Adventures In Prolog - Duplicate List Members

Today's post is about problem 14 from the list of 99 Prolog problems (see here):

(*) Duplicate the elements of a list.
Example:
?- dupli([a,b,c,c,d],X).
X = [a,a,b,b,c,c,c,c,d,d]

Since the problem is marked with one star, a solution should be within reach. Now, to duplicate each element of the list, we do the following:

1. We traverse the list element by element until we reach the last element, i.e. the empty list.


2. For each element (excluding the empty list), the element is added twice to a newly built output list.

Given the above solution statement, a possible translation to Prolog code might look like this:

dupli([], []).
dupli(L, O) :-  dupli_t(L, [], O).

dupli_t([], Current, Current).
dupli_t([H|T], CList, OutList) :-  
  append(CList, [H, H], IntermediateList),  
  dupli_t(T, IntermediateList, OutList).