> It hasn't even been decided yet which extensions are stable
It's essentially known, but it's not formally agreed. The fact that this proposal exists is evidence of that!
> GHC2021 still promises compatibility in future compiler versions. So either, you'll have GHC retain inferior semantics because of backwards compatibility, or multiple ways of doing the same thing.
GHC2021 will always provide ScopedTypeVariables. A future edition will probably provide TypeAbstractions instead. Being able to make progress to the default language like this is the point of having language editions!
But DataKinds is not stable. It's one of the most stable extensions possible! The link you provided even says it's stable:
https://github.com/telser/ghc-proposals/blob/initial-extensi...
> It hasn't even been decided yet which extensions are stable
It's essentially known, but it's not formally agreed. The fact that this proposal exists is evidence of that!
> GHC2021 still promises compatibility in future compiler versions. So either, you'll have GHC retain inferior semantics because of backwards compatibility, or multiple ways of doing the same thing.
GHC2021 will always provide ScopedTypeVariables. A future edition will probably provide TypeAbstractions instead. Being able to make progress to the default language like this is the point of having language editions!