{-# LANGUAGE OverloadedStrings #-}
import Reflex.Dom
import Data.Text
import Data.Time.Clock
import Data.Monoid ((<>))
import Control.Monad.Fix
import Control.Monad.Trans

timerWidget :: MonadWidget t m => Integer -> m ()
timerWidget maxTime = do
  t0 <- liftIO getCurrentTime
  eStart <- button "Start"
  ePause <- button "Pause"
  eTick <- tickLossy 1.0 t0
  let dTimer' = timer' t0 maxTime eStart ePause eTick
  dynText =<< dTimer'

timer' :: MonadWidget t m =>
  UTCTime -> Integer -> Event t () -> Event t () -> Event t TickInfo -> m (Dynamic t Text)
timer' t maxTime eStart ePause eTick = do
  beStartStop <- hold never . leftmost $ [ ((0+) <$ eTick) <$ ePause, ((1+) <$ eTick) <$ eStart ]
  let eSwitch = switch beStartStop
  fmap (formatS maxTime) <$> foldDyn ($) 0 eSwitch

formatS :: Integer -> Integer -> Text
formatS max cur =
  case properFraction (fromInteger (max-cur) / 60) of
    (mins, s) ->  let secs = round (s * 60) in
      pack $ fill mins <> ":" <> fill secs where
        fill n = if n `elem` [0..9] then "0" <> show n else show n

main = mainWidget $ timerWidget 120