Writing list's first elements

Question

I try to write a prolog list's first two elements in recursion.. for example:

if i have

List:[a,b,c,d,e,f]

when i enter

first_two(F,S,List).
It returns 
F:a, S:b ;
F:b, S:c ;
F:c, S:d ;
F:d, S:e ;
F:e, S:f .

I try to do this with:

first_two(F,S,[F|[S|Tail]]):-first_two(F,S,Tail).

but it fails..

Thank you very much..

Answer 1

you forgot the base case of recursion, i.e. the termination condition:

first_two(F, S, [F, S|_Tail]).
first_two(F, S, [_|Tail]) :-
    first_two(F, S, Tail).

I've also simplified your rule, with friendly syntax and spacing.

I prefer simpler code to read, don't you?

Answer 2

If all you want is the first two, then your solution is pretty close, but doesn't need the recursion:

first_two( F, S, [F|[S|_]] ).