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<H2><A NAME="sec:5.4"><SPAN class="sec-nr">5.4</SPAN> <SPAN class="sec-title">Defining
a meta-predicate</SPAN></A></H2>
<A NAME="sec:metapred"></A>
<P>A meta-predicate is a predicate that calls other predicates
dynamically, modifies a predicate or reasons about properties of a
predicate. Such predicates use either a compound term or a <EM>predicate
indicator</EM> to describe the predicate they address, e.g., <CODE>assert(name(jan))</CODE>
or <CODE>abolish(<A NAME="idx:name1:1239"></A><SPAN class="pred-ext">name/1</SPAN>)</CODE>.
With modules, this simple schema no longer works as each module defines
its own mapping from name+arity to predicate. This is resolved by
wrapping the original description in a term <<VAR>module</VAR>>:<<VAR>term</VAR>>,
e.g., <CODE>assert(person:name(jan))</CODE> or
<CODE>abolish(person:name/1)</CODE>.
<P>Of course, calling <A NAME="idx:assert1:1240"></A><A class="pred" href="db.html#assert/1">assert/1</A>
from inside a module, we expect to assert to a predicate local to this
module. In other words, we do not wish to provide this <CODE><CODE>:</CODE>/2</CODE>
wrapper by hand. The <A NAME="idx:metapredicate1:1241"></A><A class="pred" href="metapred.html#meta_predicate/1">meta_predicate/1</A>
directive tells the compiler that certain arguments are terms that will
be used to lookup a predicate and thus need to be wrapped (qualified)
with <<VAR>module</VAR>>:<<VAR>term</VAR>>, unless they are
already wrapped.
<P>In the example below, we use this to define <A NAME="idx:maplist3:1242"></A><A class="pred" href="apply.html#maplist/3">maplist/3</A>
inside a module. The argument `2' in the meta_predicate declaration
means that the argument is module sensitive and refers to a predicate
with an arity that is two more than the term that is passed in. The
compiler only distinguishes the values 0..9 and <CODE><CODE>:</CODE></CODE>,
which denote module-sensitive arguments, from <CODE><CODE>+</CODE></CODE>, <CODE><CODE>-</CODE></CODE>
and <CODE><CODE>?</CODE></CODE> which denotes
<EM>modes</EM>. The values 0..9 are used by the
<EM>cross-referencer</EM> and syntax highlighting. Note that the
helper-predicate <A NAME="idx:maplist3:1243"></A><SPAN class="pred-ext">maplist_/3</SPAN>
does not need to be declared as a meta-predicate because the <A NAME="idx:maplist3:1244"></A><A class="pred" href="apply.html#maplist/3">maplist/3</A>
wrapper already ensures that
<VAR>Goal</VAR> is qualified as <<VAR>module</VAR>>:<VAR>Goal</VAR>.
See the description of
<A NAME="idx:metapredicate1:1245"></A><A class="pred" href="metapred.html#meta_predicate/1">meta_predicate/1</A>
for details.
<PRE class="code">
:- module(maplist, [maplist/3]).
:- meta_predicate maplist(2, ?, ?).
%% maplist(:Goal, +List1, ?List2)
%
% True if Goal can successfully be applied to all
% successive pairs of elements from List1 and List2.
maplist(Goal, L1, L2) :-
maplist_(L1, L2, G).
maplist_([], [], _).
maplist_([H0|T0], [H|T], Goal) :-
call(Goal, H0, H),
maplist_(T0, T, Goal).
</PRE>
<DL class="latex">
<DT class="pubdef"><A NAME="meta_predicate/1"><STRONG>meta_predicate</STRONG> <VAR>+Head, \ldots</VAR></A></DT>
<DD class="defbody">
Define the predicates referenced by the comma-separated list <VAR>Head</VAR>
as <EM>meta-predicates</EM>. Each argument of each head is a
<EM>meta argument specifier</EM>. Defined specifiers are given below.
Only 0..9 and <CODE><CODE>:</CODE></CODE> are interpreted; the mode
declarations <CODE><CODE>+</CODE></CODE>,
<CODE><CODE>-</CODE></CODE> and <CODE><CODE>?</CODE></CODE> are ignored.
<DL class="latex">
<DT><STRONG>0..9</STRONG></DT>
<DD class="defbody">
The argument is a term that is used to reference a predicate with <VAR>N</VAR>
more arguments than the given argument term. For example: <CODE>call(0)</CODE>
or <CODE>maplist(1, +)</CODE>.
</DD>
<DT><STRONG><CODE>:</CODE></STRONG></DT>
<DD class="defbody">
The argument is module sensitive, but does not directly refer to a
predicate. For example: <CODE>consult(:)</CODE>.
</DD>
<DT><STRONG><CODE>-</CODE></STRONG></DT>
<DD class="defbody">
The argument is not module sensitive and unbound on entry.
</DD>
<DT><STRONG><CODE>?</CODE></STRONG></DT>
<DD class="defbody">
The argument is not module sensitive and the mode is unspecified.
</DD>
<DT><STRONG><CODE>+</CODE></STRONG></DT>
<DD class="defbody">
The argument is not module sensitive and bound (i.e., nonvar) on entry.
</DD>
</DL>
<P>Each argument that is module-sensitive (i.e., marked 0..9 or <CODE><CODE>:</CODE></CODE>)
is qualified with the context module of the caller if it is not already
qualified. The implementation ensures that the argument is passed as
<<VAR>module</VAR>>:<<VAR>term</VAR>>, where <<VAR>atom</VAR>>
is an atom denoting the name of a module and <<VAR>term</VAR>>
itself is not a <CODE><CODE>:</CODE>/2</CODE> term. Below is a simple
declaration and a number of queries.
<PRE class="code">
:- meta_predicate
meta(0, +).
meta(Module:Term, _Arg) :-
format('Module=~w, Term = ~q~n', [Module, Term]).
</PRE>
<PRE class="code">
?- meta(test, x).
Module=user, Term = test
?- meta(m1:test, x).
Module=m1, Term = test
?- m2:meta(test, x).
Module=m2, Term = test
?- m1:meta(m2:test, x).
Module=m2, Term = test
?- meta(m1:m2:test, x).
Module=m2, Term = test
?- meta(m1:42:test, x).
Module=42, Term = test
</PRE>
<P>The <A NAME="idx:metapredicate1:1246"></A><A class="pred" href="metapred.html#meta_predicate/1">meta_predicate/1</A>
declaration is the portable mechanism for defining meta-predicates and
replaces the old SWI-Prolog specific mechanism provided by the
deprecated predicates <A NAME="idx:moduletransparent1:1247"></A><A class="pred" href="ctxmodule.html#module_transparent/1">module_transparent/1</A>,
<A NAME="idx:contextmodule1:1248"></A><A class="pred" href="ctxmodule.html#context_module/1">context_module/1</A>
and <A NAME="idx:stripmodule3:1249"></A><A class="pred" href="ctxmodule.html#strip_module/3">strip_module/3</A>.
See also <A class="sec" href="modulecompat.html">section 5.15</A>.
</DD>
</DL>
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