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Filter to a list of function

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Igor Pinheiro Leão

7 Jul 2013 7 Jul '13

1:26 p.m.

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 Pinheiro Leão

7 Jul 7 Jul

1:32 p.m.

Hum.. really sorry about the grammar mistakes. 2013/7/7 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

-- Igor Vinícius Graduando em Ciência da Computação

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Benjamin Edwards

1:49 p.m.

But lists only contain one type, so I am not sure what you want to do here. Get elements that are true for all filters? For at least one filter? There is no sub typing relationship... On 7 Jul 2013 18:34, "Igor Pinheiro Leão" wrote:

Hum.. really sorry about the grammar mistakes.

2013/7/7 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

-- Igor Vinícius Graduando em Ciência da Computação

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

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Brent Yorgey

1:53 p.m.

No need to apologize for grammar. =) There are lots of people in the Haskell community with many different levels of English fluency, we all just try to communicate as best we can. As for your question, I am not quite sure I understand what you want. Do you mean that you want to "zip" the lists together, applying each function to one element? As in specificFilter [(>3), even, (<19)] [6, 7, 8] = [6,8] because 6>3 and 8>19 (but 7 is not even)? If that is what you mean, then you could do something like specificFilter ps xs = map snd . filter fst $ zipWith (\p x -> (p x, x)) ps xs If you mean something else, then maybe you could give some examples of what you want? -Brent On Sun, Jul 07, 2013 at 02:32:21PM -0300, Igor Pinheiro Leão wrote:

Hum.. really sorry about the grammar mistakes.

2013/7/7 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

-- Igor Vinícius Graduando em Ciência da Computação

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

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Igor Pinheiro Leão

2:09 p.m.

Thank you Benjamin and Brent! Brent, yes, that was exactly what i wanted to do. Thank you! Kind regards, Igor 2013/7/7 Brent Yorgey

No need to apologize for grammar. =) There are lots of people in the Haskell community with many different levels of English fluency, we all just try to communicate as best we can.

As for your question, I am not quite sure I understand what you want. Do you mean that you want to "zip" the lists together, applying each function to one element? As in

specificFilter [(>3), even, (<19)] [6, 7, 8] = [6,8]

because 6>3 and 8>19 (but 7 is not even)?

If that is what you mean, then you could do something like

specificFilter ps xs = map snd . filter fst $ zipWith (\p x -> (p x, x)) ps xs

If you mean something else, then maybe you could give some examples of what you want?

-Brent

On Sun, Jul 07, 2013 at 02:32:21PM -0300, Igor Pinheiro Leão wrote:

...
Hum.. really sorry about the grammar mistakes.

2013/7/7 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

-- Igor Vinícius Graduando em Ciência da Computação

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

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

-- Igor Vinícius Graduando em Ciência da Computação

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Edgar Klerks

2:25 p.m.

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

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

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Igor Pinheiro Leão

10:12 p.m.

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

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

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

-- Igor Vinícius Graduando em Ciência da Computação

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Benjamin Edwards
Brent Yorgey
Edgar Klerks
Igor Pinheiro Leão