The document discusses writing testable code. It begins by introducing the speaker and then discusses challenges with writing tests, such as determining what and how much to test. It notes unit testing only tests individual units and not integration or system-level errors. The document advocates for writing pure, composable functions without side effects that are easily testable. It discusses lessons from functional programming like separating pure from impure code and using types properly. Overall, the document argues for writing code in a functional style to enable better testing.

1. Writing
Testable Code
Alvaro Videla - Cloud Foundry
Wednesday, April 10, 13

2. About Me
• Cloud Foundry Developer Advocate
• Blog: http://videlalvaro.github.com/
• Twitter: @old_sound
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3. About Me
Co-author
RabbitMQ in Action
http://bit.ly/rabbitmq
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4. I’m not a:
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5. I’m not a:
• Application Testing Guru
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6. I’m not a:
• Application Testing Guru
• TDD Advocate
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7. Why is it so hard
to write tests?
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8. Unit Testing
The goal of unit testing is
to isolate each part of
the program and show
that the individual parts
are correct
http://en.wikipedia.org/wiki/Unit_testing
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9. Unit Testing
[…] unit testing by definition only
tests the functionality of the units
themselves.
http://en.wikipedia.org/wiki/Unit_testing
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10. Unit Testing
[…] Therefore, it will not catch
integration errors or broader
system-level errors (such as
functions performed across
multiple units, or non-functional
test areas such as performance)
http://en.wikipedia.org/wiki/Unit_testing
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11. Dogma
vs.
Reality
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12. A world of
Trade Offs
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13. What should
we test?
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14. How much
should
we test?
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15. “I get paid for code that works,
not for tests, so my philosophy is
to test as little as possible to
reach a given level of confidence”
– Kent Beck
http://stackoverflow.com/questions/153234/how-deep-are-your-unit-tests/153565#153565
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16. The Hidden
Secret
Of TDD
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17. The Secret of TDD
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18. The Secret of TDD
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19. Some books by Kent Beck
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20. To write good
tests first we need
to learn how to
program
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21. Wednesday, April 10, 13

22. We developers are
like those users we
like to complain so
much about
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23. Design evolves and
matures with time
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24. Good Code sits in
the small details
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25. TIPS
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26. Separate pure code
from impure or stateful
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27. Pure Functions
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28. Pure Functions
• Referential Transparency
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29. Pure Functions
• Referential Transparency
• Don’t modify external state
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30. Pure Functions
• Referential Transparency
• Don’t modify external state
• Don’t produce side effects
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31. What’s wrong with this code?
if($player->getScore() > 0) {
$player->setSwizzle(7);
} else {
$player->setSwizzle(
$player->getSwizzle() + 1
);
}
https://dl.dropboxusercontent.com/u/7810909/docs/what-does-fp-mean/what-does-fp-mean/chunk-html/ar01s05.html
Wednesday, April 10, 13

32. What’s wrong with this code?
$newScore = $player->getScore() > 0
? 7
: $player->getSwizzle() + 1;
$player->setSwizzle($newScore);
https://dl.dropboxusercontent.com/u/7810909/docs/what-does-fp-mean/what-does-fp-mean/chunk-html/ar01s05.html
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33. Score calculation
can be moved into
its own function
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34. Score calculation
can be tested now
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35. First write
Pure Code
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36. Add impure code
step by step when
needed
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37. Write
Composable
Code
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38. Function Composition
http://en.wikipedia.org/wiki/Function_(mathematics)
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39. Function Composition
http://en.wikipedia.org/wiki/Function_(mathematics)
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40. This looks familiar
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41. “Many UNIX programs do quite
trivial tasks in isolation, but,
combined with other programs,
become general and useful
tools.”
http://math.albany.edu/math/pers/hammond/unixphil.html
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42. Number of open connections per IP
netstat -ntu | awk '{print $5}' |
cut -d: -f1 | sort | uniq -c | sort -n
http://www.commandlinefu.com/commands/view/1767/number-of-open-connections-per-ip.
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43. Why don’t we just
code in this style?
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44. This seems familiar
again…
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45. Welcome to
Functional
Programming
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46. “Writing unit tests is reinventing
functional programming
in non-functional languages”
http://noss.github.io/2009/02/25/writing-unit-tests-is-reinventing-functional-programming-in-non-functional-languages.html
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47. What can we learn from
Functional Programming?
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48. The proper use of Types
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49. What does ‘null’ mean?
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50. What does
‘true|false’ mean?
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51. Functions with just one
responsibility
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52. Radical separation of pure
code from impure code
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53. Let’s see an example
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54. Food for Thought
http://thinking-forth.sourceforge.net
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55. “Inside every well-
written large program
is a well-written small
program”
http://www.linfo.org/q_programming.html
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56. Questions?
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57. Thanks!
http://twitter.com/old_sound
http://github.com/videlalvaro
http://www.slideshare.net/old_sound
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