2016-09-26 9:00 GMT+02:00 Ben Gamari <ben@smart-cactus.org>:

Simon Peyton Jones via ghc-devs <ghc-devs@haskell.org> writes:

> Friends
>
> GHC has a flag -XImpredicativeTypes that makes a half-hearted attempt
> to support impredicative polymorphism. But it is vestigial.... if it
> works, it's really a fluke. We don't really have a systematic story
> here at all.
>
Out of curiosity, what ever happened to the most recent attempt at
addressing impredicativity [1]?

It turned our that the new system being proposed was not better than the current one. In particular, it was almost impossible to know upfront whether a program using impredicative types would need an annotation to typecheck. Furthermore, I think it would imply a humongous amount of changes to GHC, for not a very large gain.

As far as the proposal process is concerned, it would likely be a good
idea to put together a proposal so we have somewhere to collect
comments. That being said, it needn't be terribly lengthy given that we
are merely removing a feature.

Incidentally, that reminds me that I have some documentation fixes to
the ghc-proposals repository that I should proof-read and push.

Cheers,

- Ben

[1] https://ghc.haskell.org/trac/ghc/wiki/ImpredicativePolymorphism/Impredicative-2015

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