#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 don't see how you can use the implication given the way you've described. (Look at your "use given:" line. How is it working?) Indeed, I think the program, as written, should be rejected. Role inference correctly deems the final index to `StateT` to have a nominal role, as it appears as an argument to a type variable. Therefore, I think the solver is doing the right thing. On the other hand, what happens if you defined `StateT` to be a newtype? If `StateT` were a newtype, then the solver could (plausibly) unwrap the newtype, and then the given would apply. I don't know if the solver does this -- it's quite conceivable that changing `StateT` to newtype won't change the behavior, but then at least we'd have a possible way out. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9123#comment:52 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler