Search code examples
prologclause

Prolog - return whole clause

I have some basic formulas like

female(camilla).
female(diana).
...
parent(person, child).
...

and predicate language/2 that specifies which predicates will I be using:

langugage(female, 1).
language(parent, 2).

What I have to do is to create predicate called body_lit/1 that returns whole clauses specified in language like this:

?- body_lit(X).
X = parent(charles, harry) ;
X = parent(diana, harry) ;
...
X = female(camilla) ;
X = female(diana) ;
...

I know I need to use call/n function and functor/3. I know how functor/3 works but I cant seem to figure out how to return the whole clause instead of just the name of the predicate.

Solution

  • I presume that body_lit is supposed to return all answers to predicates given in language with their arities. 

    body_lit(X) :- language(N,A), functor(F,N,A), call(F), X=F.

    language/2 selects a predicate, functor/3 constructs an actual call from the predicate name and its arity, call/1 executes the call, and finally X=F records the call as the output argument. The last unification could have been removed:

    body_lit(X) :- language(N,A), functor(X,N,A), call(X).