Determinig position of a element in a list

[Przemo Drochomirecki]

Hello,
i'm wondering if there is any tricky way for doing following thing:
    A - list of distinct integers  (e.x. A = [1,3,7,11,14,15])
    very fast function determinig position of number x in list A
    or -1 if x doesnt belong to A
Operator IN returns only false/true values
i can implement function index (e.x. index(5,A) = -1, index(7,A) = 2), but
maybe there's is simpler(builtin?) solution

Thanks