[Haskell-cafe] Sequence differences

I have a Scheme function that calculates sequence differences, i.e., it returns a sequence that is the difference between the 2nd and the 1st element, the 3rd and the 2nd, the 4th and the 3rd, etc. (define s   (lambda (f l)     (cond ((null? (cdr l)) '())           (else (cons (f (cadr l) (car l))                       (s f (cdr l))))))) where (s - '(0,1,3,6,10,15,21,28)) => (1,2,3,4,5,6,7) I'm thinking the same function in Haskell would be something like s :: s f [] = [] s f [x] = [x] s f l = [ a f b | (a,b) <- zip (init l) (tail l)] but can't figure out what the function typing would be. Michael