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93 lines
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<title>List Predicates (Guile Reference Manual)</title>
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<div class="subsubsection-level-extent" id="List-Predicates">
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Next: <a href="List-Constructors.html" accesskey="n" rel="next">List Constructors</a>, Previous: <a href="List-Syntax.html" accesskey="p" rel="prev">List Read Syntax</a>, Up: <a href="Lists.html" accesskey="u" rel="up">Lists</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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<h4 class="subsubsection" id="List-Predicates-1"><span>6.6.9.2 List Predicates<a class="copiable-link" href="#List-Predicates-1"> ¶</a></span></h4>
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<p>Often it is useful to test whether a given Scheme object is a list or
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not. List-processing procedures could use this information to test
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whether their input is valid, or they could do different things
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depending on the datatype of their arguments.
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</p>
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<a class="index-entry-id" id="index-list_003f-2"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-list_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">list?</strong> <var class="def-var-arguments">x</var><a class="copiable-link" href="#index-list_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005flist_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_list_p</strong> <var class="def-var-arguments">(x)</var><a class="copiable-link" href="#index-scm_005flist_005fp"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">x</var> is a proper list, else <code class="code">#f</code>.
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</p></dd></dl>
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<p>The predicate <code class="code">null?</code> is often used in list-processing code to
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tell whether a given list has run out of elements. That is, a loop
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somehow deals with the elements of a list until the list satisfies
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<code class="code">null?</code>. Then, the algorithm terminates.
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</p>
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<a class="index-entry-id" id="index-null_003f-3"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-null_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">null?</strong> <var class="def-var-arguments">x</var><a class="copiable-link" href="#index-null_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fnull_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_null_p</strong> <var class="def-var-arguments">(x)</var><a class="copiable-link" href="#index-scm_005fnull_005fp"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">x</var> is the empty list, else <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_005fnull"><span class="category-def">C Function: </span><span><code class="def-type">int</code> <strong class="def-name">scm_is_null</strong> <code class="def-code-arguments">(SCM x)</code><a class="copiable-link" href="#index-scm_005fis_005fnull"> ¶</a></span></dt>
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<dd><p>Return 1 when <var class="var">x</var> is the empty list; otherwise return 0.
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</p></dd></dl>
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