stefan:
Al,
Has there been any work on extending Haskell's type system with structural subtyping?
Koji Kagawaga. Polymorphic variants in Haskell. In Andres Loeh, editor, Proceedings of the 2006 ACM SIGPLAN Workshop on Haskell, Portland, Oregon, USA, September 17, 2006, pages 37--47. ACM Press, 2006. [1]
What is the canonical solution to the expression problem in Haskell?
Not canonical but Loeh and Hinze have proposed open data types:
For a short term solution, we used Typeable + type classes to provide a open Message data type. Similar techniques are used in Simon Marlow's extensible exceptions paper. -- An open Message type class Typeable a => Message a -- -- A wrapped value of some type in the Message class. -- data SomeMessage = forall a. Message a => SomeMessage a -- -- And now, unwrap a given, unknown Message type, performing a (dynamic) -- type check on the result. -- fromMessage :: Message m => SomeMessage -> Maybe m fromMessage (SomeMessage m) = cast m -- Don