Hi all, I am the shepherd for proposal #425, https://github.com/ghc-proposals/ghc-proposals/pull/425, proposing to add invisible binders in type declarations. The main payload of the proposal is to allow definitions like
data Proxy @k (a :: k) = Proxy
instead of today's
data Proxy (a :: k) = Proxy
which has no explicit binding site for k. This new syntax solves a number of smallish syntax conundra, as very well outlined in the proposal. In addition, the proposal includes two small unrelated tweaks to the syntax of type family instances; these are points (6) and (7) in the proposal. Both changes will break some obscure (but still realistic, knowing Haskell) programs, but both fixes are backward compatible. --- I recommend acceptance. The proposal is motivated nicely (do check out the examples) and solves a real problem. The new syntax fits in with other similar features. The small cleanups to existing syntax will lead to better error messages. The one drawback, as I see it, is that this proposal includes point (5), which is a breaking change to the way type synonyms work. Right now, we can say
type P = (Proxy :: k -> Type)
and GHC will infer P :: forall k. k -> Type. Under this proposal, you would have to write
type P @k = (Proxy :: k -> Type)
bringing k into scope explicitly. The fix is not backward compatible, and so I think this proposal should come with a migration strategy, where we warn about the former version for some releases before banning it. (Continuing to support it is possible, but it's very awkward to have a variable mentioned only in a synonym's right-hand side.) Sorry for the delay in producing this recommendation! Richard