Hi Igor,
You can create a new type for a -> Bool with which you can compose to
create complex filters. I find that an elegant approach:
import Data.Monoid
newtype Filter a = Filter {
runFilter :: a -> Bool
}
-- We can turn filter into a monoid, behaving like and.
instance Monoid (Filter a) where
mempty = Filter $ const True
mappend (Filter a) (Filter b) = Filter $ \x -> a x && b x
andF :: Filter a -> Filter a -> Filter a
andF = mappend
-- We can also create or
orF :: Filter a -> Filter a -> Filter a
orF (Filter a) (Filter b) = Filter $ \x -> a x || b x
mkFilter :: (a -> Bool) -> Filter a
mkFilter = Filter
-- Some example filters
evenFilter = mkFilter even
lessThanHundred = mkFilter (<100)
specificFilter :: Filter a -> [a] -> [a]
specificFilter xs ys = filter (runFilter xs) ys
-- get less then hundred and even
testa = specificFilter (evenFilter <> lessThanHundred) [1..1000]
-- get less then hundred or even
testb = specificFilter (evenFilter `orF` lessThanHundred) [1..1000]
Cheers,
Edgar
On Sun, Jul 7, 2013 at 7:26 PM, Igor Pinheiro Leão
Hi Guys, sorry in upsetting you again. Is there a way in which I can filter one list of function, one function at time, to a list of elements, declaring this and only this function.
It would be exactly like this:
specifcFilter :: [(a->Bool)] -> [a] -> [a]
where for each element of type 'a' on the second list it would exist a function on the first list that would be applied to it filtering.
Kind regards, Igor
-- Igor Vinícius Graduando em Ciência da Computação
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