[Haskell-cafe] [announcement] filtrable: class of filtrable containers

[Oleg Grenrus]

Hi M,

- I’d also like to see instances for types in containers, unordered-containers, vector and semigroup.
- The OtherLicense seems a bit scary (even the contents aren’t),
  is there a good reason why you don’t use more familiar MIT or BSD3?

If you don’t mind I can make a PR for the instances.


And one more comment:

The law:

    filter f = mapMaybe (liftA2 (<$) id (guard ∘ f))

is very hard to understand.

    filter f = mapMaybe (\x -> if f x then Just x else Nothing)

is longer, but IMHO much simpler. Or if you really want to code golf, then maybe:

    filter f = mapMaybe (mfilter f . Just)

- Oleg

> On 17 Feb 2016, at 10:32, Joachim Breitner <mail at joachim-breitner.de> wrote:
> 
> Hi M,
> 
> Am Dienstag, den 16.02.2016, 23:02 -0800 schrieb M Farkas-Dyck:
>> I quietly posted this library to Hackage nearly a year ago, but lately
>> learned that some seeking such a package had difficulty finding it, so
>> i announce it now ☺
>> 
>> https://hackage.haskell.org/package/filtrable
>> 
>> class Functor f => Filtrable f where
>>     mapMaybe :: (a -> Maybe b) -> f a -> f b
>>     catMaybes :: f (Maybe a) -> f a
>>     filter :: (a -> Bool) -> f a -> f a
>> 
>> For laws, see docs on Hackage.
> 
> You might want to add laws in the style of
> 
>    If this is also a Foldable, then
>       toList . mapMaybe f = mapMapybe f . toList
>       toList . catMaybes = catMaybes . toList
>       toList . filter f = filter f . toList
> 
> which would fix the behavior quite tightly.
> 
> I wonder if these laws (together with a “well behaved” Foldable) still
> allow any unexpected behavior.
> 
> And also whether they follow from your laws (but I don’t think so;
> mapMaybe could do something mean such as duplicating elements if there
> is at least one Nothing in the result).
> 
> Do you plan to add instances for all the other data structures in base
> that are filtrable?
> 
> Greetings,
> Joachim
> 
> --
> Joachim “nomeata” Breitner
>   mail at joachim-breitner.de • https://www.joachim-breitner.de/
>   XMPP: nomeata at joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F
>   Debian Developer: nomeata at debian.org
> 
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