I'm not sure what Philip meant there, but I don't see this as a particularly notable weakness of Haskell's type class system. We can already encode an open wor

I'm not sure what Philip meant there, but I don't see this as a particularly notable weakness of Haskell's type class system. We can already encode an open world through the standard "HasX"-style classes. There's a good reason that we don't use them much, though: type classes are typically not just interfaces but come with laws expressing how they should behave. It's not clear to me how those laws interact with an open world.