New implementation for `ImpredicativeTypes`

Alex Rozenshteyn rpglover64 at
Fri Sep 6 02:30:54 UTC 2019

I didn't say anything when you were requesting use cases, so I have no
right to complain, but I'm still a little disappointed that this doesn't
fix my (admittedly very minor) issue:

For those who don't want to click on the reddit link: I would like to be
able to write something like map show ([1, 'a', "b"] :: [forall a. Show a
=> a]), and have it work.

On Wed, Sep 4, 2019 at 8:13 AM Alejandro Serrano Mena <trupill at>

> Hi all,
> As I mentioned some time ago, we have been busy working on a new
> implementation of `ImpredicativeTypes` for GHC. I am very thankful to
> everybody who back then sent us examples of impredicativity which would be
> nice to support, as far as we know this branch supports all of them! :)
> If you want to try it, at
> you can find the result of the pipeline, which includes builds for several
> platforms (click on the "Artifacts" button, the one which looks like a
> cloud, to get them). The code is being developed at
> Any code should run *unchanged* except for some eta-expansion required for
> some specific usage patterns of higher-rank types. Please don't hesitate to
> ask any questions or clarifications about it. A merge request for tracking
> this can be found at
> Kind regards,
> Alejandro
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> ghc-devs mailing list
> ghc-devs at
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