Guarded Impredicativity

[Matthew Pickering]

Here is a nix one liner to test the current state of the branch.

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

On Fri, Jun 28, 2019 at 1:12 PM Alejandro Serrano Mena
<trupill at gmail.com> wrote:
>
> Dear all,
>
> We are trying to bring back `ImpredicativeTypes` into GHC by using the ideas in the "Guarded Impredicative Polymorphism" paper [https://www.microsoft.com/en-us/research/publication/guarded-impredicative-polymorphism/].
>
> For now I have produced a first attempt, which lives in https://gitlab.haskell.org/trupill/ghc. It would be great if those interested in impredicative polymorphism could give it a try and see whether it works as expected or not.
>
> The main idea behing "guarded impredicativity" is that you can infer an impredicative instantiation for a type variable in a function call if you have at least one given argument where that type variable appears under a type constructor different from (->).
> For example, consider the call `(\x -> x) : ids`, where `ids :: [forall a. a -> a]`. Since in the type of `(:)`, namely `forall a. a -> [a] -> [a]`, the variable `a` appears under the `[]` constructor and that second argument is given, we are allowed to instantiate `a := forall a. a -> a`. On the other hand, if we try to do `ids <> ids`, where `(<>)` is monoid concatenation with type `forall m. Monoid m => m -> m -> m`, we are forced to instantiate `m` with a not-polymorphic type because at no point the variable appears under a type constructor.
>
> Just for reference, the best to get a working clone is to follow these steps:
> > git clone --recursive https://gitlab.haskell.org/ghc/ghc impredicative-ghc
> > cd impredicative-ghc
> > git remote add trupill git at gitlab.haskell.org:trupill/ghc.git
> > git fetch trupill
> > git checkout trupill master
>
> Thanks very much in advance,
> Alejandro
>
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