Polymorphism in Clojure

Question

Suppose I have a bunch of Clojure data structures, all of the same type - for example an object type defined by defrecord.

What is the best way to get polymorphic behaviour across these structures?

Would it be good practice to embed a function within the structure so that I can do something like:

((:my-method my-object) param1 param2)

Or is there a better way?

Answer 1

You can use multimethods for dispatch,

(derive ::subtype ::type)

then you can dispatch based on type

(defmulti my-method
  (fn [type] ((meta type) :type)))

(defmethod my-method ::subtype []
  ;;stuff...
 )

You need to add :type to the objects metadata when you create it,

(with-meta [1 2 3] {:type ::subtype})

or I haven't tried this but since protocols and records creates java classes behind the scenes you dispatch based on that name.

Answer 2

If I understand your question, protocols are what you want to use.

user=> (defprotocol P (foo [p x]))
P
user=> (defrecord R [a b] P (foo [_ x] [a b x]))
user.R
user=> (defrecord S [a b] P (foo [_ x] [x b a]))
user.S
user=> (def r (R. 1 2))
#'user/r
user=> (def s (S. 1 2))
#'user/s
user=> (foo r 3)
[1 2 3]
user=> (foo s 3)
[3 2 1]

In the above both R and S implements the foo function defined by protocol P, but they each do so differently. When the foo function is called, it dispatches on type of the first arg.