Check if all numbers in a list are different in prolog

Question

I want to create a rule in prolog that checks if there\'s a repeated number in a list.

For example:

• for [1,2,3,4] it will return tru

Answer 1

a compact definition could be

all_diff(L) :- \+ (select(X,L,R), memberchk(X,R)).

i.e. all elements are different if we can't peek one and find it in the rest...

edit

Let's (marginally) improve efficiency: it's useless to check if X is member of the prefix sublist, so:

all_diff(L) :- \+ (append(_,[X|R],L), memberchk(X,R)).