peano_basic.ml

type nat = Z | S of nat

let rec nat_of_int = function
  | 0 -> Z
  | n -> S (nat_of_int (n-1))

let rec plus n m =
  match n with
  | Z -> m
  | S n' -> plus n' (S m)

let times n m =
  let rec calc n res =
    match n with
    | Z -> res
    | S n' -> calc n' (plus m res)
  in calc n Z