On Mon, Feb 07, 2005 at 01:37:53PM +0000, Malcolm Wallace wrote:
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
I often define the following, and usually just define them as IO as I haven't found a non-IO use for them yet (okay, ST and the recent STM): -- clashes with your for for :: [a] -> (a -> IO b) -> IO () for = flip mapM_ loop :: IO a -> IO () loop = sequence_ . repeat I have no idea whether GHC is smart enough to optimize the lists away (in loop and in e.g. "for [1..100] ..."), though it hasn't caused me problems in practice. I'm rather fond of lists of IO-actions, which is probably why I once wrote the following OpenGL code: renderPrimitive Polygon $ do sequence_ $ concat $ transpose [colors, vertices] where both `colors' and `vertices' are lists of actions which respectively change the current OpenGL color and draw a vertex. Greetings, Remi -- Nobody can be exactly like me. Even I have trouble doing it.