Delete two consecutive constants from a list

Question

I have a list and I should to delete consecutive duplicates in the list.

So for example:

compress([a,a,b,c,c,d,d,d,e], New_List) 
New_List = [a,b,c,d,e].

I have write some code, but when I try I receive false, and don't understand why (I'm a little bit newbie in prolog).

compress([],[]).

compress([Head| Rest], New_List):-
    compress(Head, Rest, New_List).

compress([], [], []).

compress(Head, [Head_N | Rest_N], New_List) :-
    Head \== Head_N,
    append([Head], New_List),
    compress(Head_N, Rest_N, New_List).

compress(Head, [Head_N | Rest_N], New_List) :-
    Head == Head_N,
    compress(Head_N, Rest_N, New_List).

EDIT 1.

I noticed right now, while i'm writing, that with this code the last letter will never append.

Answer 1

You can make a helper predicate that drops elements of a list until it finds the next element that is different, or reaches the end of the list:

dropEq([], _, []).
dropEq([H|T], X, [H|T]) :-
    dif(H, X).
dropEq([H|T], H, R) :-
    dropEq(T, H, R).

next we can use this to compress the list:

compress([], []).
compress([H|T], [H|R]) :-
    dropEq(T, H, Q),
    compress(Q, R).