#14131: Difference between newtype and newtype instance -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This works {{{#!hs {-# Language RankNTypes, PolyKinds, GADTs #-} newtype Nat :: (k -> *) -> (k -> *) -> * where Nat :: (forall xx. f xx -> g xx) -> Nat f g }}} but this {{{#!hs {-# Language RankNTypes, PolyKinds, GADTs, TypeFamilies #-} data family Nat :: k -> k -> * newtype instance Nat :: (k -> *) -> (k -> *) -> * where Nat :: (forall xx. f xx -> g xx) -> Nat f g }}} fails on GHC 8.3.20170815, and requires a `forall k.` to work {{{ $ ./ghc-stage2 -ignore-dot-ghci --interactive /tmp/kind.hs GHCi, version 8.3.20170815: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( /tmp/kind.hs, interpreted ) /tmp/kind.hs:5:26: error: Not in scope: type variable âkâ | 5 | newtype instance Nat :: (k -> *) -> (k -> *) -> * where | ^ /tmp/kind.hs:5:38: error: Not in scope: type variable âkâ | 5 | newtype instance Nat :: (k -> *) -> (k -> *) -> * where | ^ Failed, 0 modules loaded. }}} Related to #12369? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14131 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler