In article <20060901102733.GF21467@iai.uni-bonn.de>,
Andres Loeh
I think the stress here is on *new* extensions. I agree that an open type of type representations might be a more beautiful solution to the problem that Typeable solves. Nevertheless, the fact is that Simon's solution can be used in current GHC without further implementation work.
OK, fair enough. Open datatypes and functions will not, as you point out, be in Haskell'. I'd also like to query O'Haskell here. Simon writes in the paper:
O'Haskell extends Haskell with object-oriented subtyping. As such, it would be entirely possible to implement extensible exceptions using inheritance in O'Haskell.
I believe O'Haskell (like OOHaskell) doesn't provide the required dynamic downcasting operation either. AFAICT the extensions are essentially syntactic sugar: the subtyping is strictly static. -- Ashley Yakeley, Seattle WA