Prolog combining predicates

Question

Just a small question about Prolog. Say I have used the built in predicate findall/3 to obtain a list and used the variable X as my output.

I'm wondering how I could then use this list in another predicate such as last/2 to find the last element of this list. If you could include a small example too that would help greatly.

Answer 1

First of all, since Prolog aims to be a logic programming programming language, there is nu such thing as output variables.

Nevertheless, say you know a variable X is bounded after a certain predicate and you intend to use this value when calling a new predicate, you can use Prolog's logical "and" ,/2. I'm putting "and" between quotes because this and differs sometimes from the natural understanding of how "and" in natural language behaves.

You can thus use a predicate:

findall(A,foo(A),X),last(X,L).

To first find all occurences of foo/1, extract the variable A, put these into a list X and finally get the last/2 element of X.

You can then for instance use this in a defined predicate:

last_foo(L) :-
    findall(A,foo(A),X),
    last(X,L).

If you run this for instance with:

foo(a).
foo(9).
foo(b).

The results are:

?- foo(A).
A = a ;
A = 9 ;
A = b.

and:

?- findall(A,foo(A),X).
X = [a, 9, b].

Now the result to obtain the last is:

?- findall(A,foo(A),X),last(X,L).
X = [a, 9, b],
L = b.

or:

?- last_foo(L).
L = b.