Make equality of `Reference` independent of the contents about quickcheck-state-machine HOT 3 OPEN

Perhaps it would make sense to move your trick into the library? That way we can also use Map (Reference ...)-like models...

from quickcheck-state-machine.

I thought a bit more about this, and I'm beginning to wonder if post-conditions shouldn't have the type model Symbolic -> act Symbolic resp -> resp -> Bool?

Or perhaps the Var from Symbolic can be threaded through into Concrete and used for equality checks?

from quickcheck-state-machine.

I played around a bit more with this in #209, see:

data Concrete a where
  Concrete :: Typeable a => a -> Unique -> Concrete a

instance Eq (Concrete a) where
  Concrete _ u1 == Concrete _ u2 = u1 == u2

instance Eq (r a) => Eq (Reference a r) where
  Reference r1 == Reference r2 = r1 == r2

reference :: Typeable a => a -> IO (Reference a Concrete)
reference x = Reference <$> (Concrete x <$> newUnique)

It seems to work, but I needed:

transition :: forall r. ForallF Ord r => model r -> cmd r -> resp r -> model r 

Which is terrible to work with, but maybe we can revisit this approach when GHC gets qualified contexts.

from quickcheck-state-machine.