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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>6.8 Object Equality and Hashing</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">►</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">The Racket Reference</a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2 </td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3 </td><td><a href="syntax.html" class="tocviewlink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4 </td><td><a href="data.html" class="tocviewlink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6 </td><td><a href="mzlib_class.html" class="tocviewselflink" 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.htm
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are instances of a non-transparent class are <span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span> only if
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they are <span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._eq~3f%29%29" class="RktValLink" data-pltdoc="x">eq?</a></span>. Like transparent structures, two objects that
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are instances of the same transparent class (i.e., every superclass of
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the class has <span class="RktVal">#f</span> as its inspector) are <span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span> when
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their field values are <span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span>.</p><p>To customize the way that a class instance is compared to other
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instances by <span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span>, implement the <span class="RktSym"><a href="objectequality.html#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._equal~3c~25~3e%29%29" class="RktValLink" data-pltdoc="x">equal<%></a></span>
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interface.</p><p><div class="SIntrapara"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>interface</p></div></div><p class="RForeground"><a name="(def._((lib._racket/private/class-internal..rkt)._equal~3c~25~3e))"></a><span title="Provided from: racket/class, racket | Package: base"><span class="RktSym"><a href="objectequality.html#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._equal~3c~25~3e%29%29" class="RktValDef RktValLink" data-pltdoc="x">equal<%></a></span></span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="objectutils.html#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._interface~3f%29%29" class="RktValLink" data-pltdoc="x">interface?</a></span></p></blockquote></td></tr></table></div><div class="SIntrapara">The <span class="RktSym"><a href="objectequality.html#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._equal~3c~25~3e%29%29" class="RktValLink" data-pltdoc="x">equal<%></a></span> interface includes three methods, which are
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analogous to the functions provided for a structure type with
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<span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._prop~3aequal%2Bhash%29%29" class="RktValLink" data-pltdoc="x">prop:equal+hash</a></span>:</div><div class="SIntrapara"><ul><li><p><span class="RktSym">equal-to?</span> —<wbr></wbr> Takes two arguments. The first argument
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is an object that is an instance of the same class (or a subclass
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that does not re-declare its implementation of <span class="RktSym"><a href="objectequality.html#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._equal~3c~25~3e%29%29" class="RktValLink" data-pltdoc="x">equal<%></a></span>)
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and that is being compared to the target object. The second argument
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is an <span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span>-like procedure of two arguments that should be
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used for recursive equality testing. The result should be a true
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value if the object and the first argument of the method are equal,
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<span class="RktVal">#f</span> otherwise.</p></li><li><p><span class="RktSym">equal-hash-code-of</span> —<wbr></wbr> Takes one argument, which is a
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procedure of one argument that should be used for recursive hash-code
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computation. The result should be an exact integer representing the
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target object’s hash code.</p></li><li><p><span class="RktSym">equal-secondary-hash-code-of</span> —<wbr></wbr> Takes one argument,
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which is a procedure of one argument that should be used for
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recursive hash-code computation. The result should be an exact
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integer representing the target object’s secondary hash code.</p></li></ul></div><div class="SIntrapara">The <span class="RktSym"><a href="objectequality.html#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._equal~3c~25~3e%29%29" class="RktValLink" data-pltdoc="x">equal<%></a></span> interface is unusual in that declaring the
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implementation of the interface is different from inheriting the
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interface. Two objects can be equal only if they are instances of
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classes whose most specific ancestor to explicitly implement
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<span class="RktSym"><a href="objectequality.html#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._equal~3c~25~3e%29%29" class="RktValLink" data-pltdoc="x">equal<%></a></span> is the same ancestor.</div><div class="SIntrapara">See <span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._prop~3aequal%2Bhash%29%29" class="RktValLink" data-pltdoc="x">prop:equal+hash</a></span> for more information on equality
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comparisons and hash codes. The <span class="RktSym"><a href="objectequality.html#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._equal~3c~25~3e%29%29" class="RktValLink" data-pltdoc="x">equal<%></a></span> interface is
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implemented with <span class="RktSym"><a href="createinterface.html#%28form._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._interface%2A%29%29" class="RktStxLink" data-pltdoc="x">interface*</a></span> and <span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._prop~3aequal%2Bhash%29%29" class="RktValLink" data-pltdoc="x">prop:equal+hash</a></span>.</div><div class="SIntrapara"><blockquote class="leftindent"></blockquote></div></p><p><div class="SIntrapara">Example:
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</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=guide&rel=Module_Syntax.html%23%2528part._hash-lang%2529&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="RktMod">#lang</span></a><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket</span></a><span class="RktMeta"></span></td></tr><tr><td><span class="RktMeta"> </span></td></tr><tr><td><span class="RktMeta"></span><span class="RktCmt">;;</span><span class="hspace"> </span><span class="RktCmt">Case</span><span class="hspace"> </span><span class="RktCmt">insensitive</span><span class="hspace"> </span><span class="RktCmt">words:</span><span class="RktMeta"></span></td></tr><tr><td><span class="RktMeta"></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="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktSym">ci-word%</span><span class="RktMeta"></span></td></tr><tr><td><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktPn">(</span><span class="RktSym"><a href="createclass.html#%28form._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._class%2A%29%29" class="RktStxLink" data-pltdoc="x">class*</a></span><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktSym"><a href="createclass.html#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._object~25%29%29" class="RktValLink" data-pltdoc="x">object%</a></span><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktPn">(</span><span class="RktSym"><a href="objectequality.html#%28def._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._equal~3c~25~3e%29%29" class="RktValLink" data-pltdoc="x">equal<%></a></span><span class="RktPn">)</span><span class="RktMeta"></span></td></tr><tr><td><span class="RktMeta"> </span></td></tr><tr><td><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktCmt">;;</span><span class="hspace"> </span><span class="RktCmt">Initialization</span><span class="RktMeta"></span></td></tr><tr><td><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktPn">(</span><span class="RktSym"><a href="createclass.html#%28form._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._init-field%29%29" class="RktStxLink" data-pltdoc="x">init-field</a></span><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktSym">word</span><span class="RktPn">)</span><span class="RktMeta"></span></td></tr><tr><td><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktPn">(</span><span class="RktSym"><a href="objcreation.html#%28form._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._super-new%29%29" class="RktStxLink" data-pltdoc="x">super-new</a></span><span class="RktPn">)</span><span class="RktMeta"></span></td></tr><tr><td><span class="RktMeta"> </span></td></tr><tr><td><span class="RktMeta"></span><span class="hspace"> </span><span class="RktMeta"></span><span class="RktCmt">;;</span><span class="hspace"> </span><span class="RktCmt">We</span><span class="hspace"> </span><span class="RktCmt">define</span><span class="hspace"> </span><span class="RktCmt">equality</span><span class="hspace"> </span><span class="RktCmt">to</span><span class="hspace"> </span><span class="RktCmt">ignore</span><span class="hsp
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