#9518: Improve error message for unacceptable role annotations -------------------------------------+------------------------------------- Reporter: dmcclean | Owner: Type: feature | Status: infoneeded request | Milestone: Priority: low | Version: 7.8.3 Component: Compiler | Keywords: (Type checker) | Architecture: Unknown/Multiple Resolution: | Difficulty: Unknown Operating System: | Blocked By: Unknown/Multiple | Related Tickets: Type of failure: | None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by dmcclean): I'm sorry guys, I haven't been haskelling much in the last couple of months. I can't remember how to construct an example, in part because I am still confused by what the rules are, which is why I had trouble interpreting the original error message. To elaborate on what I find confusing about it, I think that it would help if it said *where* `a` was used in a context that required a nominal type role, in the same way that some of the other type error messages do. Annotation says representation but role nominal is required, arising from use of (type constructor) in the type expression (...) at (...) I have a terrible headache or I would research the rules again to figure out how to write a program that produces this error on purpose. I'll attempt it tomorrow if there's time. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9518#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler