polish_rev.fsx

type Op =
    | Add of (Op * Op)
    | Mul of (Op * Op)
    | Divide of (Op * Op)
    | Substract of (Op * Op)
    | Value of int 

let mul a b = Mul (a, b)
let sub a b = Substract (a, b)
let v = Value


let rec eval = function
    | Add (Eval (+) value)
    | Mul (Eval (*) value)
    | Divide (Eval (/) value)
    | Substract  (Eval (-) value)
    | Value value -> value
and (|Eval|) op (a, b) = op (eval a) (eval b)

//× − 5 6 7
let example = 
    mul (sub (v 5) (v 6)) (v 7)
    |> eval