setof in prolog

what is t开发者_JS百科he source code of setof in prolog?

?- listing(setof).
:- meta_predicate setof(?,0,-).

setof(A, B, F) :-
    free_variable_set(A, B, D, C),
    (   C==v
    ->  findall(A, D, E),
        E\==[],
        sort(E, F)
    ;   findall(C-A, D, E),
        (   ground(E)
        ->  sort(E, G),
        pick(G, C, F)
        ;   bind_bagof_keys(E, _),
        sort(E, G),
        pick(G, C, H),
        sort(H, F)
        )
    ).

true.

In case you are looking for the Sicstus built-in predicate implementation, it can be found here: http://www.sics.se/sicstus/docs/4.2.1/html/sicstus/mpg_002dref_002dsetof.html as:

setof(+Template, +Generator, -Set)

Unlike findall/3 and bagof/3, setof does not return duplicates and does give sorted order.

I.