UndoMonad Calculator Example

UndoMonadCalc.fs

open System
open FSharpx.Undo

let add x = undoable {
  let! currentVal = getCurrent
  do! putToHistory (currentVal + x) 
  return currentVal}

let undo = undoable {
  let! _ = undo
  return! getCurrent}  

let rec handleInput calculator input =
  let notifyAndContinue result =
    printfn "The current total is %O" (result |> current)      
    Console.ReadLine() |> handleInput result

  match input with         
  | "undo" -> undo calculator
  | _ -> let _, x = Double.TryParse(input) 
         add x calculator
  |> snd |> notifyAndContinue

Console.ReadLine() |> handleInput (empty 0.)