Guarded Impredicativity

Matthew Pickering matthewtpickering at
Fri Jun 28 12:17:01 UTC 2019

Have you modified how typed quotations are type checked? For example,
with your patch I would hope that

[| id |] :: Code (forall a . a -> a)

would be accepted?

I'll try it out. This patch will have big ramifications for the typed
template haskell community.


On Fri, Jun 28, 2019 at 1:12 PM Alejandro Serrano Mena
<trupill at> wrote:
> Dear all,
> We are trying to bring back `ImpredicativeTypes` into GHC by using the ideas in the "Guarded Impredicative Polymorphism" paper [].
> For now I have produced a first attempt, which lives in 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 impredicative-ghc
> > cd impredicative-ghc
> > git remote add trupill git at
> > git fetch trupill
> > git checkout trupill master
> Thanks very much in advance,
> Alejandro
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at

More information about the ghc-devs mailing list