#12045: Visible kind application -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Iceland_jack Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | TypeApplications TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): ​Phab:D2216 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Here are some examples that are actually useful, used with a non-standard version of [https://github.com/ekmett/hask/ hask]: {{{#!hs type Fix f = (f -> f) -> f newtype Mu1 :: Fix Type where In1 :: f (Mu1 f) -> Mu1 f newtype Mu2 :: Fix (k -> Type) where In2 :: f (Mu2 f) a -> Mu2 f a }}} If you want to reference the kind variable `k` you can write {{{#!hs -- instance Functor (Mu :: ((k -> Type) -> (k -> Type)) -> (k -> Type)) where instance Functor (Mu @k) where type Dom (Mu @k) = ... type Cod (Mu @k) = ... }}} instead of {{{#!hs instance Functor (Mu :: Fix (k -> Type)) where type Dom (Mu :: Fix (k -> Type)) = (Void :: Cat ((k -> Type) -> (k -> Type))) type Cod (Mu :: Fix (k -> Type)) = (Void :: Cat (k -> Type)) }}} ---- {{{#!hs instance Functor (Const @k a) where type Dom (Const @k a) = UNIT type Cod (Const @k a) = (->) fmap :: UNIT b b' -> (Const a b -> Const a b') fmap UNIT (Const a) = Const a }}} instead of {{{#!hs instance Functor (Const a :: k -> Type) where type Dom (Const a :: k -> Type) = UNIT type Cod (Const a :: k -> Type) = (->) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12045#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler