Changes to Typeable

Edward Kmett ekmett at
Tue Feb 14 16:21:13 CET 2012

Sent from my iPhone

On Feb 14, 2012, at 3:00 AM, Roman Leshchinskiy <rl at> wrote:

> On 13/02/2012, at 11:10, Simon Peyton-Jones wrote:
>> |  Should there perhaps be a NewTypeable module which could then be renamed
>> |  into Typeable once it is sufficiently well established?
>> I started with that idea, but there would be a 2-stage process:
>> * Step 1: (when PolyTypable becomes available) People change to import Data.PolyTypeable
>> * Step 2: (when PolyTypeable becomes Typeable) People change back to Data.Typeable
> The problem is that libraries generally have to support multiple versions of GHC and this would become harder. But that isn't too bad, preprocessor magic solves it. It would be easier if we could define Typeable1 etc. as an alias for Typeable (since they now mean the same thing) but we don't have class aliases.

No, but we do have the ability to make type aliases for classes now that we have constraint kinds, and typeOf1, etc. are amenable to the same implementation technique as typeOf.

> My main objection is still the fact that a central library will now rely on a highly experimental language feature which isn't even really available in a GHC release yet (my understanding is that support for polykinds in 7.4 is shaky at best). IMO, this should be avoided as a matter of policy. I realise that others are much less conservative than me in this respect, though.
> Roman
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