Programming Clojure, 3rd Edition

Defining Multimethods

Multimethods capture the same pattern we explored in the previous section but support adding new cases without changing the existing code. Multimethods also provide some additional features that we’ll look at later on. Multimethods consist of two parts: a dispatch function (created with defmulti) and a set of methods (created with defmethod).

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 ...