Testing heterogeneous equality

[Icelandjack]

testEquality compares two keys for equality that possibly have a different type, but it doesn't allow comparing Key s Int and Key s Maybe by producing a proof HRefl

-- this definition to go into Data.Type.Equality
data (a :: k1) :~~: (b :: k2) where
   HRefl :: a :~~: a

testHEquality :: Key s a -> Key s b -> Maybe (a :~~: b) 

Consider adding it when it when they get added to base.

[Icelandjack]

I'm not sure what practical use this has, but STRef has kind STRef :: Type -> Type -> Type so even if it added per this remark

testEquality :: STRef s a -> STRef s b -> Maybe (a :~: b)

we couldn't get a kind heterogeneous testHEquality for STRef.