Interpret stream of digits

gistfile1.fs

let digitToint i =
   match i with
     '0' -> 0
   | '1' -> 1
   | '2' -> 2
   | '3' -> 3
   | '4' -> 4
   | '5' -> 5
   | '6' -> 6
   | '7' -> 7
   | '8' -> 8
   | '9' -> 9
   | (_) -> 0 ;;

let mkint s =
 let rec mkintAux s a =
     match s with
      [] -> a
    | (x::xs) ->  mkintAux xs ((a*10) + (digitToint x))
 mkintAux s 0;;