Guarded Impredicativity

[Ryan Scott]

Good to know. Thanks for checking!

Ryan S.

On Fri, Jul 19, 2019 at 11:22 AM Artem Pelenitsyn
<a.pelenitsyn at gmail.com> wrote:
>
> Hello Ryan,
>
> Your example seems to work out of the box with the GI branch.
>
> With the oneliner Matthew posted before:
> nix run -f https://github.com/mpickering/ghc-artefact-nix/archive//master.tar.gz \
> ghc-head-from -c ghc-head-from \
> https://gitlab.haskell.org/mpickering/ghc/-/jobs/114593/artifacts/raw/ghc-x86_64-fedora27-linux.tar.xz
> It is really easy to check. Also, I didn't see anywhere mentioned that one need to provide -XImpredicativeTypes. The whole example, therefore, is:
>
> {-#LANGUAGE ImpredicativeTypes, ConstraintKinds #-}
> module M where
> type F f = (Functor f, forall a. Eq (f a))
>
> --
> Best, Artem
>
> On Fri, 19 Jul 2019 at 09:18, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
>>
>> I have another interesting application of guarded impredicativity that
>> I want to bring up. Currently, GHC #16140 [1] makes it rather
>> inconvenient to use quantified constraints in type synonyms. For
>> instance, GHC rejects the following example by default:
>>
>>     type F f = (Functor f, forall a. Eq (f a))
>>
>> This is because F is a synonym for a constraint tuple, so mentioning a
>> quantified constraint in one of its arguments gets flagged as
>> impredicative. In the discussion for #16140, we have pondered doing a
>> major rewrite of the code in TcValidity to permit F. But perhaps we
>> don't need to! After all, the quantified constraint in the example
>> above appears directly underneath a type constructor (namely, the type
>> constructor for the constraint 2-tuple), which should be a textbook
>> case of guarded impredicativity.
>>
>> I don't have the guarded impredicativity branch built locally, so I am
>> unable to test if this hypothesis is true. In any case, I wanted to
>> mention it as another motivating use case.
>>
>> Ryan S.
>> -----
>> [1] https://gitlab.haskell.org/ghc/ghc/issues/16140
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