#2850: GeneralizedNewtypeDeriving + TypeFamilies doesn't work -------------------------------------------------+------------------------- Reporter: ajd | Owner: Type: feature request | Status: Priority: normal | closed Component: Compiler | Milestone: 6.12 Resolution: fixed | branch Operating System: Unknown/Multiple | Version: Type of failure: None/Unknown | 6.10.1 Test Case: | Keywords: indexed_types/should_compile/T2850 | Architecture: Blocking: | Unknown/Multiple | Difficulty: | Unknown | Blocked By: | Related Tickets: -------------------------------------------------+------------------------- Changes (by nomeata): * failure: => None/Unknown Comment: If we implement GND via `Coercible`, this will no longer work: Coercing `bar :: B Bool -> B Bool` to the instance for `Int` will print {{{ No instance for (GHC.Types.Coercible (B Bool) (B Int)) because the first type argument of ‛B’ has role Nominal, but the arguments ‛Bool’ and ‛Int’ differ arising from a use of ‛GHC.Prim.coerce’ }}} Richard, as the expert on roles, is that avoidable? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/2850#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler