#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): Sorry for not piping up sooner, but this change would be unsound. Consider this accepted program: {{{ data T a type role T nominal foo :: Coercible (T a) (T b) => a -> b foo x = x }}} You can see that, if `T` were abstract, instantiating `T` with a concrete datatype that had a phantom role would be Wrong. While there is a sub- roling relationship, it does not induce a sub-typing relationship like the one you seek. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13140#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler