#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 simonpj): Actually there is ''some'' justification for the status quo. Here it is: * If A imports B, and A has `type instance F Int = Bool` and B has `type instance F Int = Char`, then any attempt to use `F Int`, in A or in any module A imports, will get a "overlapping instance" error on lookup. I think. So the conflict would be reported, but lazily. * In contrast, if A imports B and C, and B and C have those instance declarations, then compiling A might never need to reduce `F Int`, so we must eagerly check for consistency. I'm intrigued about how you managed to write `unsafeCoerce`. But regardless, I think it'd be better and more consistent to eagerly check consistency of all the new family instances in A with all those in modules it imports. Then instance lookup should never find more than one match. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13092#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler