F# State

State.fs

(* State *)
type IState<'data> =
    abstract member Data: 'data
    abstract member Update: 'data -> unit

    [<CLIEvent>]
    abstract member Updated: IEvent<'data>

(* Tracked State *)
type ITrackedState<'data> =
    inherit IState<'data>
    abstract member Undo: unit -> unit
    abstract member Redo: unit -> unit

(* State *)
type State<'data> (init: 'data) =
    let mutable data = init
    let updated = new Event<'data> ()

    interface IState<'data> with
        member __.Data = data

        member __.Update state =
            data <- state
            updated.Trigger data

        [<CLIEvent>] member __.Updated = updated.Publish

(* Tracked State *)
type TrackedState<'data> (init: 'data) =
    let mutable history = [init]
    let mutable index = 0
    let updated = new Event<'data> ()

    interface ITrackedState<'data> with
        member __.Data = history.Item index

        member __.Update state =
            history <- state :: history
            index <- 0
            history.Item index |> updated.Trigger

        member __.Redo () =
            if (index - 1) >= 0 then
                index <- index - 1
                history.Item index |> updated.Trigger

        member __.Undo () =
            if (index + 1) <= history.Length - 1 then
                index <- index + 1
                history.Item index |> updated.Trigger
        
        [<CLIEvent>] member __.Updated = updated.Publish