Object Mentor Blog: TDD and FP

TDD and FP

Uncle Bob — Sat, 08 Aug 2009 16:31:10 -0500

No, this isn’t a blog about how to do TDD in a functional language. Rather, it’s a blog about the similarities between the two disciplines.

First, a quick definition. Functional Programming is programming with functions. In particular, functions that have no side effects. A function that has no side effects is a function that returns the same value given the same inputs irrespective of the state of any other part of the system.

Does this mean that functions in a functional program are truly free of all side-effects? Clearly not, since they are usually running on machines where the machine instructions are all about side effects. It’s just that all the side effects have been cancelled out by the time the a true function returns. The function leaves no residual state behind.

A unit test, properly written in TDD style, has no side effects. Every time you run it, it will give you the same (hopefully green) result. You can run as many tests as you like, in any order you like, but they’ll always give you the same result. Why? Because they leave no state behind. They have no side-effects.

It’s not that the operations executed by the unit tests don’t have side effects. Most of the time they do, since most of the time we are writing them in languages (like Java) that are rife with side effects. But that’s not relevant since all those side effects get cancelled out by the time the unit test completes.

Does the function you are testing create files? That’s ok, your test will delete them before it completes. Does the function you are testing create rows in a database? That’s OK, your tests will remove them before it finishes. Does the function you are testing change any state at all? If so, the unit test will put that state back where it was before the test was run. Unit tests are atomic operations that leave the state of the system unchanged!

In short, code written with TDD is code that has been assembled into pairs that have no side effects. One side of the pair is a unit test, and the other side of the pair is production code. Together they are functional, even if separate they are not.

Thus, TDD is Functional Programming (of a kind). If your unit tests are very granular, you are more likely to write your production code in a functional style; or at least in a style in which side-effects are so localized that they can be managed within the scope of a single test.

Certainly TDD is not a replacement for learning a functional language. I think there’s much to be gained by learning Haskell, or Scala, or Clojure. However, TDD is a style of programming that encourages programmers to severely limit side-effects and keep them manageable within very small scopes.

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Very nice observation.

TDD could be compared to functional programming only if it’s done perfectly right :)

(some of testing frameworks discourage to have no side-effects of tests, unfortunately)

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I don’t even know the names nor definitions of all the different tests. Why?