On 4/8/11 8:55 AM, Twan van Laarhoven wrote:
-- this class is useful beyond this FRP library,
-- you might already be able to find it on hackage somewhere
class Functor f => Filterable f where
filter :: (a -> Bool) -> f a -> f a
-- filter p . fmap f == fmap f . filter (p . f)
-- filter (const True) == id
-- filter p . filter q == filter (\x -> p x && q x)
There are a few other methods that should be added to this typeclass. In
particular,
filterMap :: (a -> Maybe b) -> f a -> f b
which fuses the first rule and saves the redundant passes and evaluations.
In a similar vein you may want a class for versions that allow
applicative/monadic functions along with the sequenceA/sequence
capabilities of Traversable.
--
Live well,
~wren
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