StartStop.fsx

type Connection = {
    Start: unit -> unit
    Stop: unit -> unit
}
type ActiveConnection = {
    Stop: unit -> ConnectionWrapper
}
and InActiveConnection = {
    Start: unit -> ConnectionWrapper
}
and ConnectionWrapper =
    | InActive of InActiveConnection
    | Active of ActiveConnection
    with 
        static member Create() =
            let connection = {
                Start = (fun () -> printfn "Starting")
                Stop = (fun () -> printfn "Stopping")
            }
            let rec start () =
                connection.Start()
                Active {
                    Stop = stop
                }
            and stop () =
                connection.Stop()
                InActive {
                    Start = start
                }
            InActive { Start = start }

let mutable conn = ConnectionWrapper.Create()

let startClick() =
    conn <-
        match conn with
        | InActive inActiveConn -> inActiveConn.Start()
        | Active _ -> printfn "Already started"; conn

let stopClick() =
    conn <-
        match conn with
        | InActive _ -> printfn "Already closed"; conn
        | Active activeConn -> activeConn.Stop()

startClick() // Prints "Starting"
startClick() // Prints "Already started"
stopClick() // Prints "Stopping"
stopClick() // Prints "Already stopped"