let next = (state, event) => {
  // Compiler understand what type you want here, without additional annotations
  switch (state, event) {
    | (Paused(data), Start | Resume) => Running(data)
    | (Running({ count }), Increase) => Running({ count: count + 1 })
    | (Running({ count }), Decrease) => Running({ count: count - 1 })
    | (Running({ count }), Pause) => Paused({ count: count })
    | (_, Reset) => init()
  }
}