#13140: Handle subtyping relation for roles in Backpack -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: feature request | Status: patch Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Keywords: backpack hs- Resolution: | boot Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3123 Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): If, from your perspective, the added complexity is worth it, then this looks OK to me. What's frustrating here is that we don't really need a new `Abstract` equality relation: the current `Phantom` one works fine. All we need is a new role ''annotation''. Is it worth creating a new `data RoleAnnotation = Abstract | Concrete Role`? Perhaps. You would also then need to be careful about uses of `tyConRoles` and `tyConRolesX`, as they are currently used both for construction and decomposition -- up until this proposal, both directions were happy with the same roles. I do agree that your approach is type safe. But I have to wonder if there isn't a simpler solution somehow, as I'm hesitant to advocate yet another wrinkle to the role system. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13140#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler