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 neat way I came up with is the following:

If all members of a list are different from each other, then if I tell prolog to choose all pairs (I,J) such that I,J are members of the list and also I is equal to J, then for each element in the list it will only be able to find one such pair, which is the element with itself.

Therefore, if we can put all such pairs in a list, then the length of this list should be of the same length of the original list.

Here's my prolog code:

all_diff(L) :-
        findall((I,J), (member(I, L), member(J, L), I == J), List),
        length(L, SupposedLength),
        length(List, CheckThis),
        SupposedLength == CheckThis.