type Expr =
    |Const of int
    |Var of string
    |Add of Expr * Expr
    |Sub of Expr * Expr
    |Mul of Expr * Expr
    |Div of Expr * Expr

let rec calc expr =
    match expr with
    |Const a -> a
    |Add(a, b) -> calc a + calc b
    |Sub(a, b) -> calc a - calc b
    |Mul(a, b) -> calc a * calc b
    |Div(a, b) -> calc a / calc b

calc (Add((Const 1)(Const 2)))