reproxy is quite negotiable.

Ever since its signature was generalized a version or two ago in tagged, it has started to feel quite silly.

If I move

coerce :: (Functor f, Contravariant f) => f a -> f b

coerce = contramap absurd . fmap absurd -- using absurd from 'void'
-- or equivalently: coerce = contramap (const ()) . fmap (const ())

from lens up into the contravariant package, then the role that function plays can be replaced entirely with that more general purpose combinator in "user land" without needing any funny looking functions in base.

Proxy is both Contravariant and a normal covariant Functor as it doesn't use its argument, just like Const m. reproxy originally witnessed this fact just for Proxy, but that fact in its broader generality has since been exploited to make getters and folds in lens, so reproxy is basically just residue of an old approach.

-Edward

On Tue, Jul 30, 2013 at 6:44 PM, Shachaf Ben-Kiki <shachaf@gmail.com> wrote:

On Mon, Jul 22, 2013 at 1:23 AM, José Pedro Magalhães <jpm@cs.uu.nl> wrote:
> Thanks for bringing this up again. This was started in my data-proxy branch
> of base,
> but never really finished. We definitely want to have this in 7.8, and I
> think there's
> only some minor finishing work to do (check if we have all the instances we
> want,
> document, etc.). Perhaps you can look through what's there already, and what
> you
> think is missing? I'm more than happy to accept contributing patches too :-)
>
>

I'm looking at the current state of Data.Proxy in base (it looks like
Richard merged data-proxy into master) and it looks pretty good
instance-wise. Issues I'm aware of:

* It looks like there's a SafeHaskell issue -- should this be marked
Trustworthy? See https://github.com/ekmett/tagged/pull/13
* tagged's Data.Proxy defines some useful functions that aren't
present in base. Two of them can move into tagged's Data.Tagged, but
the other two should probably go in base's Data.Proxy. In particular

asProxyTypeOf :: a -> proxy a -> a
asProxyTypeOf = const

reproxy :: proxy s -> Proxy t
reproxy _ = Proxy

When these are fixed, tagged can probably shuffle a couple of
functions around and then use base's Data.Proxy rather than exporting
its own module (for GHC ≥ 7.7).

(By the way: Some instances are slightly different from what GHC would
derive -- e.g. «_ == _ = True» is different from «Proxy == Proxy =
True», which is ()'s Eq behavior. I think this is OK but I wanted to
mention it.)

Thanks,

Shachaf

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