Evaluation of postfixed expressions using pattern matching and piles

postfix.ml

(* Evaluate postfixed expressions *)
type 'a status = 
    Number of 'a
   |Operator of ('a -> 'a -> 'a)
   |Function of ('a -> 'a)

let eval exp =
    let s = Stack.create () in
    let n = Array.length exp in

    for i = 0 to n - 1 do
        match exp.(i) with
            | Number(n) -> Stack.push n s
            | Operator(op) -> Stack.push (op (Stack.pop s) (Stack.pop s)) s
            | Function(f) -> Stack.push (f (Stack.pop s)) s
    done;

    Stack.pop s