#13092: family instance consistency checks are too pessimistic -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: rwbarton Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by rwbarton): Actually this is wrong! I managed to write `unsafeCoerce` in my compiler with this change. The reason is that type family instances defined in a module `A` are not actually checked for consistency with all its family instance dependencies `B`, `C`, ...; that seems to only happen when the interface file for (say) `B` is read for some other reason. I'm not sure yet whether that means even the existing consistency checks are inadequate; but it's at least surprising that you can write a module that has a type family instance that conflicts with an indirect dependency. Just to say it again explicitly, because it seems strange: when compiling a module containing a type family instance, GHC does not read all its indirect dependencies which have type family instances to ensure consistency. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13092#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler