#9097: Change GHC.Exts.Any to a type family ------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- I just had this slightly alarming interchange with GHCi: {{{ Prelude> import Data.Type.Equality Prelude Data.Type.Equality> import GHC.Exts Prelude Data.Type.Equality GHC.Exts> :kind! ((Any :: Bool) == (Any :: Bool)) ((Any :: Bool) == (Any :: Bool)) :: Bool = 'False }}} After staring at the result in disbelief, I figured out why. The instance for `==` at kind `Bool` looks like this: {{{ type family EqBool a b where EqBool False False = True EqBool True True = True EqBool a b = False type instance (a :: Bool) == (b :: Bool) = EqBool a b }}} Well, `Any` isn't `False`, `Any` isn't `True`, so `Any == Any` must be `False`! The solution to this, of course, is to make `Any` a type family, not a datatype. Then, it wouldn't be apart from the equations in `EqBool`. I believe this idea has been floated previously but was not implemented as it would have disturbed !TypeLits and/or singletons. These libraries have been updated, and it's time. I'm happy to do this myself in due course. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9097 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler