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
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.
Trac metadata
Trac field | Value |
---|---|
Version | 7.8.2 |
Type | Task |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
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Blocking | |
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