Remove duplicates in list (Prolog)

Question

I am completely new to Prolog and trying some exercises. One of them is:

Write a predicate set(InList,OutList) which takes as input an arbitrary l

Answer 1

Using the support function mymember of Tim, you can do this if the order of elements in the set isn't important:

mymember(X,[X|_]).
mymember(X,[_|T]) :- mymember(X,T).

mkset([],[]).
mkset([T|C], S) :- mymember(T,C),!, mkset(C,S).
mkset([T|C], S) :- mkset(C,Z), S=[T|Z].

So, for example ?- mkset([1,4,1,1,3,4],S) give you as output:

S = [1, 3, 4]

but, if you want a set with the elements ordered like in the list you can use:

mkset2([],[], _).
mkset2([T|C], S, D) :- mkset2(C,Z,[T|D]), ((mymember(T,D), S=Z,!) ; S=[T|Z]).
mkset(L, S) :- mkset2(L,S,[]).

This solution, with the same input of the previous example, give to you:

S = [1, 4, 3]

This time the elements are in the same order as they appear in the input list.