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<h2 id="sec:preddesc"><a id="sec:4.1"><span class="sec-nr">4.1</span> <span class="sec-title">Notation 
of Predicate Descriptions</span></a></h2>

<a id="sec:preddesc"></a>

<p>We have tried to keep the predicate descriptions clear and concise. 
First, the predicate name is printed in bold face, followed by the 
arguments in italics. Arguments are preceded by a mode indicator. There 
is no complete agreement on mode indicators in the Prolog community. We 
use the following definitions:<sup class="fn">34<span class="fn-text">These 
definitions are taken from PlDoc. The current manual has only one mode 
declaration per predicate and therefore predicates with mode (+,-) and 
(-,+) are described as (?,?). The <code>@</code>-mode is often replaced 
by <code><code>+</code></code>.</span></sup>

<p><table class="latex frame-hsides center">
<tr><td>+</td><td>Argument must be fully instantiated to a term that 
satisfies the required argument type. Think of the argument as <em>input</em>. </td></tr>
<tr><td>-</td><td>Argument must be unbound. Think of the argument as <em>output</em>. </td></tr>
<tr><td>?</td><td>Argument must be bound to a <em>partial term</em> of 
the indicated type. Note that a variable is a partial term for any type. 
Think of the argument as either <em>input</em> or <em>output</em> or
<em>both</em> input and output. For example, in
<code>stream_property(S, reposition(Bool))</code>, the <code>reposition</code> 
part of the term is input and the uninstantiated <var>Bool</var> is 
output. </td></tr>
<tr><td>:</td><td>Argument is a meta-argument. Implies <code><code>+</code></code>. 
See <a class="sec" href="modules.html">chapter 5</a> for more 
information on module handling. </td></tr>
<tr><td>@</td><td>Argument is not further instantiated. Typically used 
for type tests.</td></tr>
<tr><td>!</td><td>Argument contains a mutable structure that may be 
modified using
<a id="idx:setarg3:302"></a><a class="pred" href="manipterm.html#setarg/3">setarg/3</a> 
or <a id="idx:nbsetarg3:303"></a><a class="pred" href="manipterm.html#nb_setarg/3">nb_setarg/3</a>. </td></tr>
</table>

<p><a id="idx:predicateindicator:304"></a>Referring to a predicate in 
running text is done using a
<em>predicate indicator</em>. The canonical and most generic form of a 
predicate indicator is a term &lt;<var>module</var>&gt;:&lt;<var>name</var>&gt;/&lt;<var>arity</var>&gt;. 
If the module is irrelevant (built-in predicate) or can be inferred from 
the context it is often omitted. Compliant to the ISO standard draft on 
DCG (see
<a class="sec" href="DCG.html">section 4.12</a>), SWI-Prolog also allows 
for [&lt;<var>module</var>&gt;]:&lt;<var>name</var>&gt;//&lt;<var>arity</var>&gt; 
to refer to a grammar rule. For all non-negative arity, &lt;<var>name</var>&gt;//&lt;<var>arity</var>&gt; 
is the same as &lt;<var>name</var>&gt;/&lt;<var>arity</var>&gt;+2, 
regardless of whether or not the referenced predicate is defined or can 
be used as a grammar rule. The //-notation can be used in all places 
that traditionally allow for a predicate indicator, e.g., the module 
declaration, <a id="idx:spy1:305"></a><a class="pred" href="debugger.html#spy/1">spy/1</a>, 
and <a id="idx:dynamic1:306"></a><a class="pred" href="dynamic.html#dynamic/1">dynamic/1</a>.

<p></body></html>
swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / preddesc.html
