#13398: Associated type family instance validity checking is too conservative -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11450 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Upon further thought, I'm not even sure why this particular restriction was added in the first place. #11450 was about making the type variables used in a class instance head the same as the variables used in the same positions in the associated type family instances. This business about not allowing multiple associated type family instances feels completely orthogonal to that issue (indeed, I wasn't even aware of its existence until just now, as it seems to have been snuck in without a mention in the GHC 8.2 release notes). Moreover, this additional check is causing code in the wild to fail, and for reasons that I don't comprehend. Simon, what exactly is the problem with allowing code like what is shown in the original program? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13398#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler