GHC 6.12.1 and impredicative polymorphism

[Sittampalam, Ganesh]

Simon Peyton-Jones wrote:

> Fortunately, I don't think a lot of people use the feature in anger. 
> Please yell if you *are* using impredicative polymorphism for
> something serious.  But if you are, we need to think of a workaround.
> The current situation seems unsustainable.   

I think darcs is using it. At least, I had to enable
ImpredicativePolymorphism to successfully build darcs with GHC 6.11 (a
snapshot from about February), although the flag is not required with
GHC 6.10. I haven't had a chance to try with the RC yet, but will do
this weekend.

I'll have to check the full details of why it's needed, but from memory
I think it can be worked around at the cost of more verbosity by using
some newtypes in appropriate places.

Cheers,

Ganesh

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