r-wos.org

Some months ago, Racket 5.3 introduced a new generics library. As you would expect, it’s basically a convenient way to implement generic interfaces for related type-specific operations (for example generic queue operations for different queue implementations).

That’s all fine and works well, but it doesn’t (as of now) support dispatching for more than one function parameter, i.e. multiple dispatch.

However, there is a reasonably easy way one can get something equivalent to multiple dispatch in Racket: pattern matching. (This may seem obvious to others but it wasn’t obvious to me until very recently.)

To take the example from the wikipedia page – in Common Lisp one could do:

(defmethod collide-with ((x asteroid) (y asteroid))
  ;; deal with asteroid hitting asteroid
  )
(defmethod collide-with ((x asteroid) (y spaceship))
  ;; deal with asteroid hitting spaceship
  )
(defmethod collide-with ((x spaceship) (y asteroid))
  ;; deal with spaceship hitting asteroid
  )
(defmethod collide-with ((x spaceship) (y spaceship))
  ;; deal with spaceship hitting spaceship
  )

While in Racket one could use pattern matching to get the same effect:

(define/match (collide-with x y)
  ((asteroid asteroid)
    #| deal with asteroid hitting asteroid |#)
  ((asteroid spaceship)
    #| deal with asteroid hitting spaceship |#)
  ((spaceship asteroid)
    #| deal with spaceship hitting asteroid |#)
  ((spaceship spaceship)
    #| deal with spaceship hitting spaceship |#))

I don’t know enough Common Lisp or Racket to say whether these two versions are completely equivalent (especially performance-wise), but they can both be used in the same way – which is good enough for me.