Re: Guarded Impredicativity

No, up to now the only changes are in the type checking of applications and variables. However, I guess that it would be possible to give [| id |] the type Code (forall a. a -> a) with a explicit type signature (the system always allows impredicative instantiation is explicitly marked), but without the annotation it would the usual type forall a. Code (a -> a). El vie., 28 jun. 2019 a las 14:17, Matthew Pickering (< matthewtpickering@gmail.com>) escribió:

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.

Matt

On Fri, Jun 28, 2019 at 1:12 PM Alejandro Serrano Mena 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-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

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