/usr/lib/swi-prolog/doc/Manual/preddesc.html

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<H2><A NAME="sec:4.1"><SPAN class="sec-nr">4.1</SPAN> <SPAN class="sec-title">Notation 
of Predicate Descriptions</SPAN></A></H2>

<A NAME="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">21<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>
<CENTER>
<TABLE BORDER=2 FRAME=hsides RULES=groups>
<TR VALIGN=top><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 VALIGN=top><TD>-</TD><TD>Argument must be unbound. Think of the 
argument as <EM>output</EM>. </TD></TR>
<TR VALIGN=top><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. E.g. 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 VALIGN=top><TD>:</TD><TD>Argument is a meta-argument. Implies <CODE><CODE>+</CODE></CODE>. 
See <A class="sec" href="modules.html">section 5</A> for more 
information on module-handing. </TD></TR>
<TR VALIGN=top><TD>@</TD><TD>Argument is not further instantiated. 
Typically used for type-tests.</TD></TR>
<TR VALIGN=top><TD>!</TD><TD>Argument contains a mutable structure that 
may be modified using
<A NAME="idx:setarg3:314"></A><A class="pred" href="manipterm.html#setarg/3">setarg/3</A> 
or <A NAME="idx:nbsetarg3:315"></A><A class="pred" href="manipterm.html#nb_setarg/3">nb_setarg/3</A>. </TD></TR>
</TABLE>

</CENTER>

<P><A NAME="idx:predicateindicator:316"></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 4.11</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;arity+2&gt;, regardless on 
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 NAME="idx:spy1:317"></A><A class="pred" href="debugger.html#spy/1">spy/1</A>, 
and <A NAME="idx:dynamic1:318"></A><A class="pred" href="dynamic.html#dynamic/1">dynamic/1</A>.

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