#14101: Type synonyms can make roles too conservative -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: roles Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #8234 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Sorry -- I wrote a post here but somehow it got lost. (I'm sure this is my fault.) This is by design. Any non-nominal role on a type parameter must be assigned to a proper type parameter to that type. When you eta-reduce, there's no place to put the non-nominal role. Happily, Ryan's patch solves the problem and looks utterly correct to me. So, while the synonym's argument's role is still nominal, that fact becomes irrelevant during role inference. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14101#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler