Generalize filterM to Applicative

[Mario Blažević]

On 14-12-30 12:34 PM, David Feuer wrote:
> I realized just now that we can actually make the type a little bit
> more general still, interpreting `filterM` for lists as being an
> applicative filter *producing* lists:
>
> filterM :: (Applicative f, Foldable t) => (a -> f Bool) -> t a -> f [a]

	I'm +1 on the more limited generalization proposal, but not on this 
one. I've been feeling for a while that we need a subclass of 
Traversable to generalize functions like filter, mapMaybe, and partition 
(potentially span, splitAt and others as well). If you call this missing 
class Filterable, the properly-generalized signature would be

 > filterM :: (Applicative f, Filterable t) =>
 > (a -> f Bool) -> t a -> f (t a)



>
> On Mon, Dec 29, 2014 at 3:17 PM, Edward Kmett <ekmett at gmail.com> wrote:
>> I think this is the interpretation I think we're probably best left with.
>>
>> I agree that names matter and that anachronistic labels confuse, but I think
>> it is the lesser of evils to widen the definition of the 'M' suffix than it
>> is to double up on almost all the names taken in the environment and force
>> the entire community to go through a positively gigantic deprecation cycle.
>>
>> -Edward
>>
>> On Mon, Dec 29, 2014 at 1:31 PM, Andreas Abel <andreas.abel at ifi.lmu.de>
>> wrote:
>>>
>>> Or you can widen the interpretation of suffix ...M as "effectful", which
>>> could be a monadic or applicative effect.
>>>
>>> On 29.12.2014 16:50, Kim-Ee Yeoh wrote:
>>>>
>>>>
>>>> On Mon, Dec 29, 2014 at 7:58 PM, Edward Kmett <ekmett at gmail.com
>>>> <mailto:ekmett at gmail.com>> wrote:
>>>>
>>>>      Is it "madness" to want to avoid namespace proliferation and
>>>>      maximize the usefulness of an existing combinator now that the
>>>>      constraints that forged it have changed to allow it to be slightly
>>>>      more permissive?
>>>>
>>>>
>>>> Madness is such strong language for this august list.
>>>>
>>>> May I speak on behalf of haskell newcomers for a time?
>>>>
>>>> Haskell places such an emphasis on uniformity and regularity. Functions
>>>> with names that end with M once meant they were monadic variants of
>>>> those that don't. That's no longer uniformly the case, because of the
>>>> FAM restructuring.
>>>>
>>>> The names of functions matter.
>>>>
>>>> Anachronistic labels confuse.
>>>>
>>>> Leaving filterM with a type signature of Applicative cannot be the
>>>> long-term solution.
>>>>
>>>> -- Kim-Ee
>>>>
>>>>
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>>>
>>>
>>> --
>>> Andreas Abel  <><      Du bist der geliebte Mensch.
>>>
>>> Department of Computer Science and Engineering
>>> Chalmers and Gothenburg University, Sweden
>>>
>>> andreas.abel at gu.se
>>> http://www2.tcs.ifi.lmu.de/~abel/
>>
>>
>>
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-- 
Mario Blazevic
mblazevic at stilo.com
Stilo International

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