#12087: Inconsistency in GADTs? -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: GADTs Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11540 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * cc: RyanGlScott (added) * status: closed => new * resolution: invalid => * related: => #11540 Comment: I disagree that this not "valid" Haskell. It's certainly not //standard// Haskell, but I don't see why it ought to be rejected. Perhaps one day after #11540 is fixed, we'll need some extra language extension enabled to use this feature, but I think GHC is more than capable of handling such a thing. For background context: the structure of GADT type signatures are currently validity checked in a rather simple-minded way in [http://git.haskell.org/ghc.git/blob/86abe0e03f6cc2392758d6b45390177d44896113... gadtDeclDetails]. It splits apart a single `forall` with `splitLHsSigmaTy` and checks if the return type is of the form `T a_1 ... a_n`, where `T` is the GADT tycon. But since there's nested sigma types here, GHC mistakenly believes that the return type is `Eq a => a -> F a`, which is bad news. I think it would be possible to tweak this check to accommodate nested foralls, though. Essentially, I think we'd need to create a version of [http://git.haskell.org/ghc.git/blob/86abe0e03f6cc2392758d6b45390177d44896113... tcSplitNestedSigmaTys] that works over `LHsType` instead of `Type`, and replace the use of `splitLHsSigmaTy` with that in `gadtDeclDetails`. Then we'd typecheck `MkF` as if it had the type `forall a. (Ord a, Eq a) => a -> F a`. This isn't exactly what the user //wrote//, but it's equivalent and serviceable. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12087#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler