Protocols in Clojure

Multimethods are just one of the options for polymorphism you have in Clojure, there are other ways to implement polymorphic functions.

Protocols are a little easier to understand and they might feel more similar to Java interfaces.

Lets try to define our shape program using protocols:

(defprotocol Shape
  "This is a protocol for shapes"
  (perimeter [this] "Calculates the perimeter of this shape")
  (area [this] "Calculates the area of this shape"))

Here, we have defined a protocol and it is called shaped and everything that implements this protocol must implement the following two functions: perimeter and area.

There are a number of ways to implement a protocol; one interesting feature is that you can even extend Java classes to implement a protocol without an access to the Java source and without having to recompile anything.

Let's start by creating a record that implements the type.

Records in Clojure

Records work exactly like maps, but they are much faster if you stick...