Change GHC.Exts.Any to a type family
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
Any == Any must be
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.