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Haskell's type classes generalize further than multi methods do. You get dispatch based on any part of the type signature, not just the arguments.

• chongli 14y ago
  Not further than Clojure's multimethods. They dispatch on the value of an arbitrary function you write. This can be anything; it's not even restricted to types. Couple this with Clojure's ad hoc hierarchy system and even Haskell's types seem limited by comparison.
  • Peaker 14y ago
    Can you implement the Read class in Clojure? class Read r where read :: String -> r (Let's ignore parse errors for our purposes here) Haskell supports return-type polymorphism. Clojure, being dynamically typed, cannot in general support this kind of overloading.
    • chongli 14y ago
      Good point, though Clojure's extensible reader takes care of most of the use for this. I think it's a trade-off though. Can Haskell dispatch on a value rather than a type?
      • Peaker 14y ago
        Note that this feature is far from being useful only for class Read. The Monad class actually depends on it too: class Monad m where (>>=) :: m a -> (a -> m b) -> m b return :: a -> m a The type of "return" can only be expressed with return-type polymorphism. This means you cannot really implement Haskell-like monads with dynamic typing. You have to implement them in a less general way. As for dispatching on runtime values, that is ordinary pattern matching: f (RuntimeValue1 x y) = ... case 1 ... f (RuntimeValue2 x y) = ... case 2 ...
        • chongli 14y ago
          >This means you cannot really implement Haskell-like monads with dynamic typing. Ahh, yeah I've heard this before. It totally slipped my mind. >As for dispatching on runtime values, that is ordinary pattern matching Can you extend the patterns of a function at runtime (or even across modules)?
          • Peaker 14y ago
            No, you can't write a function spread across modules depending on the value of its input, short of implementing a dispatch system.