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 <ivpcl@cin.ufpe.br> wrote:

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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