Thinking Functionally

I've been thinking about software solutions more and more as pure functions.
This quote describes perfectly the way my thinking has changed:

In fact, you can think of any impure function as having three "steps": 1) An "input" side effect, Unit => B, a pure function (A,B) => C, and an "output" side effect C => Unit. It makes perfect software engineering sense to decouple these components - and that is exactly what is done in purely functional programming.

from actors-are-not-good-concurrency-model.

One side benefit is that the core logic of the functionality is now perfectly testable; as pure functions are the easiest thing in the world to write tests for.