Overriding functions in Clojure

August 22, 2012

While testing in an OO language it is common to mock objects out of the picture. This could be do the fact that the object not yet implemented or maybe it calls out to the third party service you rather not use when testing.

Clojure provides us some tools to do the same thing with functions we may want to mock out.

Dependency Injection

Like OO languages you can use dependency injection in a functional language like Clojure. Lets say we have the following function.

(defn do-work-son [arg]
  (third-party/big-function arg))

While testing we don’t want to have to wait for the third parts big function to run. We can re-write the function to take a second argument of what function to run on the argument.

(defn do-work-son [arg worker]
  (worker arg))

Now we can pass what ever we want to the function. In production we call,

(do-work-son "arg" third-party/big-function)

While during the test we can do some thing like,

(do-work-son "arg" #(* % 2))

Just to test that the function is being called.

Redefinition

Sometimes Dependency Injection is just out of the question or would make your code too complicated. At these times we can redefine functions at run time using with-redefs. Here is the above example using this method.

;; production code
(defn do-work-son [arg]
  (third-party/big-function arg))

;; test code
(with-redefs [third-part/big-function #(* % 2)]
  (do-work-son "arg"))