never executed always true always false

    1 module UnliftIO.MessageBox.Util.Future
    2   ( Future (Future),
    3     tryNow,
    4     awaitFuture,
    5   )
    6 where
    7 
    8 import UnliftIO (MonadIO (liftIO), MonadUnliftIO)
    9 import UnliftIO.Concurrent (threadDelay)
   10 
   11 -- | A wrapper around an IO action that returns value
   12 -- in the future.
   13 newtype Future a = Future
   14   { -- | Return 'Just' the value or 'Nothing',
   15     --   when the value is not available yet.
   16     fromFuture :: IO (Maybe a)
   17   }
   18 
   19 -- | Return 'Just' the value or 'Nothing',
   20 --   when the value is not available yet.
   21 --
   22 --   Once the value is available, that value
   23 --   will be returned everytime this function is
   24 --   invoked.
   25 {-# INLINE tryNow #-}
   26 tryNow :: MonadUnliftIO m => Future a -> m (Maybe a)
   27 tryNow = liftIO . fromFuture
   28 
   29 -- | Poll a Future until the value is present.
   30 awaitFuture :: MonadUnliftIO m => Future b -> m b
   31 awaitFuture !f =
   32   tryNow f >>= maybe (threadDelay 10 >> awaitFuture f) return