Programming Clojure, 2nd Edition

Creating Ad Hoc Taxonomies

Multimethods let you create ad hoc taxonomies, which can be helpful when you discover type relationships that are not explicitly declared as such.

For example, consider a financial application that deals with checking and savings accounts. Define a Clojure struct for an account, using a tag to distinguish the two. Place the code in the namespace examples.multimethods.account. To do this, you will need to create a file named examples/multimethods/account.clj on your classpath^[57] and then enter the following code:

(ns examples.multimethods.account)

(defstruct account :id :tag :balance)

Now, you are going to create two different checking accounts, tagged as ::Checking ...