type CommandType = 
    | Deposit
    | Withdraw
type TCommand = 
    | Command of CommandType * int

let result = ref 7
let deposit x = result := !result + x
let withdraw x = result := !result - x
let Do = fun (cmd:TCommand) ->
    match cmd with
    | Command(CommandType.Deposit, n) -> deposit n
    | Command(CommandType.Withdraw,n) -> withdraw n
let Undo = fun (cmd:TCommand) ->
    match cmd with
    | Command(CommandType.Deposit, n) -> withdraw n
    | Command(CommandType.Withdraw,n) -> deposit n

printfn "current balance %d" !result
let depositCmd = Command(Deposit, 3)
Do depositCmd
printfn "after deposit: %d" !result
Undo depositCmd
printfn "after undo: %d" !result