#11715: Constraint vs * -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.0.1-rc1 checker) | Resolution: | Keywords: Typeable Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Let's imagine a future where `(->) :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep). TYPE r1 -> TYPE r2 -> Type`. Then, we have the well-typed `(->) ConstraintRep LiftedRep (C a) (a -> a)`. But if we normalize w.r.t. newtypes (e.g., in `normaliseFfiType`), then we'll get `(->) ConstraintRep LiftedRep (a -> a) (a -> a)`, which is ill-kinded. In this case, we can just change `ConstraintRep` to `LiftedRep` as we normalize, but it's not clear that it will always be so easy. (But maybe it really will be, because we tend not to unwrap newtypes under other type constructors.) There's also the niggling problem that the type safety proofs in the presence of newtype axioms explicitly assume that axioms are homogeneous. But maybe this is unnecessary. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11715#comment:49 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler