type MyEvent = Event1 | Event2 | Event3

​

type MachineState<'event when 'event:comparison> =

    {

    Transitions: Map<'event, Lazy<MachineState<'event>>>

    Data: int  

    }

    static member Default = {Transitions=Map.empty;Data=0}

​

//simple helper to add a new Transition    

let on event endState state =

    {state with Transitions = state.Transitions.Add(event, endState)}

let withData data state =

    {state with Data = data}

let transition event (state: MachineState<_>) =

    match state.Transitions.TryFind event with

    | None -> state

    | Some m -> m.Value

let rec StateA =

    lazy (MachineState<_>.Default

    |> on Event1 StateB

    |> on Event2 StateC

    |> withData 5)

and StateB =

    lazy (MachineState<_>.Default

    |> on Event2 StateC

    |> withData 10)

and StateC =

    lazy (MachineState<_>.Default

    //|> on Event3 StateA

    |> withData -1)

let printValue m =

    printfn "Current Data: %d" m.Data; m

[<EntryPoint>]

let main argv =

    let initialState = StateA.Value

    let machine = 

        initialState

        |> printValue

        |> transition Event1

        |> printValue

        |> transition Event2

        |> printValue

    printfn "%A" (machine)

    0

​