pow10-rule-04-short-functions

Rule 4 — Short Functions

Severity: high

Statement

Each function fits on one printed page — hard limit 60 source lines (excluding comments), soft limit 40. Functions exceeding the hard limit must be decomposed or carry a documented waiver.

Rationale

A function that fits on one page can be reviewed and tested as a unit. Longer functions hide bugs by combining responsibilities. Short functions also make refactoring safer and unit testing tractable.

Universal violation patterns

• Single function > 60 SLoC
• Cyclomatic complexity > 10
• Multiple distinct responsibilities in one body (parsing + validating + persisting)
• Deep nesting (> 3 levels of if/for)

Universal remediation pattern

Extract by responsibility. Each helper takes one phase of the work and returns its output to the caller, which now reads as a sequential pipeline. Each helper is independently testable.

Per-language guidance

• C: references/c.md
• Go: references/go.md
• Python: references/python.md
• Java: references/java.md
• Kotlin: references/kotlin.md

When violation is justified

Generated code, large switch dispatchers (consider table-driven instead), state machines. Document with a waiver:

// pow10: allow rule=4 until=YYYY-MM-DD owner=<handle> reason="generated state table"

Citations

• Holzmann 2006 — Rule 4