Question

How does one evaluate the result of a prolog predicate to pass as an argument?
I am trying to write code to reverse pairs of elements in a list:

swap([A,B,C,D,E,F],R).

I want the result:

[B,A,D,C,F,E]

but I get this result:

append(append(append([],[B,A],[D,C],[F,E])))

Here is my code:

swap(L,R) :- swapA(L,[],R).
swapA([],A,A).
swapA([H,H2|T],A,R) :-  swapA(T, append(A,[H2,H]), R).

Thanks.

Answer 1

Several things:

• a variable starts with a capital letter, if you want to differenciate an atom from a variable, wrap it between ': ['A','B','C','D','E','F']
• you do not need append to successfully implement this predicate. The complexity is way worse when using append.
• because you do not need append, you do not need 3 arguments either, 2 suffice

Here is a suggestion:

swapA([], []).
swapA([X, Y|T], [Y, X|R]) :- swapA(T, R).

And consider adding another base case if you want your predicate to hold when you have an odd number of elements in your list:

swapA([X], [X]).