(* A simple arithmetic evaluator. *)
open System

type Expr =
| Number of int
| Plus of Expr * Expr
| Minus of Expr * Expr
| Mult of Expr * Expr

let rec eval(e: Expr) : int =
    match e with
    | Number n -> n
    | Plus(e1, e2) -> (eval e1) + (eval e2)
    | Minus(e1, e2) -> (eval e1) - (eval e2)
    | Mult(e1, e2) -> (eval e1) * (eval e2)

let rec prettyprint(e: Expr) : string =
    match e with
    | Number n -> n.ToString()
    | Plus(e1, e2) -> (prettyprint e1) + " + " + (prettyprint e2)
    | Minus(e1, e2) -> (prettyprint e1) + " - " + (prettyprint e2)
    | Mult(e1, e2) -> (prettyprint e1) + " * " + (prettyprint e2)

[<EntryPoint>]
let main argv =
    let e = Minus(Plus(Number 1, Mult(Number 2, Number 3)), Number 1)
    printfn "You typed: %A" (prettyprint e) 
    let result = eval e
    printfn "Result is: %d" result
    0