Getting rid of -XImpredicativeTypes

Alejandro Serrano Mena trupill at
Mon Sep 26 07:10:51 UTC 2016

2016-09-26 9:00 GMT+02:00 Ben Gamari <ben at>:

> Simon Peyton Jones via ghc-devs <ghc-devs at> 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]
> Impredicative-2015
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