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-p... ]. 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