#9123: Need for higher kinded roles -------------------------------------+------------------------------------ Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 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 agree with everything Simon said. About hand-written instances of `Representational`: We need some facility for this. For example, take `ReaderT`: {{{ newtype ReaderT r m a = ReaderT (r -> m a) }}} `ReaderT` gets roles R, R, N. But, this instance is sensible: {{{ instance Representational m => Representational (ReaderT r m) }}} I suppose it wouldn't be beyond possibility to beef up the role inference algorithm to spit out such `Representational` instances. Or, we could require that users request such instances (with a standalone `deriving` perhaps) but fill in the details automatically. If we do allow users to write their own instances, we seem to lose the guarantee that `coerce` is free at runtime, no? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9123#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler