223 lines
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HTML
223 lines
12 KiB
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Equality (Guile Reference Manual)</title>
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<meta name="description" content="Equality (Guile Reference Manual)">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Utility-Functions.html" rel="up" title="Utility Functions">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Equality">
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<div class="nav-panel">
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<p>
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Next: <a href="Object-Properties.html" accesskey="n" rel="next">Object Properties</a>, Up: <a href="Utility-Functions.html" accesskey="u" rel="up">General Utility Functions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsection" id="Equality-1"><span>6.9.1 Equality<a class="copiable-link" href="#Equality-1"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-sameness"></a>
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<a class="index-entry-id" id="index-equality"></a>
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<p>There are three kinds of core equality predicates in Scheme, described
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below. The same kinds of comparisons arise in other functions, like
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<code class="code">memq</code> and friends (see <a class="pxref" href="List-Searching.html">List Searching</a>).
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</p>
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<p>For all three tests, objects of different types are never equal. So
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for instance a list and a vector are not <code class="code">equal?</code>, even if their
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contents are the same. Exact and inexact numbers are considered
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different types too, and are hence not equal even if their values are
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the same.
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</p>
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<p><code class="code">eq?</code> tests just for the same object (essentially a pointer
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comparison). This is fast, and can be used when searching for a
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particular object, or when working with symbols or keywords (which are
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always unique objects).
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</p>
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<p><code class="code">eqv?</code> extends <code class="code">eq?</code> to look at the value of numbers and
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characters. It can for instance be used somewhat like <code class="code">=</code>
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(see <a class="pxref" href="Comparison.html">Comparison Predicates</a>) but without an error if one operand isn’t a
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number.
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</p>
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<p><code class="code">equal?</code> goes further, it looks (recursively) into the contents
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of lists, vectors, etc. This is good for instance on lists that have
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been read or calculated in various places and are the same, just not
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made up of the same pairs. Such lists look the same (when printed),
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and <code class="code">equal?</code> will consider them the same.
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</p>
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<br>
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<dl class="first-deffn">
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<dt class="deffn" id="index-eq_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">eq?</strong> <var class="def-var-arguments">…</var><a class="copiable-link" href="#index-eq_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005feq_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_eq_p</strong> <var class="def-var-arguments">(x, y)</var><a class="copiable-link" href="#index-scm_005feq_005fp"> ¶</a></span></dt>
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<dd><a class="index-entry-id" id="index-eq_003f-3"></a>
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<p>The Scheme procedure returns <code class="code">#t</code> if all of its arguments are the
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same object, except for numbers and characters. The C function does the
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same but takes exactly two arguments. For example,
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</p>
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<div class="example">
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<pre class="example-preformatted">(define x (vector 1 2 3))
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(define y (vector 1 2 3))
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(eq? x x) ⇒ #t
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(eq? x y) ⇒ #f
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</pre></div>
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<p>Numbers and characters are not equal to any other object, but the
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problem is they’re not necessarily <code class="code">eq?</code> to themselves either.
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This is even so when the number comes directly from a variable,
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</p>
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<div class="example">
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<pre class="example-preformatted">(let ((n (+ 2 3)))
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(eq? n n)) ⇒ *unspecified*
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</pre></div>
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<p>Generally <code class="code">eqv?</code> below should be used when comparing numbers or
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characters. <code class="code">=</code> (see <a class="pxref" href="Comparison.html">Comparison Predicates</a>) or <code class="code">char=?</code>
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(see <a class="pxref" href="Characters.html">Characters</a>) can be used too.
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</p>
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<p>It’s worth noting that end-of-list <code class="code">()</code>, <code class="code">#t</code>, <code class="code">#f</code>, a
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symbol of a given name, and a keyword of a given name, are unique
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objects. There’s just one of each, so for instance no matter how
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<code class="code">()</code> arises in a program, it’s the same object and can be
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compared with <code class="code">eq?</code>,
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</p>
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<div class="example">
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<pre class="example-preformatted">(define x (cdr '(123)))
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(define y (cdr '(456)))
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(eq? x y) ⇒ #t
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(define x (string->symbol "foo"))
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(eq? x 'foo) ⇒ #t
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</pre></div>
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</dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fis_005feq"><span class="category-def">C Function: </span><span><code class="def-type">int</code> <strong class="def-name">scm_is_eq</strong> <code class="def-code-arguments">(SCM x, SCM y)</code><a class="copiable-link" href="#index-scm_005fis_005feq"> ¶</a></span></dt>
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<dd><p>Return <code class="code">1</code> when <var class="var">x</var> and <var class="var">y</var> are equal in the sense of
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<code class="code">eq?</code>, otherwise return <code class="code">0</code>.
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</p>
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<a class="index-entry-id" id="index-_003d_003d"></a>
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<p>The <code class="code">==</code> operator should not be used on <code class="code">SCM</code> values, an
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<code class="code">SCM</code> is a C type which cannot necessarily be compared using
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<code class="code">==</code> (see <a class="pxref" href="The-SCM-Type.html">The SCM Type</a>).
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</p></dd></dl>
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<br>
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<dl class="first-deffn">
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<dt class="deffn" id="index-eqv_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">eqv?</strong> <var class="def-var-arguments">…</var><a class="copiable-link" href="#index-eqv_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005feqv_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_eqv_p</strong> <var class="def-var-arguments">(x, y)</var><a class="copiable-link" href="#index-scm_005feqv_005fp"> ¶</a></span></dt>
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<dd><a class="index-entry-id" id="index-eqv_003f-2"></a>
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<p>The Scheme procedure returns <code class="code">#t</code> if all of its arguments are the
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same object, or for characters and numbers the same value. The C function
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is similar but takes exactly two arguments.
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</p>
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<p>On objects except characters and numbers, <code class="code">eqv?</code> is the same as
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<code class="code">eq?</code> above. <code class="code">(eqv? x y)</code> is true if <var class="var">x</var> and <var class="var">y</var> are
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the same object.
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</p>
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<p>If <var class="var">x</var> and <var class="var">y</var> are numbers or characters, <code class="code">eqv?</code> compares
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their type and value. An exact number is not <code class="code">eqv?</code> to an
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inexact number (even if their value is the same).
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</p>
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<div class="example">
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<pre class="example-preformatted">(eqv? 3 (+ 1 2)) ⇒ #t
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(eqv? 1 1.0) ⇒ #f
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</pre></div>
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</dd></dl>
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<br>
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<dl class="first-deffn">
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<dt class="deffn" id="index-equal_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">equal?</strong> <var class="def-var-arguments">…</var><a class="copiable-link" href="#index-equal_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fequal_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_equal_p</strong> <var class="def-var-arguments">(x, y)</var><a class="copiable-link" href="#index-scm_005fequal_005fp"> ¶</a></span></dt>
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<dd><a class="index-entry-id" id="index-equal_003f-2"></a>
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<p>The Scheme procedure returns <code class="code">#t</code> if all of its arguments are the
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same type, and their contents or value are equal. The C function is
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similar, but takes exactly two arguments.
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</p>
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<p>For a pair, string, vector, array or structure, <code class="code">equal?</code> compares the
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contents, and does so using the same <code class="code">equal?</code> recursively,
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so a deep structure can be traversed.
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</p>
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<div class="example">
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<pre class="example-preformatted">(equal? (list 1 2 3) (list 1 2 3)) ⇒ #t
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(equal? (list 1 2 3) (vector 1 2 3)) ⇒ #f
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</pre></div>
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<p>For other objects, <code class="code">equal?</code> compares as per <code class="code">eqv?</code> above,
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which means characters and numbers are compared by type and value (and
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like <code class="code">eqv?</code>, exact and inexact numbers are not <code class="code">equal?</code>,
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even if their value is the same).
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</p>
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<div class="example">
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<pre class="example-preformatted">(equal? 3 (+ 1 2)) ⇒ #t
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(equal? 1 1.0) ⇒ #f
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</pre></div>
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<p>Hash tables are currently only compared as per <code class="code">eq?</code>, so two
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different tables are not <code class="code">equal?</code>, even if their contents are the
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same.
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</p>
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<p><code class="code">equal?</code> does not support circular data structures, it may go
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into an infinite loop if asked to compare two circular lists or
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similar.
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</p>
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<p>GOOPS object types (see <a class="pxref" href="GOOPS.html">GOOPS</a>), including foreign object types
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(see <a class="pxref" href="Defining-New-Foreign-Object-Types.html">Defining New Foreign Object Types</a>), can have an <code class="code">equal?</code>
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implementation specialized on two values of the same type. If
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<code class="code">equal?</code> is called on two GOOPS objects of the same type,
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<code class="code">equal?</code> will dispatch out to a generic function. This lets an
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application traverse the contents or control what is considered
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<code class="code">equal?</code> for two objects of such a type. If there’s no such
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handler, the default is to just compare as per <code class="code">eq?</code>.
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</p></dd></dl>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Object-Properties.html">Object Properties</a>, Up: <a href="Utility-Functions.html">General Utility Functions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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