The Four Elements of Simple Design

• Introduced by Kent Beck in the 1990s.
• Part of his software development methodology Extreme Programming.
• His exact wording appears in the White Book.

The rules can be stated as followed:

1. Passes all tests
2. Maximizes clarity
3. Minimizes duplication
4. Has fewer elements

• The rules are listed in priority order.
  • e.g., Passes all tests trumps Maximizes clarity.
• Rules 2 and 3 often seem to swap priority.
  • Minimizes duplication precedes Maximizes clarity in most versions (including Beck's).
  • Really, rules 2 and 3 feed off of each other like a dynamo.
  • When writing code, Minimizes duplication seems to trump Maximizes clarity, but when reading code, the reverse seems to be true.
  • On average, a developer will spend 10 times longer reading and understanding code than writing it. As a result, I personally place Maximizes clarity above Minimizes duplication, but it depends on what is being done and by whom.
• Rule 0: Kent Beck says:

  In the rare case they are in conflict (in tests are the only examples I can recall), empathy wins over some strictly technical metric."

Further reading:

• BeckDesignRules
• Xp Simplicity Rules
• Essential XP: Emergent Design
• The Four Elements of Simple Design
• Understanding the Four Rules of Simple Design