Just to amplify: we are very interested to

• Get some idea of whether anyone  cares about impredicativity.  If we added it to GHC, would you use it?  Have you ever bumped up Haskell’s inability to instantiate a polymorphic function at a polytype.

• Get some idea of whether the particular form of impredicativity described in the paper would be expressive enough for your application.

Simon

From: ghc-devs <ghc-devs-bounces@haskell.org> On Behalf Of Alejandro Serrano Mena
Sent: 28 June 2019 13:12
To: ghc-devs@haskell.org
Subject: Guarded Impredicativity

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@gitlab.haskell.org:trupill/ghc.git

> git fetch trupill

> git checkout trupill master

Thanks very much in advance,

Alejandro