#8503: New GeneralizedNewtypeDeriving check still isn't permissive enough -------------------------------------+------------------------------------ Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: #8541 -------------------------------------+------------------------------------ Comment (by simonpj): For the recurisve-newtype problem (ie the main topic of this ticket, now that we have moved polykinded `Coercible` to #8541), the approach (developed by Joachim and me) is to A) Add a feature to the constraint solver to prevent recursive dictionaries for specially marked instances (for now only used for Coercible). Rationale: Such dictionaries (which are fine for most classes like Show) would make coerce diverge. Implementation: Use the depth counter and do not use lower depths to solver constraints with a higher depth. This is of course a very conservative approximation, but should be sufficient. B) Use the regular constraint depth bound to prevent looping at compile time. In order for that to be more useful, count constraint solving and type function resolving separately. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8503#comment:24 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler