Re: Proxy and new-typeable

* Shachaf Ben-Kiki [2013-04-18 23:43:41-0700]

On Thu, Apr 18, 2013 at 11:56 AM, Richard Eisenberg wrote:

...

Hi Ben,

Oops. I had updated this page [http://hackage.haskell.org/trac/ghc/wiki/TypeLevelReasoning] with details but forgot to advertise. Pedro and I are scheduled to discuss next week, and I do hope that this get implemented/committed before April is out.

Richard

There's another possible approach to the Proxy mess: Instead of using or mentioning Proxy anywhere in the Typeable API, make Maybe the "official" way to use typeRep.

The argument is something like: You want to get a TypeRep of a type, but you may or may not have a value of that type. If you do, you pass Just x. If you don't, you pass Nothing, annotated to the appropriate type. A situation where you may or may not have a value is exactly what Maybe is for, so it's not a hack. :-)

This is better than the Proxy approach because it lets typeRep have the functionality of typeOf -- typeOf x = typeRep (Just x) -- which it doesn't with plain Proxy. It's also better because Maybe is a Prelude type suited to the purpose which everybody already knows.

(Of course typeRep can keep the polymorphic type, which would make it Proxy-compatible. But we don't need to put Proxy in base, or re-export it from Data.Typeable, or even mention it in the documentation.)

I don't think it's possible. Proxy has to be kind-polymorphic, so that we can apply it to type constructors, constraints etc. But Maybe can't be kind-polymorphic, because it has a field of type 'a'. Thus, the kind of 'a' is forced to be *. Another problem with this approach is that it's not self-documenting. You have to say specifically that this argument is just a proxy and the semantics will be the same regardless of what you pass. But it was an interesting idea nevertheless. Roman