pow10-rule-06-minimum-scope

name	pow10-rule-06-minimum-scope
description	NASA Power of 10 Rule 6 — Declare data at smallest possible scope. Severity: medium.

Rule 6 — Minimum Scope

Severity: medium

Statement

Variables declared at the narrowest scope where they are used. No mutable globals or file-scope statics. When unavoidable, mark them const or guard with documented access protocols.

Rationale

Narrow scope makes data hiding explicit, simplifies debugging, prevents accidental cross-scope mutation, and helps the compiler optimize register allocation. Globals hide ownership and create implicit coupling.

Universal violation patterns

• Mutable module-level / package-level state shared across functions
• Variables declared at function top but used in only one branch
• File-scope static/var mutated from multiple functions
• Loop variables declared outside the loop (and Go ≤ 1.21 capture bugs)
• companion object / static class fields with var

Universal remediation pattern

Encapsulate state in an instance owned by a single object. Pass dependencies explicitly. Declare locals at the point of first use. Prefer immutable bindings (const/final/val) over mutable.

Per-language guidance

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

When module-level state is justified

• Truly immutable constants (const/final/val)
• Logger handles created at startup
• Singleton service handles initialized in init phase

Mark them clearly:

// Configuration loaded at init; immutable thereafter.
var config = mustLoadConfig()

Citations

• Holzmann 2006 — Rule 6