Question

I have the following list in prolog:

[[1,2],[2,3]]

Now, I can do a simple traversal in the list like this:

traverse([]).

traverse([H|T]):-
   write(H),
   traverse(T).

But, I don’t want this; I want to be able to access each element in the list as a list itself, not as a simple variable. What I want to do, out of this, is to cast each iteration of H into a list, so I can print out either the first or second value in H. However, I can’t come up with the syntax in prolog to do so. Can anyone help?

Answer 1

You need to unify (not cast) the H variable with the list constructor.

traverse([H|T]) :-
  (
     H = [],
     % The list within the list is the empty list.
  ;
     H = [HH | HT]
     % HH is the head of the list within the list.
     % HT is the tail of the list within the list (note, not the second item in the list).
  ),
  traverse(T).

This disjunction (the two operands to the ; operator), is the same as what’s happening in the heads of the two clauses of traverse.
Therefore a separate predicate could also be used to traverse the list inside the list.