#9562: Type families + hs-boot files = unsafeCoerce -------------------------------------+------------------------------------- Reporter: goldfire | Owner: ezyang Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 7.8.3 Resolution: | Keywords: TypeFamilies, | SafeHaskell hs-boot Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: #10270 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by rwbarton): With the caveat that I don't feel like I fully grok type checking in the middle of an import cycle, let me offer a variant on goldfire's and adamgundry's proposals: When we type check a module that has an hs-boot file (here `B.hs`), do so in the type family instance environment of `B .hs-boot`. In other words, allow type family instances in hs-boot files and require of the user that if they want to use a type family instance while compiling `B.hs`, it must already be present in `B.hs-boot`. This is more lenient than "''every'' type family in `B.hs` ''must'' be listed in `B.hs-boot`" and I think it's a clearer or at least more implementable version of adamgundry's "must list all type instance declarations used (transitively) in the typechecking of [...]". Maybe it's even exactly equivalent to that. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9562#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler