- GADTs:
- Stable and well documented,
- Adding indices to types is one of the main reasons one would like to have MultiParamTypeClasses and TypeFamilies on,
- I find the GADT syntax much nicer (but this is an extremely personal choice.)
I voted against GADTs and am not yet inspired to change that vote: GADTs cause trouble for type inference. For example:
data T a where
MkT :: Int -> T a
foo (MkT x) = x
GHC can infer that foo :: T a -> Int.
But if I change this to
data T a where
MkT :: Int -> T Int
foo (MkT x) = x
(where T is now a GADT) GHC can no longer infer my type. It complains about untouchable variables. This is a case of a "bad failure mode", where a simple error in input can lead to a very complicated error message in output. I thus think that users should knowledgeably opt into GADTs. Maybe if we had a much gentler error message in place here, I could be convinced otherwise. But, for now:
Vote against GADTs!
Richard
PS: A counterpoint is RankNTypes, which I voted for. It would be hard to accidentally write a higher-rank type, and I find the error messages that arise are much clearer. Yes, it's an advanced feature, but one that is appropriately (in my opinion) gated in the code.