[Haskell-cafe] run-time type testing in haskell?

Udo Stenzel u.stenzel at web.de
Sat Apr 28 11:27:35 EDT 2007

Eric wrote:
> (1) Have Handlers implement a method handle(m: Msg). To add new types of 
> message, we declare message types which extend Msg. The Handler then 
> uses runtime type testing to decide how to deal with each message. The 
> advantage of this design is that we can add new Handler and new Msg 
> types without recompiling the Dispatcher.

Note however that the type of the handle method is a lie (your handlers
don't actually accept arbitrary messages).  I guess, you actually  want
a polymorphic "Dispatcher", which will actually turn out to be a
function.  Of course, you give insufficient information to actually
solve your problem.

> Type classes allow us to adopt approach (0) in Haskell, but don't seem 
> to allow approach (1)....

I consider that a good thing.

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