On Sun, Jun 27, 2010 at 09:59:42AM +0200, Heinrich Apfelmus wrote:
Michael Mossey wrote:
Is there a way to write the function
process :: [(Location,Item)] -> [(Location,ValuableItem)]
given a function indicating which Item's to keep
transform :: Item -> Maybe ValuableItem
using functors and arrows? The value for location stays with any item that is kept.
What I have is
process inp = catMaybes (map g inp) where g (l,i) = case transform i of Nothing -> Nothing Just v -> Just (l,v)
This looks like an arrow situation to me because you want to make a function that acts on the second value in a tuple, and a little bit like a Maybe functor.
import Control.Arrow ((***))
process = catMaybes . map (uncurry (liftM2 (,)) . (return *** transform))
One could also do process = catMaybes . map (sequenceA . second transform) where Data.Traversable.sequenceA :: (Traversable t, Applicative f) => t (f a) -> f (t a) is used to "lift" the Maybe from the inside of the tuple to apply to the whole tuple, specifically sequenceA :: (a, Maybe b) -> Maybe (a,b) Unfortunately this will not work without a Traversable instance for ((,) a), which doesn't already exist, but it ought to, and it's easy to make: import Data.Foldable import Data.Traversable instance Foldable ((,) a) where foldMap = foldMapDefault instance Traversable ((,) a) where sequenceA (a,fb) = fmap ((,) a) fb -Brent
Whether this is more readable is another question.
Regards, Heinrich Apfelmus
-- http://apfelmus.nfshost.com
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