Understanding the Four Rules of Simple Design

1. Understanding the
Four Rules of Simple
Design

2. Lessons from watching thousands
of pairs work on Conway's Game
of Life by Corey Haines1
1
Buch bei Leanpub, Twitter

3. Focused on the almost continuous
decisions made around things
such as naming variables and
extracting methods

4. It's not about any particular
technique.
It's not about any particular
language.

5. It's about better design, not good
versus bad design.

6. Four Rules of Simple Design
1. Tests Pass
2. Expresses Intent
3. No Duplication (DRY)
4. Small

7. Don't Repeat Yourself
This rule isn't about code duplication; it is about knowledge
duplication:
Every piece of knowledge should have one and only one
representation.
— The Pragmatic Programmer2
2
http://pragprog.com/book/tpp/the-pragmatic-programmer

8. Small
• Any vestigial code that is no longer used?
• Any duplicate abstractions?
• Extracted too far?

9. Conway's Game of Life
1. If a living cell has less than two living neighbors, it will die.
2. If a living cell has two or three living neighbors, it stays alive.
3. If a living cell has more than three living neighbors, it will
die.
4. If a dead cell has exactly three living neighbors, it becomes
alive.