type Operator =
    | Oper of (Expr -> Expr)
    | Oper2 of (Expr -> Expr -> Expr)
and Expr =
    | Exprs of Expr * Operator * Expr
    | Expr of Operator * Expr
    | Num of int
   
type Operator with =
    member this.Invoke expr1 expr2 =
        match this with
        | Oper(x) -> x expr1
        | Oper2(x) -> x expr1 (Option.get expr2)

let rec int expr =
    match expr with
    | Num(x) -> x
    | Expr(op, x) -> int <| op.Invoke x None
    | Exprs(x, op, y) -> int <| op.Invoke x (Some(y))