[Haskell-cafe] Has anyone looked into adding subtyping to Haskell?

[Donald Bruce Stewart]

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