Proposal: unify constant functors

[wren ng thornton]

Hello all,

I recently (re)noticed the following duplication:

     base:Control.Applicative
         newtype Const a b = Const { getConst :: a }
         instance Functor (Const m) where...
         instance Monoid m => Applicative (Const m) where...

     transformers:Data.Functor.Constant
         newtype Constant a b = Constant { getConstant :: a }
         instance Functor (Constant a) where...
         instance Foldable (Constant a) where...
         instance Traversable (Constant a) where...
         instance (Monoid a) => Applicative (Constant a) where...

I don't see any reason for this redundancy. I propose we:

     (1) add the Foldable and Traversable instances to base, and
     (2) deprecate transformers:Data.Functor.Constant

This will cause breakage to any orphan instances of Foldable/Traversable 
for Const, but that seems fine by me. Eventually we'll want to remove 
transformers:Data.Functor.Constant (or have it re-export the Const stuff 
from base:Control.Applicative); but that can be handled later.

Deadline: 14 May 2012.

-- 
Live well,
~wren