#10039: Make Const (Control.Applicative) kind polymorphic in its second argument -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: ekmett Type: feature request | Status: new Priority: normal | Milestone: Component: Core Libraries | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by Iceland_jack): That would be great, other types like `Data.Functor.Compose.Compose` and `Data.Functor.Reverse.Reverse` could also be generalised (Vinyl does this with [[http://hackage.haskell.org/package/vinyl-0.5.1/docs/Data-Vinyl- Functor.html#t:Compose|Compose]]): {{{#!hs newtype Compose (f :: j -> *) (g :: k -> j) (a :: k) = Compose { getCompose :: f (g a) } newtype Reverse (f :: k -> *) (a :: k) = Reverse { getReverse :: f a } }}} A more general `Compose` is definitely useful for composing dictionaries constraints (`Compose Dict Eq`, [[http://www.reddit.com/r/haskell/comments/2qwqf2/generate_list_of_dicts_from_type_level_list/|discussion]]), non-empty vectors: {{{#!hs data Exists :: (k -> *) -> * where Never :: p x -> Exists p more :: Compose (Vector Char) Suc (Suc Zero) more = Compose (Cons 'H' (Cons 'S' Nil)) type NonEmpty a = Exists (Compose (Vector a) Suc) oh :: NonEmpty Char oh = Never more }}} working with typed lists and such (I also ran into [[http://lpaste.net/edit/75980|this]]). It's not clear to me that a generalised `Reverse` would be worth it. I wonder if any of this will break code though. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10039#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler