Guarded Impredicativity

Alejandro Serrano Mena trupill at
Fri Jun 28 12:11:58 UTC 2019

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
> 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,
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the ghc-devs mailing list