What is the advance of using type classes? A function of the form f :: Show a => ... really has an implicit argument f :: Show__Dict a -> ... that the compiler infers for us. So, the advantage of type classes is one of convenience: we don't have to pass dictionaries around, or even figure out which dictionaries we need; the compiler does that for us. But if we have a type class of the form class Foo a where mkFoo :: IO FooToken otherFun1 :: FooToken -> ... otherFun2 :: FooToken -> ... then this advantage is mostly lost; we still need to pass around an explicit FooToken object. In a case like this, I don't see the advantage of using a type class over using a data type data Foo = Foo { otherFun1 :: ... , otherFun2 :: ... } mkFoo :: .. -> Foo There are exceptions; for instance, if you want to encode 'inheritance' in some way then type classes might still be useful; for instance, see the Gtk2Hs library, which uses this extensively. Edsko