#11439: Request for comments: Allow duplicate type signatures -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 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): Similar for type families: {{{#!hs type family Sing (res :: k -> Type) | res -> k where -- Sing :: Nat -> Type Sing = SingNat -- Sing :: Nat -> Type Sing = SingBool }}} Annotations work fine here `Sing = (SingNat :: Nat -> Type)`, ... but its utility is clearer in this [https://gist.github.com/AndrasKovacs/c7d385aa117929503feb gist] where we could annotate `(@@)` instances in different ways without modifying the definitions: {{{#!hs data Con1 :: (a -> b) -> (a ~> b) data Con2 :: (a -> b -> c) -> (a ~> b ~> c) data Con3 :: (a -> b -> c -> d) -> (a ~> b ~> c ~> d) type family (@@) (f :: a ~> b) (x :: a) :: b -- (@@) :: (a ~> b) -> (a -> b) type instance Con1 c @@ a = c a -- (@@) :: (a ~> b ~> c) -> a -> (b ~> c) -- (@@) :: (a ~> (b ~> c)) -> (a -> (b ~> c)) -- (@@) :: (a ~> b ~> c) -> (a -> b ~> c) type instance Con2 c @@ a = Con1 (c a) -- (@@) :: (a ~> b ~> c ~> d) -> a -> (b ~> c ~> d) -- (@@) :: (a ~> (b ~> c ~> d)) -> (a -> (b ~> c ~> d)) -- (@@) :: (a ~> b ~> c ~> d) -> (a -> b ~> c ~> d) type instance Con3 c @@ a = Con2 (c a) }}} I think this will only become more useful as GHC's type system evolves -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11439#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler