Question

I know that multimethods are often dispatches based on class, but is there a way to dispatch based on an interface which is implemented instead?

Answer 1

Multimethods allow you to specify your own dispatch function. So you can dispatch based on any predicate! The following code dispatches based on the interface implemented by the argument:

(defmulti process-collection
  (fn [arg1 & _]
    (cond
      (instance? java.util.List arg1) :list
      (instance? java.util.Set arg1) :set
      :else :coll)))

(defmethod process-collection :list
  [list-to-process]
  ())

(defmethod process-collection :set
  [set-to-process]
  ())

(defmethod process-collection :coll
  [coll-to-process]
  ())