Proxy and new-typeable

Edward Kmett ekmett at
Fri Mar 22 06:25:33 CET 2013

On Fri, Mar 22, 2013 at 12:03 AM, Ben Gamari <bgamari.foss at> wrote:

> Roman Cheplyaka <roma at> writes:
> > Right now Data.Typeable is proxy-agnostic. The proposal is just to
> > export a Proxy type for convenience (alternatively, the user can define
> > her own Proxy or use one from tagged).
> > What exactly forces you to support both proxies in your code?
> >
> > (I'm reluctant to have many proxy types scattered around mainly because
> > of unnecessary name conflicts, but I'd like to understand your concerns
> > too.)
> >
> It seems the options before us are,
>   a) Use Proxy strictly internally in Data.Typeable. In this case users
>      will continue to use the Proxy types in tagged and elsewhere as they
>      already happily do
>   b) Export Proxy from Data.Typeable in its current state, accepting
>      that users relying on external Proxy types will need to either
>      accept a loss of functionality, explicitly hide Typeable's Proxy,
>      or rely on orphan instances
>   c) Find a way to bring Typeable's Proxy type to a level of
>      functionality comparable to that currently available outside of
>      base
> Having tried to compile a good amount of code using Data.Typeable's
> Proxy, it seems clear to me that (b) is the worst of the three
> outcomes. There are a good number of packages which rely on the
> instances provided by external Proxy types. Removing these will bring
> great deal of pain in the short turn and pose a large maintenance burden
> moving forward.
> Tonight I tried to implement (c) but found that this might be
> quite tricky without establishing some very brittle cyclic imports in
> base. As it stands, nearly everything in base imports Typeable
> somehow. Requiring Typeable to in turn import Applicative, Foldable,
> Traversable, and others places some very unfortunate cycles in the
> dependency structure. Even after an hour of hacking and 200 lines of
> changes, I still hadn't succeeded in getting base to build with a
> reasonable set of Proxy instances (although, admittedly, this might just
> be due to my inexperience with this sort of issue).

You don't need to make it cyclic, if the edge already exists one way in the
graph just follow it back and put the instance with the class.

> In light of these points, I believe that (a) is the course of least
> pain. Those users that need Proxy already happily rely on
> packages outside of base. Meanwhile base can use its own (necessarily
> minimal) Proxy internally without issue. This approach requires minimal
> changes in base and avoids unnecessary breakage of user code, all while
> depriving no one of current or future functionality. Admittedly, there
> may be a couple more Proxy types in the namespace than would otherwise
> exist, but this seems like a small price to pay to avoid the breakage
> and pain of (b) and (c) above.

I would be happy with (a) or (c).

That said, given (c) I could make Data.Proxy in tagged re-export the
Data.Typeable Proxy whenever we're on a new enough GHC and I can invert all
the other dependency edges. It'll be work for me, but it means that when a
user goes to mix the existing tagged Proxy with code from Data.Typeable it
'just works' and there aren't two types named Proxy floating around that'll
have lots of users.
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