbl)))"></a>Two structure values are
<span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._eqv~3f%29%29" class="RktValLink" data-pltdoc="x">eqv?</a></span> if and only if they are <span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._eq~3f%29%29" class="RktValLink" data-pltdoc="x">eq?</a></span>. Two structure
values are <span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span> if they are <span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._eq~3f%29%29" class="RktValLink" data-pltdoc="x">eq?</a></span>. By default, two
structure values are also <span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span> if they are instances of the
same structure type, no fields are opaque, and the results of applying
<span class="RktSym"><a href="structutils.html#%28def._%28%28quote._~23~25kernel%29._struct-~3evector%29%29" class="RktValLink" data-pltdoc="x">struct->vector</a></span> to the structs are
<span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span>. (Consequently, <span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span> testing for
structures may depend on the current inspector.) A structure type can
override the default <span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span> definition through the
<span class="RktSym"><a href="booleans.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._gen~3aequal%2Bhash%29%29" class="RktValLink" data-pltdoc="x">gen:equal+hash</a></span> <a href="struct-generics.html#%28tech._generic._interface%29" class="techoutside" data-pltdoc="x"><span class="techinside">generic interface</span></a>.</p><table cellspacing="0" cellpadding="0"><tr><td><p><span class="hspace"> </span><a href="define-struct.html" class="toclink" data-pltdoc="x">5.1<span class="hspace"> </span>Defining Structure Types: <span class="RktSym"><span class="RktStxLink">struct</span></span></a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="creatingmorestructs.html" class="toclink" data-pltdoc="x">5.2<span class="hspace"> </span>Creating Structure Types</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="structprops.html" class="toclink" data-pltdoc="x">5.3<span class="hspace"> </span>Structure Type Properties</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="struct-generics.html" class="toclink" data-pltdoc="x">5.4<span class="hspace"> </span>Generic Interfaces</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="struct-copy.html" class="toclink" data-pltdoc="x">5.5<span class="hspace"> </span>Copying and Updating Structures</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="structutils.html" class="toclink" data-pltdoc="x">5.6<span class="hspace"> </span>Structure Utilities</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="structinfo.html" class="toclink" data-pltdoc="x">5.7<span class="hspace"> </span>Structure Type Transformer Binding</a></p></td></tr></table><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, "6.1", "../");" 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("6.1");">top</a></span><span class="navright"> <a href="undefined.html" title="backward to "4.19 Undefined"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "The Racket Reference"" data-pltdoc="x">up</a> <a href="define-struct.html" title="forward to "5.1 Defining Structure Types: struct"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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