[ghc-steering-committee] a plea for PolyKinds

Spiwack, Arnaud arnaud.spiwack at tweag.io
Fri Dec 4 08:18:29 UTC 2020

On Thu, Dec 3, 2020 at 5:40 PM Richard Eisenberg <rae at richarde.dev> wrote:

> > data App f a = App (f a)
> With PolyKinds, App gets the kind (k -> Type) -> k -> Type. Without
> PolyKinds, GHC has to use defaulting to get (Type -> Type) -> Type -> Type,
> which might not work at certain use sites.

You know how I dislike this automatic kind generalisation. This is not
going to win me over :-p .  I still think that the polymorphism inference
of PolyKind was a mistake. That being said, it's something that we have
committed to. It's going to be quite hard to change any time soon.

> * Supporting NoPolyKinds is difficult within GHC, and I've been waiting
> for the right time to propose removing support for it. PolyKinds is
> conceptually *simpler* than NoPolyKinds, as it doesn't require an awkward
> defaulting step.

On the other hand, I'd say that this is a rather strong argument. Who else
wants to remove `NoPolyKinds`? Simon PJ, what do you think?

Is it even possible while still supporting Haskell2010?
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