RPN

open System

let evalRpnExpr (s : string) = 
    let solve items current =
        match (current, items) with
        | "+", y::x::t -> (x + y)::t
        | "-", y::x::t -> (x - y)::t
        | "*", y::x::t -> (x * y)::t
        | "/", y::x::t -> (x / y)::t
        | _ -> (float current)::items
    (s.Split(' ') |> Seq.fold solve []).Head

[<EntryPoint>]
let main argv = 
    [ "4 2 5 * + 1 3 2 * + /"
      "5 4 6 + /"
      "10 4 3 + 2 * -"
      "2 3 +"
      "90 34 12 33 55 66 + * - + -"
      "90 3 -" ]
    |> List.map (fun expr -> expr, evalRpnExpr expr)
    |> List.iter (fun (expr, result) -> printfn "(%s) = %A" expr result)
    Console.ReadLine() |> ignore
    0 // return an integer exit code