#9123: Need for higher kinded roles -------------------------------------+------------------------------------- Reporter: simonpj | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.2 checker) | Keywords: Roles, Resolution: | QuantifiedConstraints 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): I wonder if you stumbled on the solution there: perhaps processing a quantified constraint ''should'' proactively spit out superclass constraints. That is, we can imagine that `forall p q. Coercible p q => Coercible (m p) (m q)` proactively produces `forall p q. Coercible p q => m p ~R# m q`, based on the superclass constraint to `Coercible`. Note that this works nicely in general. If you have `forall x. D x => Ord x` and find yourself needing `Eq t`, then we can use the quantified constraint and reduce `Eq t` to `D t`. This more general solution would solve this problem here nicely, I think. And it just seems Right. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9123#comment:43 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler