Indeed, it really only simulates the function type constraint aspect of Haskell typeclasses. Fortunately most of the time I have direct access to the implementa

Indeed, it really only simulates the function type constraint aspect of Haskell typeclasses. Fortunately most of the time I have direct access to the implementation, and can throw an interface on a type when I need it. This is effectively the expression problem. You can always solve it by using a wrapper class, although it's a bit inconvenient.