Prolog: First duplicate value

Here is a pure version using dif/2 which implements sound inequality. dif/2 is offered by B-Prolog, YAP-Prolog, SICStus-Prolog and SWI-Prolog.

firstdup(E, [E|L]) :-
    member(E, L).
firstdup(E, [N|L]) :-
    non_member(N, L),
    firstdup(E, L).

member(E, [E|_L]).
member(E, [_X|L]) :-
    member(E, L).

non_member(_E, []).
non_member(E, [F|Fs]) :-
    dif(E, F),
    non_member(E, Fs).

The advantages are that it can also be used with more general queries:

?- firstdup(E, [A,B,C]).
   E = A, A = B
;  E = A, A = C
;  E = C, B = C, dif(A, C)
;  false.

Here, we get three answers: A is the duplicate in the first two answers, but on two different grounds: A might be equal to B or C. In the third answer, B is the duplicate, but it will only be a duplicate if C is different to A.

To understand the definition, read :- as an arrow ← So what is on the right-hand side is what you know and on the left is what you conclude. It is often in the beginning a bit irritating to read predicates in that direction, after all you might be tempted to follow “the thread of execution”. But often this thread leads to nowhere – it gets too complex to understand.

The first rule reads:

Provided E is an element of list L we conclude that [E|L] has E as first duplicate.

The second rule reads:

Provided E is the first duplicate of L (don’t panic here and say we don’t know that …) and provided that N is not an element of L we conclude that [N|L] has E as first duplicate.

The member/2 predicate is provided in many Prolog systems and non_member(X,L) can be defined as maplist(dif(X),L). Thus one would define firstdup/2 rather as:

firstdup(E, [E|L]) :-
    member(E, L).
firstdup(E, [N|L]) :-
    maplist(dif(N), L),
    firstdup(E, L).