Re: Generalize filterM to Applicative

29 Dec 2014

      +1 generalize filterM to Applicative.

I am not fond of ...A functions if the ...M functions can be generalized 
to Applicative, so -1 on filterA.

On 29.12.2014 00:15, Edward Kmett wrote:
...
The way we've been doing things so far is to just generalize the
existing signature and keep the existing name.
This avoids needless name proliferation and keeping needlessly
over-constrained versions of code lying around.
FWIW- I have no objection to a more general filterM.
-Edward
On Sun, Dec 28, 2014 at 1:14 PM, Yitzchak Gale mailto:gale@sefer.org> wrote:
David Feuer wrote:
    > filterM          :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
    > We could change this to
    > filterM :: (Applicative f) => (a -> f Bool) -> [a] -> f [a]
Following the usual conventions, that should be filterA.
And I'm not sure where it should go. We could also
    generalize the list parameter to a Foldable and put
    it in Data.Foldable, but I don't think that would be
    very useful, since in the end it anyway returns a list.
We could add a new "Filterable" class, but that's also
    probably not too useful for anything not list-like. For
    example, Data.Tree has two useful filters, filterGraft
    and filterPrune.
So maybe it should go in Control.Applicative? Not very
    satisfying. Any ideas?
As for the fate of the name filterM:
Personally, I would want filterM to remain a convenient
    type-specialized version for monads. But recent madness
    would turn it into yet another unsightly overly polymorphic
    namespace wasting synonym for filterA.
Thanks,
    Yitz
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