#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): GHC has always permitted types that are uninhabited (except by bottom); consider {{{ data Void -- No constructors }}} So I don't see why `(undefined :: TT a)` should be a problem. I certainly don't see a problem with `test4` or `test5`. Your instance of `C [a]` does not require `C a`, and so of course we can compute the length of a list `[TT a]` without knowing anything about the type `TT a`. That's just parametricity. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9918#comment:20 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler