GHC 6.12.1 and impredicative polymorphism
ganesh.sittampalam at credit-suisse.com
Fri Oct 30 06:22:45 EDT 2009
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
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.
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