#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 simonpj): The straw-man proposal is that with `{-# UNDECIDABLE #-}` (or some other pragma name) on a closed type family, the surely-apart check is strengthened, allowing more reductions to fire. Richard, you rightly point out that if you put that on `Equal`, then `Equal x [x]` would return `False`, as you'd expect if all types were finite. But you also claim that if you can define an infinite type, then you can get `unsafeCoerce`. I believe you (c.f Section 6 of the [http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/ Closed Type Families paper]). But can you exhibit an example? And if you can, can you translate it back into an example using overlapping classes, probably with equality superclasses? If so, perhaps our existing compiler is unsound! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9918#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler