Code tells a story, but is it the actual story the original author wanted to tell? A lot of time the original story gets lost, as new functionality gets bolted on, or key parts are extracted, leaving behind an incomplete picture. These repurposed objects might pass all the unit tests but have no actual meaning in the context of the original story. We will take a look at how we can more easily communicate the purpose of code to future developers, by introducing contracts and immutability, and using explicit context bound language, to give meaning to the code and keep the story whole.

1. Object Invariants,
Immutability & You
Making Objects Do Your Bidding
AmsterdamPHP 15-01-2015

2. Jeroen v.d. Gulik
• CTO / Co-Founder Isset (http://isset.nl)
• Builder of Artificial Stupidity
• @n0x13
• Likes Cats … a lot

3. Isset Internet Professionals
• We build custom internet technology based solutions
• We helped design and build several online video
platforms (Pathé Thuis, Videoland, NPO Plus)
• Currently focussing on video-transcoder.com
• We like a challenge so if you have interesting problems to
solve, come and talk to me
• Still Hiring

4.
“Every Great Talk should
start with a Great Quote”
- Ross Tuck
-

5. -House M.D.

6. Telephone Game

7. “Over time, the original
intent and purpose
gets lost”
- Jeroen v.d. Gulik

8. “A Customer should
always have an email
address”

13. Testing shows the
presence, not the absence
of bugs.
- Joseph Stalin

16. “We should enforce
(business) rules when
creating objects”
- Mao Zedong

17. “A Customer should have
a valid email address”

19. Outside-in validation
should be avoided
- Albert Einstein

21. Objects should be
responsible for their own
internal consistency
- Barack Obama

23. ● Single Responsibility Principle
● Duplicate code

26. Design by contract

28. Value Object
● Small Objects
● Equality based on value
● Immutable

29. Entity Object
● Usually big(-ger)
● Equality based on Identity
● Mutable
● Can change over time

30. Immutability
● Simplifies code
● Less cognitive load
● Guaranteed consistent
● Referencing without breakage
● Easy to test
● Caching (profit!)

31. Immutable Object,
Best Object
- The Hulk

32. “A Customer should have a
valid email address and a
password”

35. Make the Implicit,
Explicit

39. Objects as dumb data
stores
This is our industry standard

42. Published ?

43. Objects should have
behaviour attached
to them
- Julius Caesar

47. Objects should tell a tale about its
current state and all the possible
ways to change that state
- Bruce Lee

51. Schrödinger's cat

52. UnpaidOrder PaidOrder

55. “Sounds like a State Machine”
- Igor Wiedler

56. Immutability

57. UnpublishedArticle PublishedArticle

58. UnpublishedArticle PublishedArticle
Draft Article

60. Draft Article RetractedArticle

62. Objects should only expose
methods relevant to it's
current state
- George Bush

65. Objects should …
• Be responsible for their own internal state
• Have specific behaviour towards its responsibility
• Broadcast their state
• Only care about the state it’s currently in.

66. Magic is bad, hmmmkay
- Saddam Hussein

67. Questions?
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