Peter Simons
while :: (Monad m) => m Bool -> m a -> m () while cond f = cond >>= flip when (f >> while cond f)
forever :: (Monad m) => m a -> m () forever f = while (return True) f
forever performs unnecessary computation which can't be optimized out by the compiler before specializing the monad (because the compiler can't use monad laws). Why not forever f = f >> forever f or forever f = loop where loop = f >> loop ? I don't know which is better - I guess the second gives better space behavior with some monads if it's not inlined (avoids recomputation of identical copies), but maybe the first actually allows GHC to optimize the case of IO better by not materializing a closure. I don't like the point-free style of the first either. while cond f = do c <- cond when c (f >> while cond f) -- __("< Marcin Kowalczyk \__/ qrczak@knm.org.pl ^^ http://qrnik.knm.org.pl/~qrczak/