Re: [Haskell-beginners] Filter to a list of function

Thank you Edgar! 2013/7/7 Edgar Klerks

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 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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-- Igor Vinícius Graduando em Ciência da Computação