Functional Programming Patterns in Scala and Clojure

Pattern 11

Replacing Dependency Injection

Intent

To compose objects together using an external configuration or code, rather than having an object instantiate its own dependencies—this allows us to easily inject different dependency implementations into an object and provides a centralized place to understand what dependencies a given object has.

Overview

Objects are the primary unit of composition in the object-oriented world. Dependency Injection is about composing graphs of objects together. In its simplest form, all that’s involved in Dependency Injection is to inject an object’s dependencies through a constructor or setter.

For instance, the following class outlines a movie service that’s capable of returning a user’s favorite movies. It ...