How can I pass a predicate as parameter for another predicate in Prolog?

Question

I have these 3 predicates:

times(X, Y):-
    Result is X * Y.
minus(X, Y):-
    Result is X - Y.
plus(X, Y):-
    Result is X + Y.

and I want to pass for example times(2,2) in the plus(X, Y) like this plus(times(2,2), minus(X, Y)).

Answer 1

The relationship between the title of your question and the text of your question is unclear to me, and I think @false is probably right that there is a more fundamental misunderstanding about Prolog here. I don't know if this really addresses your need or not, but the alternative here is writing your own evaluator.

eval(times(X,Y), Result) :-
    eval(X, XResult),
    eval(Y, YResult),
    Result is XResult * YResult.
eval(minus(X,Y), Result) :-
    eval(X, XResult),
    eval(Y, YResult),
    Result is XResult - YResult.
eval(plus(X,Y), Result) :-
    eval(X, XResult),
    eval(Y, YResult),
    Result is XResult + YResult.

The recursive calls to eval/2 inside the bodies of each of these rules are needed to handle cases like plus(times(2,2), minus(X, Y)). Then you need a rule for numbers:

eval(Num, Num) :- number(Num).

This works great for cases like this:

?- eval(plus(times(2,2), minus(7,1)), Result).
Result = 10.

It doesn't do you any good for cases like this:

?- eval(plus(times(2,2), minus(X,Y)), Result).
ERROR: Out of local stack

Of course, it would work if we established bindings for X and Y before getting there, but if you want it to generate possible solutions for X and Y you're out of luck, you'll need to use clpfd. The reason for this curious error, if you trace in, is because number(X) when X is unbound is false, so it is actually generating new clauses involving the times, minus and plus structures and trying them, which isn't what you want in an evaluator.

Edit: implementing printterm/1.

The eval/2 predicate shows you how to perform a recursive tree walk. The principal is the same with making a pretty printer. I am very lazy so I will only sketch it, you'll have to fill in the details yourself.

printterm(T) :- format_term(T, Formatted), write(Formatted), nl.

format_term(plus(X,Y), Formatted) :- 
  format_term(X, XFormatted),
  format_term(Y, YFormatted),
  format(atom(Formatted), '(~a + ~a)', [XFormatted, YFormatted]).

% other format_term clauses here for other arithmetic expressions

format_term(X, X) :- number(X).

Hope this helps!