#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): `instance Rep f => Rep (Compose f)` where `newtype Compose f g a = Compose (f (g a))`: We wish to show `forall a. Coercible (m (f a)) (m (g a))` from `Rep m` and `Coercible f g`. Fix `a`. 1. Use rule (*) to get `Coercible (f a) (g a)`. 2. Use #9117's eta to get `Coercible f g`. We're done. `instance Rep p => Rep (WrappedArrow p)` where `newtype WrappedArrow a b c = WrappedArrow (a b c)`: We wish to show `forall a. Coercible (p x a) (p y a)` from `Rep p` and `Coercible x y`. Fix `a`. 1. Use #9117's eta to get `Coercible (p x) (p y)`. 2. Use rule (*), and we're done. `instance Rep (p a) => Rep (WrappedArrow p a)`: We wish to show `Coercible (p a x) (p a y)` from `Rep (p a)` and `Coercible x y`. 1. Use rule (*), and we're done. -------------------------- Unfortunately, I can't counter the point about `StateT` -- we still can't express that `s` should be representational if `m`'s argument is. But, what about this: {{{ newtype Flip0 f a b = Flip0 (f b a) -- not used; just to better understand Flip1 newtype Flip1 f a b c = Flip1 (f b a c) instance Rep m => Rep (Flip1 StateT m) }}} Would that work for you? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9123#comment:21 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler