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<h2 id="sec:typetest"><a id="sec:4.6"><span class="sec-nr">4.6</span> <span class="sec-title">Verify
Type of a Term</span></a></h2>
<a id="sec:typetest"></a>
<p>Type tests are semi-deterministic predicates that succeed if the
argument satisfies the requested type. Type-test predicates have no
error condition and do not instantiate their argument. See also library
<code>library(error)</code>.
<dl class="latex">
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="var/1"><strong>var</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> currently is a free variable.
</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="nonvar/1"><strong>nonvar</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> currently is not a free variable.
</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="integer/1"><strong>integer</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is bound to an integer.
</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="float/1"><strong>float</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is bound to a floating point number.
</dd>
<dt class="pubdef"><a id="rational/1"><strong>rational</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is bound to a rational number. Rational numbers
include integers.
</dd>
<dt class="pubdef"><a id="rational/3"><strong>rational</strong>(<var>@Term,
-Numerator, -Denominator</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is a rational number with given <var>Numerator</var>
and
<var>Denominator</var>. The <var>Numerator</var> and <var>Denominator</var>
are in canonical form, which means <var>Denominator</var> is a positive
integer and there are no common divisors between <var>Numerator</var>
and <var>Denominator</var>.
</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="number/1"><strong>number</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is bound to an integer or floating point number.<sup class="fn">45<span class="fn-text">As
rational numbers are not atomic in the current implementation and we do
not want to break the rule that <a id="idx:number1:556"></a><a class="pred" href="typetest.html#number/1">number/1</a>
implies <a id="idx:atomic1:557"></a><a class="pred" href="typetest.html#atomic/1">atomic/1</a>, <a id="idx:number1:558"></a><a class="pred" href="typetest.html#number/1">number/1</a>
fails on rational numbers. This will change if rational numbers become
atomic.</span></sup></dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="atom/1"><strong>atom</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is bound to an atom.</dd>
<dt class="pubdef"><a id="blob/2"><strong>blob</strong>(<var>@Term,
?Type</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is a <em>blob</em> of type <var>Type</var>. See
<a class="sec" href="foreigninclude.html">section 10.4.7</a>.</dd>
<dt class="pubdef"><a id="string/1"><strong>string</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is bound to a string. Note that string here
refers to the built-in atomic type string as described in <a class="sec" href="strings.html">section
5.2</a>. Starting with version 7, the syntax for a string object is
text between double quotes, such as <code>"hello"</code>.<sup class="fn">46<span class="fn-text">In
traditional Prolog systems, double quoted text is often mapped to a list
of <em>character codes</em>.</span></sup> See also the Prolog flag
<a class="flag" href="flags.html#flag:double_quotes">double_quotes</a>.</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="atomic/1"><strong>atomic</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is bound (i.e., not a variable) and is not
compound. Thus, atomic acts as if defined by:
<pre class="code">
atomic(Term) :-
nonvar(Term),
\+ compound(Term).
</pre>
<p>SWI-Prolog defines the following atomic datatypes: atom (<a id="idx:atom1:559"></a><a class="pred" href="typetest.html#atom/1">atom/1</a>),
string (<a id="idx:string1:560"></a><a class="pred" href="typetest.html#string/1">string/1</a>),
integer (<a id="idx:integer1:561"></a><a class="pred" href="typetest.html#integer/1">integer/1</a>),
floating point number (<a id="idx:float1:562"></a><a class="pred" href="typetest.html#float/1">float/1</a>)
and blob (<a id="idx:blob2:563"></a><a class="pred" href="typetest.html#blob/2">blob/2</a>).
In addition, the symbol <code>[]</code> (empty list) is atomic, but not
an atom. See <a class="sec" href="ext-lists.html">section 5.1</a>.</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="compound/1"><strong>compound</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is bound to a compound term. See also <a id="idx:functor3:564"></a><a class="pred" href="manipterm.html#functor/3">functor/3</a>
=../2, <a id="idx:compoundnamearity3:565"></a><a class="pred" href="manipterm.html#compound_name_arity/3">compound_name_arity/3</a>
and <a id="idx:compoundnamearguments3:566"></a><a class="pred" href="manipterm.html#compound_name_arguments/3">compound_name_arguments/3</a>.</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="callable/1"><strong>callable</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is bound to an atom or a compound term. This was
intended as a type-test for arguments to <a id="idx:call1:567"></a><a class="pred" href="metacall.html#call/1">call/1</a>
and <a id="idx:call2:568"></a><a class="pred" href="metacall.html#call/2">call/2</a>..
Note that callable only tests the <em>surface term</em>. Terms such as
(22,true) are considered callable, but cause <a id="idx:call1:569"></a><a class="pred" href="metacall.html#call/1">call/1</a>
to raise a type error. Module-qualification of meta-argument (see <a id="idx:metapredicate1:570"></a><a class="pred" href="metapred.html#meta_predicate/1">meta_predicate/1</a>)
using
<b>:/2</b> causes callable to succeed on any meta-argument.<sup class="fn">47<span class="fn-text">We
think that <a id="idx:callable1:571"></a><a class="pred" href="typetest.html#callable/1">callable/1</a>
should be deprecated and there should be two new predicates, one
performing a test for callable that is minimally module aware and
possibly consistent with type-checking in <a id="idx:call1:572"></a><a class="pred" href="metacall.html#call/1">call/1</a>
and a second predicate that tests for atom or compound.</span></sup>
Consider the program and query below:
<pre class="code">
:- meta_predicate p(0).
p(G) :- callable(G), call(G).
?- p(22).
ERROR: Type error: `callable' expected, found `22'
ERROR: In:
ERROR: [6] p(user:22)
</pre>
</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="ground/1"><strong>ground</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> holds no free variables.</dd>
<dt class="pubdef"><a id="cyclic_term/1"><strong>cyclic_term</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> contains cycles, i.e. is an infinite term. See
also <a id="idx:acyclicterm1:573"></a><a class="pred" href="typetest.html#acyclic_term/1">acyclic_term/1</a>
and <a class="sec" href="cyclic.html">section 2.16</a>.<sup class="fn">48<span class="fn-text">The
predicates <a id="idx:cyclicterm1:574"></a><a class="pred" href="typetest.html#cyclic_term/1">cyclic_term/1</a>
and <a id="idx:acyclicterm1:575"></a><a class="pred" href="typetest.html#acyclic_term/1">acyclic_term/1</a>
are compatible with SICStus Prolog. Some Prolog systems supporting
cyclic terms use is_cyclic/1 .</span></sup></dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="acyclic_term/1"><strong>acyclic_term</strong>(<var>@Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> does not contain cycles, i.e. can be processed
recursively in finite time. See also <a id="idx:cyclicterm1:576"></a><a class="pred" href="typetest.html#cyclic_term/1">cyclic_term/1</a>
and <a class="sec" href="cyclic.html">section 2.16</a>.
</dd>
</dl>
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