Beck Design Rules
Author / context
Martin Fowler documenting Kent Beck’s four rules of simple design, martinfowler.com, March 2015. Beck formulated the rules during the development of Extreme Programming in the late 1990s.
Summary
Beck’s four rules give a concrete, priority-ordered definition of “good design” — a replacement for vague taste-based judgements. The rules are: (1) passes the tests, (2) reveals intention, (3) no duplication, (4) fewest elements. Priority order matters: passing tests is non-negotiable; readability beats DRY when they conflict; and anything that doesn’t serve the first three rules should be deleted. The rules are language-agnostic and hold across paradigms.
Key precepts
- Good design is not subjective; these four rules are evaluable right now, not post-hoc.
- Priority order is the whole point: passes tests > reveals intention > no duplication > fewest elements.
- “Reveals intention” means code communicates the author’s purpose to the next reader.
- “No duplication” = Once and Only Once; duplication to increase clarity is papering over a design problem.
- “Fewest elements” = delete speculative abstractions; extra complexity added for future flexibility usually makes things less flexible in practice.
Verbatim anchors
“Passes the tests / Reveals intention / No duplication / Fewest elements”
“At the time there was a lot of ‘design is subjective’, ‘design is a matter of taste’ bullshit going around. I disagreed. There are better and worse designs.” — Kent Beck
“anything that doesn’t serve the three prior rules should be removed”