[Haskell-beginners] Filter to a list of function

[Igor Pinheiro Leão]

Thank you Edgar!


2013/7/7 Edgar Klerks <edgar.klerks at gmail.com>

> 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 at 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
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
>


-- 
Igor Vinícius
Graduando em Ciência da Computação
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20130707/6115059b/attachment.htm>