So back to the Plan A vs Plan B discussion.
I'm sorry, I don't completely understand why Plan A is an "optimization". As far as I know, the typechecker doesn't merely have the task of answering the question "does Typeable X" hold, but rather it has to come up with evidence that "Typeable X" holds. I fail to see how even the knowledge that every concrete datatype is in principle Typeable makes it any easier to come up with the type representation that's required during run-time. For example, if you consider data AnyTypeable = forall a. (Typeable a) => AnyTypeable a and you unpack and use such a value in a function, then clearly the only place to get the type representation from is the constructor itself, and similarly, data Any = forall a. Any a should really not have any type representation available at runtime at all. Similarly, for less extreme cases, we'd still need dictionary transformers and plug together type representations from different components. So why can we get rid of the instance table? What am I missing? Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com