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
NijmegenPHP 15-04-2015

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/NLZiet)
• 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.
- Kanye West

16. “We should enforce (business)
rules when creating objects”
- Justin Bieber

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. Best Solution ?
● Single Responsibility Principle
● Duplicate code

26. Design by contract

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

29. Example Value Objects
• Money
• Currency
• Time
• DateRange / Period

31. Money Value Object
https://github.com/mathiasverraes/money

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

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

34. Immutable Object,
Best Object
- The Hulk

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

40. Make the Implicit,
Explicit !

44. Assertion Library
https://github.com/beberlei/assert

46. “As an Author I want to be able to Create
an Article. Before Publishing it, I want to
have a Draft. I also would like to be able
to Retract an Article with a Reason”

48. Objects as dumb data stores
a.k.a.
Anemic Domain Model
This is our industry standard

50. Published ?

51. Objects should have
behaviour attached
to them
- Lex Luther

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

59. Schrödinger's cat

60. UnpaidOrder PaidOrder

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

64. Immutability
but of a different kind

65. UnpublishedArticle PublishedArticle

66. UnpublishedArticle PublishedArticle
Draft Article

68. Draft Article RetractedArticle

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

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

74. But there is still a piece
of the puzzle missing…

75. As a Publisher I want to
know which Article has been
retracted more than X times

77. As an Editor I want to see
all the changes that have
been made

79. Store all the Changes
(Δ delta’s),that have
happened

81. EventSourcing!
But that is a talk on its own

82. Magic is bad, hmmmkay
- Saddam Hussein

83. Questions?
https://joind.in/event/nijmegenphp-201504