Turnstile State Machine

type State = Locked | Unlocked

type Input = Coin | Push

let next state input =
  match state, input with
    | Locked, Coin -> Unlocked
    | Unlocked, Push -> Locked
    | _ -> state
    
printfn "%A" (next Locked Coin)

printfn "%A" (List.ofSeq (Seq.scan next Locked [Coin; Coin; Push; Push; Coin]))