# Applying Code Principles

> Fundamental software principles - SOLID, DRY, Occam's Razor, Miller's Law, YAGNI. Triggers: 原則, シンプル, 複雑, アーキテクチャ, リファクタリング, 保守性, コード品質, design pattern, best practice, clean code

- Skill: `majiayu000/applying-code-principles-2` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds add majiayu000/applying-code-principles-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/applying-code-principles-2/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/applying-code-principles-2

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# Code Principles

Claude knows all principles. This file defines project-specific thresholds and priority.

## Priority Order

When principles conflict:

1. **Safety First** - Security, data integrity
2. **YAGNI** - Don't build what you don't need
3. **Occam's Razor** - Simplest solution
4. **SOLID** - For complex systems
5. **DRY** - Eliminate duplication (not at cost of clarity)
6. **Miller's Law** - Respect 7±2 cognitive limit

## Quick Decision Questions

- "Is there a simpler way?" (Occam's Razor)
- "Understandable in <1 min?" (Miller's Law)
- "Duplicating knowledge?" (DRY)
- "Needed now?" (YAGNI)
- "Single reason to change?" (SOLID/SRP)

## Thresholds

### Miller's Law

| Target | Ideal | Max | Limit |
| --- | --- | --- | --- |
| Function args | 3 | 5 | 7 |
| Class methods | 5 | 7 | 9 |
| Conditionals | 3 | 5 | 7 |

### DRY

- **Rule of Three**: Abstract on 3rd duplication

### SOLID

- Create interface only when 2nd implementation appears
- No single-implementation interfaces

### YAGNI

Before adding, confirm:

1. Problem exists now? → No = don't build
2. Failed in production? → No = no error handling yet
3. User requested? → No = no feature yet
4. Measured evidence? → No = no optimization yet