Proposal: unify constant functors

[wren ng thornton]

On 4/30/12 8:38 AM, Brent Yorgey wrote:
> On Mon, Apr 30, 2012 at 01:33:39AM -0400, wren ng thornton wrote:
>> 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
>
> Why do it this way?  For backwards compatibility?  It seems to me it
> would be much more consistent to remove the one in
> Control.Applicative and have all the fundamental functor combinators
> in one place.


I think that Const/Constant should be in base because it's such a 
primitive thing and used all over the place. Other than that, I was 
mainly aiming for minimal breakage in resolving the duplication.

-- 
Live well,
~wren