Programming Clojure, 2nd Edition

Defining Multimethods

To define a multimethod, use defmulti:

(defmulti name dispatch-fn)

name is the name of the new multimethod, and Clojure will invoke dispatch-fn against the method arguments to select one particular method (implementation) of the multimethod.

Consider my-print from the previous section. It takes a single argument, the thing to be printed, and you want to select a specific implementation based on the type of that argument. So, dispatch-fn needs to be a function of one argument that returns the type of that argument. Clojure has a built-in function matching this description, namely, class. Use class to create a multimethod called my-print:

(defmulti my-print class)

At this point, ...