6.4 Defining a meta-predicate

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 predicate indicator to describe the predicate they address, e.g., assert(name(jan)) or abolish(name/1). 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 <module>:<term>, e.g., assert(person:name(jan)) or abolish(person:name/1).

Of course, when calling assert/1 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 :/2 wrapper by hand. The meta_predicate/1 directive tells the compiler that certain arguments are terms that will be used to look up a predicate and thus need to be wrapped (qualified) with <module>:<term>, unless they are already wrapped.

In the example below, we use this to define maplist/3 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 :, which denote module-sensitive arguments, from +, - and ?, which denote modes. The values 0..9 are used by the cross-referencer and syntax highlighting. Note that the helper predicate maplist_/3 does not need to be declared as a meta-predicate because the maplist/3 wrapper already ensures that Goal is qualified as <module>:Goal. See the description of meta_predicate/1 for details.

:- 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, Goal).

maplist_([], [], _).
maplist_([H0|T0], [H|T], Goal) :-
        call(Goal, H0, H),
        maplist_(T0, T, Goal).

meta_predicate +Head, ...

Define the predicates referenced by the comma-separated list Head as meta-predicates. Each argument of each head is a meta argument specifier. Defined specifiers are given below. Only 0..9, : and ^ are interpreted; the mode declarations +, - and ? are ignored.

0..9

The argument is a term that is used to reference a predicate with N more arguments than the given argument term. For example: call(0) or maplist(1, +).

:

The argument is module-sensitive, but does not directly refer to a predicate. For example: consult(:).

-

The argument is not module-sensitive and unbound on entry.

?

The argument is not module-sensitive and the mode is unspecified.

*

The argument is not module-sensitive and the mode is unspecified. The specification * is equivalent to ?. It is accepted for compatibility reasons. The predicate predicate_property/2 reports arguments declared using * with ?.

+

The argument is not module-sensitive and bound (i.e., nonvar) on entry.

^

This extension is used to denote the possibly ^-annotated goal of setof/3, bagof/3, aggregate/3 and aggregate/4. It is processed similar to `0', but leaving the ^/2 intact.

//

The argument is a DCG body. See phrase/3.

Each argument that is module-sensitive (i.e., marked 0..9, : or ^) is qualified with the context module of the caller if it is not already qualified. The implementation ensures that the argument is passed as <module>:<term>, where <module> is an atom denoting the name of a module and <term> itself is not a :/2 term where the first argument is an atom. Below is a simple declaration and a number of queries.

:- meta_predicate
        meta(0, +).

meta(Module:Term, _Arg) :-
        format('Module=~w, Term = ~q~n', [Module, Term]).

?- 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

The meta_predicate/1 declaration is the portable mechanism for defining meta-predicates and replaces the old SWI-Prolog specific mechanism provided by the deprecated predicates module_transparent/1, context_module/1 and strip_module/3. See also section 6.15.