#9918: GHC chooses an instance between two overlapping, but cannot resolve a clause within the similar closed type family -------------------------------------+------------------------------------- Reporter: qnikst | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by goldfire): Replying to [comment:17 simonpj]:
It's tantalising that overlapping type classes achieve the same end without risking unsoundness. Surely we must be able to replicate this in type families, somehow?
However, a mistake with type family reduction is unsound. A mistake with class instance selection is unpredictable/incoherent, but not unsound. I posit that we have such a mistake lurking here -- a loophole that, if we closed it, would prevent Oleg's working case from working. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9918#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler