Haskell --- average of a list.

Rules:

✡ must compute arithmetic average of a list
✡ only one pass is allowed
✡ constant memory

Never used Haskell. I use Scheme though, and since TCO's required by the implementation,

(define (average lst)
(let loop ((sum 0) (len 0) (lst lst))
(if (null? lst)
(/ sum len)
(loop (+ (car lst) sum) (+ len 1) (cdr lst)))))

Similar hip functional programming shit though.