[Haskell-cafe] nested maybes
Donald Bruce Stewart
dons at cse.unsw.edu.au
Sun Feb 4 18:39:34 EST 2007
> J. Garrett Morris wrote:
> > On 2/4/07, Udo Stenzel <u.stenzel at web.de> wrote:
> > >> 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!
> > 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
(<=<) :: Monad m => (b -> m c) -> (a -> m b) -> (a -> m c)
(<=<) = flip (>=>)
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