Re: [Haskell-cafe] nested maybes

u.stenzel:

J. Garrett Morris wrote:

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On 2/4/07, Udo Stenzel wrote:

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exists s wmap = isJust $ Map.lookup (sort s) wmap >>= find (== s) . snd

If you're going to write it all on one line, I prefer to keep things going the same direction:

Hey, doing it this way saved me a full two keystrokes!!!1

Sure, you're right, everything flowing in the same direction is usually nicer, and in central Europe, that order is from the left to the right. What a shame that the Haskell gods chose to give the arguments to (.) and ($) the wrong order!

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exists s wmap = isJust $ find (==s) . snd =<< Map.lookup (sort s) wmap

Normally, from there I would be tempted to look for a points-free implementation, but in this case I have a strong suspicion that would simply be unreadable.

Well, depends on whether we are allowed to define new combinators. I sometimes use

-- Kleisli composition infixl 1 @@ (@@) :: Monad m => (a -> m b) -> (b -> m c) -> (a -> m c) f @@ g = join . liftM g . f

By the way, this is now in Control.Monad (in darcs). Though since we also want the flipped version, it becomes: -- | Left-to-right Kleisli composition of monads. (>=>) :: Monad m => (a -> m b) -> (b -> m c) -> (a -> m c) f >=> g = \x -> f x >>= g -- | Right-to-left Kleisli composition of monads. '(>=>)', with the arguments flipped (<=<) :: Monad m => (b -> m c) -> (a -> m b) -> (a -> m c) (<=<) = flip (>=>) Cheers, Don