type Command = { Redo: unit->unit; Undo: unit->unit }

​

let mutable result =  7

​

let add  n = { 

    Redo = (fun _ -> result <- result + n); 

    Undo = (fun _ -> result <- result - n) }

​

let minus  n = { 

    Redo = (fun _ -> result <- result - n); 

    Undo = (fun _ -> result <- result + n) }

​

let cmd = (add 3)

printfn "current state = %d" result

​

cmd.Redo()

printfn "after redo: %d" result

​

cmd.Undo()

printfn "after undo: %d" result

​