#2532: Add Typeable instance to Data.Unique

[Sittampalam, Ganesh]

Jim Apple wrote:

> http://hackage.haskell.org/trac/ghc/ticket/2532
>
> Discussion deadline: Sept 7

I believe that the combination of this instance, the behaviour of Data.Unique, and the behaviour of hs-plugins is broken.

In particular, as I understand the behaviour of hs-plugins, it is already possible to generate Unique values inside a plugin that conflict with Unique values generated in the main program, and this instance will allow those values to be passed across the plugin boundary.

It's not clear to me what is to blame here. hs-plugins does something dodgy, in that it hacks the behaviour of Data.Typeable instances so that the same types get identical instances on both sides of the plugin boundary. Data.Unique is doing something dodgy by using a global variable. (There's a long discussion about this on haskell-café at the moment.) The combination is what leads to the brokenness.

I will test this to confirm as soon as I can, but as the deadline is approaching I thought I should raise the issue immediately.

Cheers,

Ganesh

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