Even though I share with Arnaud a bit of sadness about GADTs, I think that the set of extensions we ended up with is quite coherent:
- We switch on by default some “simple syntactic” extensions, like new forms of numbers, empty cases, and so on.
- We put all generically-derivable type classes on the same ground; no more special treatment of Eq, Ord… just because they were there earlier.
- We enable the most battle-tested extensions to type classes which never compromise the safety.
- We make the kind system more regular by enabling PolyKinds, ConstraintKinds, and KindSignatures; but no fancy promotion from DataKinds.
I think it’s good that we’ve decided to keep *both* GADTs and TypeFamilies out. For me this gives the message that type level computation is still something we considered advanced and not fully settled; hence it’s still under a pragma.