new-typeable, new cast?

Edward Kmett ekmett at
Wed Jul 31 01:35:47 CEST 2013

I would really want to keep asProxyTypeOf around as it is commonly used and
has no other plausible home within tagged.

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.


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

> On Mon, Jul 22, 2013 at 1:23 AM, José Pedro Magalhães <jpm at>
> 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
> * 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
> _______________________________________________
> Libraries mailing list
> Libraries at
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the Libraries mailing list