Re: [ghc-steering-committee] Proposal #209: Levity polymorphic lift. Recommendation: accept

Too bad. It sounds like we agree then that removing the default implementation is the best option we have. I've proposed that on GitHub and will approve the modified proposal assuming there aren't any last objections. On Thu, Mar 14, 2019, at 15:29, Richard Eisenberg wrote:

...

On Mar 7, 2019, at 10:27 PM, Eric Seidel wrote:

I imagine we could get away with

default liftTyped :: (r ~ 'LiftedRep, Data t) => t -> Q (TExp t) liftTyped = unsafeTExpCoerce . liftData

Sadly, this won't work. GHC just isn't clever enough. The problem is that, for (Data t) to be well-typed, we need (r ~ LiftedRep). This means that the typedness of one constraint depends on another. The internal language can do this fine, but type inference isn't up to the challenge. We'll get there some day. But I think this way is a dead-end for now.

So it sounds (to me) that the best way forward is to remove the default implementation of liftTyped altogether....

Richard