Add Zero and One to Data.Functor (mirroring V1, U1)

[Edward Kmett]

I'm similarly weakly +1 on adding Data.Functor.Zero, but -1 on One because
of the existence of Proxy.

-Edward

On Wed, Jan 25, 2017 at 8:36 PM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:

> You linked to the Trac issue [1], but for the purposes of motivating
> this proposal, I think it's important that we reiterate why you want
> this.
>
> First of all, I think we should be clear that you're proposing to add
> these in the modules:
>
> * Data.Functor.One
> * Data.Functor.Zero
>
> Right? Not literally "Data.Functor", as the title might mislead you
> into believing.
>
> Also, for a reason why you might want these - there are currently
> analogs in the GHC.Generics [2] module:
>
> * data U1 a = U1 -- isomorphic to One
> * data V1 a -- isomorphic to Zero
>
> But this is an unfortunate situation, since I'd rather not recommend
> that folks import a generics library to use what should be
> general-purpose datatypes.
>
> Now, as for the proposal itself:
>
> * I'm weakly +1 in favor of Zero. I think it'd be nice to have a
> proper * -> * counterpart to Void in base (that isn't V1, obviously).
> * I'm weakly -1 in favor of One, simply because we already have
> Data.Proxy, which came first.
>
> Ryan S.
> -----
> [1] https://ghc.haskell.org/trac/ghc/ticket/13177
> [2] http://hackage.haskell.org/package/base-4.9.1.0/docs/GHC-Generics.html
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