[GHC] #9097: Change GHC.Exts.Any to a type family

[GHC]

#9097: Change GHC.Exts.Any to a type family
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       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:                    |
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 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>
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