Re: Proposal: add ifM and whenM to Control.Monad

On Sun, Apr 20, 2014 at 04:10:34PM -0500, John Wiegley wrote:

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Edward Kmett writes: However, given that they keep getting reinvented with the exact same names and functionality. I'm finally ready to give in.

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+1 from me.

How about a more general combinator, like om (name needed)?

om :: Monad m => (a -> b -> m c) -> m a -> b -> m c om f m = (m >>=) . flip f

whenM = om when unlessM = om unless

How about a completely general way of feeding arguments into a function, some of which are in a monad, some not? All you need is a combinator (<*>|) which indicates when you've supplied all the arguments. It can be read as "done" or something like that. Using it looks a bit odd but could be neatened up at the expense of adding even *more* combinators! Anyway, it completely obviates the need for ifM/whenM/unlessM and om, as far as I can tell. Tom import Control.Monad import System.Directory import Control.Applicative import Control.Lens infixl 4 <*>| (<*>|) :: Monad m => m (a -> m b) -> m a -> m b f <*>| x = join (f `ap` x) -- Here's how you can mix arguments in a monad with those that are not f :: Monad m => a -> b -> m c -> d -> m e -> m f f = undefined g :: (Monad m, Applicative m) => m a -> m b -> m c -> m d -> m e -> m f g a b c d e = f <$> a <*> b <*> pure c <*> pure d <*>| pure e -- Here's another example f' :: Monad m => a -> b -> m c -> d -> m e -> f -> m g f' = undefined g' :: (Monad m, Applicative m) => m a -> m b -> m c -> m d -> m e -> m f -> m g g' a b c d e f = f' <$> a <*> b <*> pure c <*> pure d <*> pure e <*>| f -- Implementing whenM and unlessM is easy. No om required. whenM :: (Monad m, Applicative m) => m Bool -> m () -> m () whenM cond action = when <$> cond <*>| pure action unlessM :: (Monad m, Applicative m) => m Bool -> m () -> m () unlessM cond action = unless <$> cond <*>| pure action -- But the point is that you don't actually need whenM and unlessM, since -- it's easy to feed arguments to when and unless directly. path = undefined usingUnless = unless <$> doesDirectoryExist path <*>| pure (do putStrLn $ "Creating directory " ++ path createDirectory path)