---

name: clean-code
description: 'Write readable, maintainable code through disciplined naming, small functions, and clean error handling. Use when the user mentions "code review", "naming conventions", "function too long", "code smells", "readable code", "boy scout rule", "single responsibility", or "unit test quality". Also trigger when reviewing pull requests for readability, refactoring messy functions, debating comment styles, or improving error handling patterns. Covers SRP, comment discipline, formatting, and unit testing. For refactoring techniques, see refactoring-patterns. For architecture, see clean-architecture.'
license: MIT
metadata:
author: wondelai
version: "1.2.0"

Clean Code Framework

A disciplined approach to writing code that communicates intent, minimizes surprises, and welcomes change. Apply these principles when writing new code, reviewing pull requests, refactoring legacy systems, or advising on code quality.

Core Principle

Code is read far more often than it is written — optimize for the reader. The read-to-write ratio is well over 10:1, so every naming choice, function boundary, and formatting decision either adds clarity or adds cost. Clean code reads like well-written prose: names reveal intent, functions tell a story one step at a time, and the Boy Scout Rule applies — always leave the code cleaner than you found it.

Scoring

Goal: 10/10. Rate any code 0-10 against the principles below. Report the current score and the specific improvements needed to reach 10/10.

• 9-10: Names reveal intent, functions are small and focused, error handling is consistent, tests are clean and comprehensive
• 7-8: Mostly clean with minor naming ambiguities or a few long functions; tests may lack edge cases
• 5-6: Mixed — good patterns alongside unclear names, duplicated logic, or inconsistent error handling
• 3-4: Long multi-purpose functions, misleading names, poor or missing tests
• 1-2: Nearly unreadable — magic numbers, cryptic abbreviations, no structure, no tests

The Clean Code Framework

Six disciplines for writing code that communicates clearly and adapts to change:

1. Meaningful Names

Core concept: Names should reveal intent, avoid disinformation, and make the code read like prose. If a name requires a comment to explain it, the name is wrong.

Why it works: Names are the most pervasive form of documentation — a well-chosen name eliminates the need to read the implementation; a poor one forces every reader to reverse-engineer intent.

Key insights:

• A name should answer why it exists, what it does, and how it is used
• No encodings, prefixes, or type information (no Hungarian notation); single letters only for tiny-scope loop counters
• Classes are nouns; methods are verbs
• One word per concept: don't mix fetch, retrieve, and get
• Longer scope demands a longer, more descriptive name
• Rename freely — IDEs make it trivial

Code applications:

Context	Pattern	Example
Variables	Intention-revealing	elapsedTimeInDays not d
Booleans	Predicate phrasing	isActive, hasPermission, canEdit
Functions	Verb + noun	calculateMonthlyRevenue() not calc()
Classes	Noun naming the responsibility	InvoiceGenerator not InvoiceManager

See: references/naming-conventions.md

2. Functions

Core concept: Functions should be small, do one thing, and do it well — ideally 4-6 lines, zero to two arguments, one level of abstraction.

Why it works: Small single-purpose functions are easy to name, understand, test, and reuse; long functions hide bugs, resist testing, and accumulate responsibilities.

Key insights:

• Step-Down Rule: code reads top-down, each function calling the next level of abstraction
• Argument count: zero best, one fine, two acceptable, three+ requires justification
• Flag arguments are a smell — the function does two things; split it
• Command-Query Separation: change state or return a value, never both
• Extract till you drop: if you can pull out a named function, do it
• No hidden side effects — the name must tell the whole truth

Code applications:

Context	Pattern	Example
Long function	Extract named steps	validateInput(); transformData(); saveRecord();
Flag argument	Split into two functions	renderForPrint() / renderForScreen() not render(isPrint)
Error cases	Guard clauses at top	Early return for errors, single happy path
Many arguments	Introduce parameter object	new DateRange(start, end) not report(start, end, format, locale)
Side effects	Make effects explicit	checkPassword() that starts a session → rename or separate

See: references/functions-and-methods.md

3. Comments and Formatting

Core concept: A comment is a failure to express yourself in code. When comments are necessary, they explain why, never what. Formatting creates the visual structure that makes code scannable.

Why it works: Comments rot — code changes but comments often don't, creating documentation worse than none. Clean formatting lets developers scan code like a newspaper: headlines first, details on demand.

Key insights:

• The best comment is a well-named extracted function
• Acceptable: legal headers, TODOs, public API docs, genuine "why" explanations
• Commented-out code and journal comments: delete — version control remembers
• Vertical openness between concepts; vertical density within them; declare variables near usage
• Newspaper metaphor: high-level functions at the top of the file, details below

Code applications:

Context	Pattern	Example
Explaining "what"	Replace with better name	// check if eligible → isEligible()
Explaining "why"	Keep as comment	// RFC 7231 requires this header for proxies
Commented-out code	Delete it	Trust version control
Team formatting	Decide once, automate	Prettier, Black, gofmt

See: references/comments-formatting.md

4. Error Handling

Core concept: Error handling is a separate concern from business logic. Use exceptions rather than return codes, provide context with every exception, and never return or pass null.

Why it works: Return codes clutter the happy path with checks; exceptions separate the two cleanly. Returning null forces null checks on every caller, and one missing check crashes far from the source.

Key insights:

• Write the try-catch first — it defines a transaction boundary
• Prefer unchecked exceptions — checked ones violate the Open/Closed Principle
• Define exception classes by the caller's needs, not the failure type
• Don't return null (use empty collections, Optional, or throw); don't pass null either
• Special Case / Null Object pattern: return an object with default behavior instead of null

Code applications:

Context	Pattern	Example
Null returns	Empty collection or Optional	return Collections.emptyList() not return null
Error codes	Replace with exceptions	throw new InsufficientFundsException(balance, amount)
Third-party APIs	Wrap with adapter	PortfolioService wraps the vendor API, translates its exceptions
Special cases	Null Object pattern	GuestUser with default behavior instead of null checks
Context in errors	Include operation + state	"Failed to save invoice #1234 for customer 'Acme'"

See: references/error-handling.md

5. Unit Testing

Core concept: Tests are first-class code, kept clean with the same discipline as production code. Dirty tests are worse than no tests — they become a liability that slows every change.

Why it works: Clean tests are executable documentation and a safety net for refactoring; dirty tests make every modification a fight through incomprehensible test code.

Key insights:

• Three Laws of TDD: write a failing test first; only enough test to fail; only enough code to pass
• One concept per test — one logical assertion, not necessarily one assert
• F.I.R.S.T.: Fast, Independent, Repeatable, Self-validating, Timely
• Build a domain-specific testing language: helpers that read like a DSL
• Refactor test code as readily as production code

Code applications:

Context	Pattern	Example
Test structure	Arrange-Act-Assert	Setup, execute, verify — clearly separated
Test naming	Scenario + expected behavior	shouldRejectExpiredToken not test1
Shared setup	Builder/factory helpers	aUser().withRole(ADMIN).build()
Flaky tests	Remove external dependencies	Mock time, network, file system

See: references/testing-principles.md

6. Code Smells and Heuristics

Core concept: Smells are surface indicators of deeper design problems — learn to recognize them quickly and apply targeted refactorings instead of vague "cleanup".

Why it works: Smells are heuristics that point toward likely problems without deep analysis, turning code review instinct into specific, repeatable moves.

Key insights:

• Function smells: too many arguments, output arguments, flag arguments, dead functions
• General smells: duplication, wrong level of abstraction, feature envy, magic numbers
• Test smells: insufficient coverage, skipped tests, untested boundary conditions and failure paths
• Refactor in small, tested steps — never refactor and add features simultaneously
• Boy Scout Rule: leave the code cleaner than you found it

Code applications:

Context	Pattern	Example
Duplication	Extract shared logic	Common validation → validateEmail() helper
Feature envy	Move method to the data's class	order.calculateTotal() not calculator.total(order)
Dead code	Delete it	Remove unused functions, unreachable branches
Magic numbers	Named constants	MAX_LOGIN_ATTEMPTS = 5 not bare 5
Shotgun surgery	Consolidate related changes	Group scattered logic into a single module

See: references/code-smells.md

Common Mistakes

Mistake	Why It Fails	Fix
Abbreviating names	Saves seconds writing, costs hours reading	Full descriptive names; IDEs autocomplete
"Clever" one-liners	Impressive to write, impossible to debug	Expand into readable named steps
Comments instead of refactoring	Comments rot; code is the truth	Extract a well-named function instead
Catching generic exceptions	Swallows bugs along with expected errors	Catch specific exceptions; let the rest propagate
No tests for error paths	Happy path works, edge cases crash	Test every branch, boundary, and failure mode
Premature optimization	Obscures intent for marginal gains	Clean first; optimize measured bottlenecks
God classes	One 2000-line class does everything	Apply SRP — split by responsibility
Refactoring without tests	No safety net for regressions	Write characterization tests first
Inconsistent conventions	Every file feels like a different codebase	Agree on style; enforce with linters and formatters
Returning null everywhere	Null checks spread like a virus	Optional, empty collections, or Null Object

Quick Diagnostic

Question	If No	Action
Can you understand each function without reading its body?	Names don't reveal intent	Rename to describe what it does
Are all functions under 20 lines?	Functions do too many things	Extract sub-operations into named helpers
Zero commented-out code blocks?	Dead code creating confusion	Delete — version control has history
Is error handling separate from business logic?	Try-catch clutters the main flow	Extract handlers; exceptions over return codes
Does every class have a single responsibility?	Classes accumulate unrelated duties	Split into focused, well-named classes
Is there a test for every public method?	No safety net for changes	Add tests before changing further
Are test names descriptive of behavior?	Failures are hard to interpret	Rename to shouldDoXWhenY
Is duplication below 3 occurrences?	Copy-paste spreading bugs	Extract a shared function or module
Are magic numbers named constants?	Intent hidden behind raw values	Extract descriptive constants
Do all tests run in under 10 seconds?	Slow tests don't get run	Mock external deps; split integration tests

Reference Files

• naming-conventions.md: Intention-revealing names, avoiding disinformation, class vs. method naming, before/after examples
• functions-and-methods.md: Small functions, argument counts, command-query separation, the step-down rule, side effects
• comments-formatting.md: Good vs. bad comments, the newspaper metaphor, vertical formatting, team rules
• error-handling.md: Exceptions over return codes, null handling, Special Case pattern, wrapping third-party APIs
• testing-principles.md: TDD laws, F.I.R.S.T. principles, clean test patterns, test readability
• code-smells.md: Comprehensive smell catalog organized by category, with targeted refactorings

Further Reading

Based on Robert C. Martin's seminal guide to software craftsmanship:

• "Clean Code: A Handbook of Agile Software Craftsmanship" by Robert C. Martin
• "The Clean Coder: A Code of Conduct for Professional Programmers" by Robert C. Martin
• "Clean Architecture: A Craftsman's Guide to Software Structure and Design" by Robert C. Martin
• "Refactoring: Improving the Design of Existing Code" by Martin Fowler

About the Author

Robert C. Martin ("Uncle Bob") has been programming since 1970, co-authored the Agile Manifesto, and founded Uncle Bob Consulting and Clean Coders. His books — Clean Code, The Clean Coder, Clean Architecture, and Clean Agile — shaped how a generation of developers think about code quality, and his core stance is that the only way to go fast is to go well.