#8177: Roles for type families -------------------------------------+------------------------------------ Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by goldfire): I'm not convinced that this is useless, actually. For example, I don't think Simon's last example `F` could be written without role annotations on type families, even if we switch the order of parameters. And, the kind signature trick doesn't work at all with data families: {{{ data family Q1 :: * -> * }}} is equivalent to {{{ data family Q2 a }}} In particular, under the current implementation, instances of `Q1` ''can'' pattern-match on the parameter. Accordingly, `Q1`'s parameter is currently given role N, just like `Q2`'s. So, I do think this idea would yield a real expressiveness gain, and I mused about it while implementing the main chunk of roles. Is it worth the extra complexity? Perhaps. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8177#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler