#8984: Improve output of failed GeneralizedNewtypeDeriving coercion due to type roles -------------------------------------+------------------------------------- Reporter: haasn | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.1 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 goldfire): With my update to the solver (preview available at http://github.com/goldfirere/ghc, `wip/rae-new-coercible` branch), the new error message is thus: {{{ T8984.hs:7:46: Couldn't match representation of type ‘cat a (N cat a Int)’ with that of ‘cat a (cat a Int)’ arising from the coercion of the method ‘app’ from type ‘cat a (cat a Int)’ to type ‘N cat a (N cat a Int)’ Relevant role signatures: type role N representational nominal nominal NB: We cannot know what roles the parameters to ‘cat a’ have; we must assume that the role is nominal When deriving the instance for (C (N cat a)) }}} I like it, and will close this ticket when I merge. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8984#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler