180 lines
11 KiB
HTML
180 lines
11 KiB
HTML
<!DOCTYPE html>
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<html>
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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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under the terms of the GNU Free Documentation License, Version 1.3 or
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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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Documentation License." -->
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<title>Booleans (Guile Reference Manual)</title>
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<meta name="description" content="Booleans (Guile Reference Manual)">
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<link href="index.html" rel="start" title="Top">
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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="Data-Types.html" rel="up" title="Data Types">
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<link href="Numbers.html" rel="next" title="Numbers">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Booleans">
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<div class="nav-panel">
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<p>
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Next: <a href="Numbers.html" accesskey="n" rel="next">Numerical data types</a>, Up: <a href="Data-Types.html" accesskey="u" rel="up">Data Types</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="Booleans-1"><span>6.6.1 Booleans<a class="copiable-link" href="#Booleans-1"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-Booleans"></a>
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<p>The two boolean values are <code class="code">#t</code> for true and <code class="code">#f</code> for false.
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They can also be written as <code class="code">#true</code> and <code class="code">#false</code>, as per R7RS.
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</p>
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<p>Boolean values are returned by predicate procedures, such as the general
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equality predicates <code class="code">eq?</code>, <code class="code">eqv?</code> and <code class="code">equal?</code>
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(see <a class="pxref" href="Equality.html">Equality</a>) and numerical and string comparison operators like
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<code class="code">string=?</code> (see <a class="pxref" href="String-Comparison.html">String Comparison</a>) and <code class="code"><=</code>
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(see <a class="pxref" href="Comparison.html">Comparison Predicates</a>).
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(<= 3 8)
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⇒ #t
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(<= 3 -3)
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⇒ #f
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(equal? "house" "houses")
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⇒ #f
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(eq? #f #f)
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⇒
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#t
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</pre></div>
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<p>In test condition contexts like <code class="code">if</code> and <code class="code">cond</code>
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(see <a class="pxref" href="Conditionals.html">Simple Conditional Evaluation</a>), where a group of subexpressions will be
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evaluated only if a <var class="var">condition</var> expression evaluates to “true”,
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“true” means any value at all except <code class="code">#f</code>.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(if #t "yes" "no")
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⇒ "yes"
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(if 0 "yes" "no")
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⇒ "yes"
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(if #f "yes" "no")
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⇒ "no"
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</pre></div>
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<p>A result of this asymmetry is that typical Scheme source code more often
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uses <code class="code">#f</code> explicitly than <code class="code">#t</code>: <code class="code">#f</code> is necessary to
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represent an <code class="code">if</code> or <code class="code">cond</code> false value, whereas <code class="code">#t</code> is
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not necessary to represent an <code class="code">if</code> or <code class="code">cond</code> true value.
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</p>
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<p>It is important to note that <code class="code">#f</code> is <strong class="strong">not</strong> equivalent to any
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other Scheme value. In particular, <code class="code">#f</code> is not the same as the
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number 0 (like in C and C++), and not the same as the “empty list”
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(like in some Lisp dialects).
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</p>
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<p>In C, the two Scheme boolean values are available as the two constants
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<code class="code">SCM_BOOL_T</code> for <code class="code">#t</code> and <code class="code">SCM_BOOL_F</code> for <code class="code">#f</code>.
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Care must be taken with the false value <code class="code">SCM_BOOL_F</code>: it is not
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false when used in C conditionals. In order to test for it, use
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<code class="code">scm_is_false</code> or <code class="code">scm_is_true</code>.
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</p>
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<a class="index-entry-id" id="index-not-2"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-not"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">not</strong> <var class="def-var-arguments">x</var><a class="copiable-link" href="#index-not"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fnot"><span class="category-def">C Function: </span><span><strong class="def-name">scm_not</strong> <var class="def-var-arguments">(x)</var><a class="copiable-link" href="#index-scm_005fnot"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">x</var> is <code class="code">#f</code>, else return <code class="code">#f</code>.
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</p></dd></dl>
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<a class="index-entry-id" id="index-boolean_003f-2"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-boolean_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">boolean?</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-boolean_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fboolean_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_boolean_p</strong> <var class="def-var-arguments">(obj)</var><a class="copiable-link" href="#index-scm_005fboolean_005fp"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">obj</var> is either <code class="code">#t</code> or <code class="code">#f</code>, else
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return <code class="code">#f</code>.
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</p></dd></dl>
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<dl class="first-deftypevr">
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<dt class="deftypevr" id="index-SCM_005fBOOL_005fT"><span class="category-def">C Macro: </span><span><code class="def-type">SCM</code> <strong class="def-name">SCM_BOOL_T</strong><a class="copiable-link" href="#index-SCM_005fBOOL_005fT"> ¶</a></span></dt>
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<dd><p>The <code class="code">SCM</code> representation of the Scheme object <code class="code">#t</code>.
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</p></dd></dl>
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<dl class="first-deftypevr">
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<dt class="deftypevr" id="index-SCM_005fBOOL_005fF"><span class="category-def">C Macro: </span><span><code class="def-type">SCM</code> <strong class="def-name">SCM_BOOL_F</strong><a class="copiable-link" href="#index-SCM_005fBOOL_005fF"> ¶</a></span></dt>
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<dd><p>The <code class="code">SCM</code> representation of the Scheme object <code class="code">#f</code>.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fis_005ftrue"><span class="category-def">C Function: </span><span><code class="def-type">int</code> <strong class="def-name">scm_is_true</strong> <code class="def-code-arguments">(SCM obj)</code><a class="copiable-link" href="#index-scm_005fis_005ftrue"> ¶</a></span></dt>
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<dd><p>Return <code class="code">0</code> if <var class="var">obj</var> is <code class="code">#f</code>, else return <code class="code">1</code>.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fis_005ffalse"><span class="category-def">C Function: </span><span><code class="def-type">int</code> <strong class="def-name">scm_is_false</strong> <code class="def-code-arguments">(SCM obj)</code><a class="copiable-link" href="#index-scm_005fis_005ffalse"> ¶</a></span></dt>
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<dd><p>Return <code class="code">1</code> if <var class="var">obj</var> is <code class="code">#f</code>, else return <code class="code">0</code>.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fis_005fbool"><span class="category-def">C Function: </span><span><code class="def-type">int</code> <strong class="def-name">scm_is_bool</strong> <code class="def-code-arguments">(SCM obj)</code><a class="copiable-link" href="#index-scm_005fis_005fbool"> ¶</a></span></dt>
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<dd><p>Return <code class="code">1</code> if <var class="var">obj</var> is either <code class="code">#t</code> or <code class="code">#f</code>, else
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return <code class="code">0</code>.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005ffrom_005fbool"><span class="category-def">C Function: </span><span><code class="def-type">SCM</code> <strong class="def-name">scm_from_bool</strong> <code class="def-code-arguments">(int val)</code><a class="copiable-link" href="#index-scm_005ffrom_005fbool"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#f</code> if <var class="var">val</var> is <code class="code">0</code>, else return <code class="code">#t</code>.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fto_005fbool"><span class="category-def">C Function: </span><span><code class="def-type">int</code> <strong class="def-name">scm_to_bool</strong> <code class="def-code-arguments">(SCM val)</code><a class="copiable-link" href="#index-scm_005fto_005fbool"> ¶</a></span></dt>
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<dd><p>Return <code class="code">1</code> if <var class="var">val</var> is <code class="code">SCM_BOOL_T</code>, return <code class="code">0</code>
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when <var class="var">val</var> is <code class="code">SCM_BOOL_F</code>, else signal a ‘wrong type’ error.
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</p>
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<p>You should probably use <code class="code">scm_is_true</code> instead of this function
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when you just want to test a <code class="code">SCM</code> value for trueness.
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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="Numbers.html">Numerical data types</a>, Up: <a href="Data-Types.html">Data Types</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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