timerF :: F (Maybe (Int, Int)) Tick data Tick = Tick instance Eq Tick instance Show Tick
timerF provides an interface to the timers in the runtime system.
Just (interval,first) will start the timer. After first
ms, the timer will alarm, then, if interval is non-zero,
the timer will alarm repeatedly with that interval.
If input is Nothing, the timer will stop.
Tick is output when the timer alarms.