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

[Edward Kmett]

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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