---
created_at: '2013-05-14T23:00:49.000Z'
title: Writing unit tests is reinventing functional programming (2009)
url: http://noss.github.io/2009/02/25/writing-unit-tests-is-reinventing-functional-programming-in-non-functional-languages.html
author: tosh
points: 44
story_text: ''
comment_text: 
num_comments: 20
story_id: 
story_title: 
story_url: 
parent_id: 
created_at_i: 1368572449
_tags:
- story
- author_tosh
- story_5708976
objectID: '5708976'
year: 2009

---
[Source](http://noss.github.io/2009/02/25/writing-unit-tests-is-reinventing-functional-programming-in-non-functional-languages.html "Permalink to Writing unit tests is reinventing functional programming in non-functional languages")

# Writing unit tests is reinventing functional programming in non-functional languages

![Fork me on GitHub][1]

# Writing unit tests is reinventing functional programming in non-functional languages

* [Front page][2]
* [Github][3]

2009-02-25

So yesterday I posted this to the erlang list about [testable code and functional programming][4]. The mailing list followups disappointed me, since people just focused on the use of “academic”. Maybe that hurt some feelings.

Today I get up just to see a similar argument about [testable code and functional programming at hacker news][5].

Hey! Someone submitted my story! This is a big day for me. :)

### Repeated again:

I used to think that I liked erlang programming just because it was  
quite academic, and that this was just a personal preference. But I  
have come to realize that side-effect free programming is easier to  
reason about. Not always easier to write though. Being easier to  
reason about means that I can imagine more execution in my head than I  
can in languages that rebind variables and mutate values. Because  
those destructive changes defines a before-change, and after-change,  
sequence, order, and there are only so many steps one can keep the  
mind without the brain starting to lie to you, filling out the blanks  
without telling you.

I recently read an article published in idg.se where Erik Stenman of  
kreditor was quoted saying that (something like) java systems grow  
complex over time in a way that Erlang does not as much. This is  
something I experience with Java systems. In Java there is no cost in  
adding more things that keep state. It is said that you just  
encapsulate state it in your object, but that state is not as  
encapsulated as you think. Problems are fixed by adding more things  
with state, and more. and more. Pretty soon you have behavior that  
depends on several hundred mutable variables, and the code-base feels  
hard to grasp, difficult to reason back into previous code execution  
to see the “big picture”. It makes architectural fixes more difficult.  
And the problems accelerates, since more quick fixes are added. (I  
really need to get better at describing this.)

The funny thing is that the OOP world have found one way to manage the  
complexity and the code-bases that grow ugly: They are using unit  
tests, and practice the art of writing testable code. (“Testable code”  
is something that is simple to write unit tests for. )

* You make it very easy to supply the dependencies to code under test  
need. Avoid things that go out and grab values from global state.
* You avoid side-effects, and the side-effects you need (database  
updates, writing files etc) you make sure that you perform with a  
layer of indirection so that during tests you can replace the real  
object with (for example) a do-nothing mock.

What they are doing is that they are making as much of their code as  
possible to be side-effect free and placing all that code in one  
method so it can be called from a unit test. They are concentrating  
side-effects to well-defined places, carefully avoiding mixing  
side-effects and testable/test-worthy logic.

What they are doing is that they’re reinventing functional programming.

**PS**  
Anyone care to make my argument better or shoot it down?

Send comments to chsu79@gmail.com

[1]: http://s3.amazonaws.com/github/ribbons/forkme_left_red_aa0000.png
[2]: http://noss.github.io/
[3]: http://github.com/noss
[4]: http://www.erlang.org/pipermail/erlang-questions/2009-February/041969.html
[5]: http://news.ycombinator.com/item?id=493963