#12369: data families shouldn't be required to have return kind *, data instances should -------------------------------------+------------------------------------- Reporter: ekmett | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Is this another example of this {{{#!hs type f ~> g = forall a. f a -> g a type f ~~> g = forall a b. f a b -> g a b newtype Free0 :: Foo Type where Free0 :: (forall q. k q => (p -> q) -> q) -> Free0 k p newtype Free1 :: ((k -> Type) -> Constraint) -> (k -> Type) -> (k -> Type) where Free1 :: (forall q. k q => (p ~> q) -> q a) -> Free1 k p a newtype Free2 :: ((k1 -> k2 -> Type) -> Constraint) -> (k1 -> k2 -> Type) -> (k1 -> k2 -> Type) where Free2 :: (forall q. k q => (p ~~> q) -> q a b) -> Free2 k p a b }}} They all follow the pattern `type FREE ĸ = (ĸ -> Constraint) -> (ĸ -> ĸ)` {{{#!hs Free0 :: FREE Type Free1 :: FREE (k -> Type) Free2 :: FREE (k1 -> k2 -> Type) }}} Maybe you could write it as? {{{#!hs data family Free :: FREE k newtype instance Free :: FREE Type where Free0 :: (forall q. k q => (p -> q) -> q) -> Free k p newtype instance Free :: forall k. FREE (k -> Type) where Free1 :: (forall q. k q => (p ~> q) -> q a) -> Free k p a newtype instance Free :: forall k1 k2. FREE (k1 -> k2 -> Type) where Free2 :: (forall q. k q => (p ~~> q) -> q a b) -> Free2 k p a b }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12369#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler