Proposal: Migrate Data.Functor.{Sum, Product, Compose} from transformers to base.

[David Feuer]

The biggest problem with Constant is the bloody name. The one from
transformers is called Constant, while the one in Control.Applicative is
called Const, so they are incompatible. I have no idea how to fix that.
On Jul 23, 2015 7:18 PM, "Edward Kmett" <ekmett at gmail.com> wrote:

> As part of GHC 7.10 we moved Data.Functor.Identity from transformers into
> base. (We'd previously had a private, equivalent definition buried in
> Data.Traversable that we didn't export.)
>
> In a similar vein, I hereby propose moving Data.Functor.Sum,
> Data.Functor.Product, Data.Functor.Compose, and Data.Functor.Classes into
> base as they exist in transformers, without unduly bikeshedding their
> APIs.
>
> This would effectively reduce the role of transformers to simply
> supplying actual transformers and continue the general pattern of universal
> constructions migrating into base. Without bikeshedding their
> implementations this migration becomes a no-op for existing users.
>
> The most awkward of these is the Data.Functor.Classes API, but it affects
> a number of their instances, and without it, those instances would have to
> be orphans, so my gut reaction would be to say yes, we should move that as
> well. There are also some in-flight API changes in the works in the HEAD
> version of transformers for which a base-based version of the API could
> exploit DefaultSignatures to ease migration.
>
> I'm rather deliberately not including Data.Functor.Constant in this
> proposal as when it has been discussed in the past, Const from
> Control.Applicative has effectively grown to subsume that role. That
> said, we if we bring in the rest of this stuff we could migrate it to
> Data.Functor.Const and simply re-export it from Control.Applicative to
> avoid breakage.
>
> So, as a concrete proposal I propose that we:
>
> 1.) Move Data.Functor.Sum, Data.Functor.Product, Data.Functor.Compose and
> Data.Functor.Classes into base.
>
> 2.) Move Control.Applicative's Const into Data.Functor.Const, but
> continue to re-export it from Control.Applicative.
>
> 3.) Add whatever missing, sensible, instances make sense after the
> transition that were ruled out by these modules' previous placement.
> *e.g.* Data, Generic, Generic1.
>
> Discussion Period: 2 weeks
>
> -Edward Kmett
>
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