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 <[email protected]> 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 > _______________________________________________ > ghc-devs mailing list > [email protected] > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >
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