Peter Simons
On a similar note, I find myself using two rather simple combinators frequently but have no place where to put them:
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
'while', at least, would a nice addition to Control.Monad,
I agree that these sorts of combinators are frequently useful. However, there is a reasonable variety in the possible signatures one might assign to the control-flow notion of "while". For instance, how about while :: Monad m => Bool -> m Bool -> m () while True f = f >>= \b-> while b f while False f = return () There are other control-flow analogies like until :: Monad m => m Bool -> m () until f = f >>= \b-> if b then return () else until f for :: Monad m => Int -> m a -> m () for 0 f = return () for (n+1) f = f >> for n f which probably also have a few possible monadic variations. Regards, Malcolm