Generalize filterM to Applicative

[Edward Kmett]

> 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.

What would be gained for having a crippled version of filterM that only
worked for monads, but didn't use any of the additional power gained by
that constraint?

To borrow from John Maynard Keynes: "When the facts change, I change my
mind. What do you do, sir?" =)

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?

-Edward

On Sun, Dec 28, 2014 at 1:14 PM, Yitzchak Gale <gale at 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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