What is the clojure equivalent to google guice?

Question

I came across google guice and could not really understand it and what it did, although there seems to be alot of hype around it. I was hoping to get a clojurian perspective of the library and why it is needed/not needed in clojure applications and if there was anything similar built into the language.

Answer 1

Because of Java's OO and type system, dynamically switching between different underlying implementations (for test (mocking) purposes for instance) can be difficult to manage. Libraries like Google Guice are intended to handle these dependency injections in Java more gracefully.

In Clojure and other functional languages functions can be passed around, which makes using different implementations much easier.

There's several ways this can be done in Clojure:

• Using your choice of function as parameters in higher order functions.
• (Re)Binding your choice of function to a var.
• Encapsulating your choice of function inside closures that can then be passed around and called.

Chapter 12 of Clojure Programming has some nice examples of OO patterns like dependency injection and the alternative ways to handle these in Clojure.

Sean Devlin also has a Full Disclojure video on Dependency Injection in Clojure. His example might have been chosen better, though. Instead of using completely different function implementations in his closure, he uses a factory that returns different 'versions' of a function. The gist stays the same though.

Basically, dependency injection is a pattern that is a necessary evil in OOP, and can be solved easily (or is not even a problem) in FP.

Answer 2

The rough Clojure equivalents are still in development. There are two libraries currently in development (as of Oct '12): Prismatic's Graph (not yet open sourced) and Flow by Stuart Sierra.

Note that I consider Guice to be more than dependency injection. It provides a framework for application configuration / modularization. The above libraries aim to accomplish that goal.