attempt.sml

(*

Write a function number_before_reaching_sum that takes an int called sum, which you can assume
is positive, and an int list, which you can assume contains all positive numbers, and returns an int.
You should return an int n such that the first n elements of the list add to less than sum, but the first
n + 1 elements of the list add to sum or more. Assume the entire list sums to more than the passed in
value; it is okay for an exception to occur if this is not the case. 

*)

 fun number_before_reaching_sum (sum : int, xs : int list) =
     let fun sum_aux (limit, ys, acc, count) =
	     if acc >= limit
	     then count - 1
	     else
		 sum_aux (limit, tl ys, acc + hd ys, count + 1);
     in
	     sum_aux(sum, xs, 0, 0)
     end