Proposal: unify constant functors
Edward Z. Yang
ezyang at MIT.EDU
Mon Apr 30 08:38:31 CEST 2012
+1, seems reasonable.
Edward
Excerpts from wren ng thornton's message of Mon Apr 30 01:33:39 -0400 2012:
> 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.
>
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