clean-code

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":"1.1.0"}

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 improvements.

Core Principle

Code is read far more often than it is written. Optimize for the reader. Every naming choice, function boundary, and formatting decision either adds clarity or adds cost. The ratio of time spent reading code to writing code is well over 10:1. Making code easier to read makes it easier to write, easier to debug, and easier to extend.

The foundation: Clean code is not about following rules mechanically -- it is about caring for the craft. A clean codebase reads like well-written prose: names reveal intent, functions tell a story one step at a time, and there are no surprises lurking in dark corners. The Boy Scout Rule applies: always leave the code cleaner than you found it.

Scoring

Goal: 10/10. When reviewing or writing code, rate it 0-10 based on adherence to the principles below. A 10/10 means full alignment with all guidelines; lower scores indicate gaps to address. Always provide the current score and 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 exist but may lack edge cases.
• 5-6: Mixed quality -- some good patterns alongside unclear names, duplicated logic, or inconsistent error handling.
• 3-4: Significant readability issues -- long functions doing multiple things, misleading names, poor or missing tests.
• 1-2: Code works but is 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 poorly chosen name forces every reader to reverse-engineer the author's intent.

Key insights:

• Names should answer why it exists, what it does, and how it is used
• Avoid single-letter variables except for loop counters in tiny scopes
• Avoid encodings, prefixes, and type information in names (no Hungarian notation)
• Class names should be nouns or noun phrases; method names should be verbs or verb phrases
• Use one word per concept consistently: don't mix fetch, retrieve, and get
• Longer scopes demand longer, more descriptive names
• Don't be afraid to rename -- IDEs make it trivial

Code applications:

Context	Pattern	Example
Variables	Intention-revealing name	elapsedTimeInDays not d or elapsed
Booleans	Predicate phrasing	isActive, hasPermission, canEdit
Functions	Verb + noun describing action	calculateMonthlyRevenue() not calc()
Classes	Noun describing responsibility	InvoiceGenerator not InvoiceManager
Constants	Searchable, all-caps with context	MAX_RETRY_ATTEMPTS = 3 not 3 inline
Collections	Plural nouns or descriptive phrases	activeUsers not list or data

See: references/naming-conventions.md

2. Functions

Core concept: Functions should be small, do one thing, and do it well. The ideal function is 4-6 lines long, takes zero to two arguments, and operates at a single level of abstraction.

Why it works: Small functions are easy to name, easy to understand, easy to test, and easy to reuse. When a function does one thing, its name can describe exactly what it does, eliminating the need to read the body. Long functions hide bugs, resist testing, and accumulate responsibilities over time.

Key insights:

• Functions should do one thing, do it well, and do it only
• The Step-Down Rule: code should read like a top-down narrative, each function calling the next level of abstraction
• Ideal argument count is zero (niladic), then one (monadic), then two (dyadic); three or more (polyadic) requires justification
• Flag arguments (booleans) are a code smell -- they mean the function does two things
• Command-Query Separation: a function should either change state or return a value, never both
• Extract till you drop: if you can extract a named function from a block, do it
• Functions should have no side effects -- no hidden changes to state

Code applications:

Context	Pattern	Example
Long function	Extract into named steps	validateInput(); transformData(); saveRecord();
Flag argument	Split into two functions	renderForPrint() and renderForScreen() not render(isPrint)
Deep nesting	Extract inner blocks	Move nested if/for bodies into named functions
Multiple returns	Guard clauses at top	Early return for error cases, single happy path
Many arguments	Introduce parameter object	new DateRange(start, end) not report(start, end, format, locale)
Side effects	Make effects explicit	Rename checkPassword() to checkPasswordAndInitSession() or separate

See: references/functions-and-methods.md

3. Comments and Formatting

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

Why it works: Comments rot. Code changes but comments often do not, creating misleading documentation that is worse than no documentation. Clean formatting -- consistent indentation, vertical spacing between concepts, and logical ordering -- lets developers scan code the way readers scan a newspaper: headlines first, details on demand.

Key insights:

• The best comment is the code itself -- extract a well-named function instead of writing a comment
• Legal comments (copyright headers) and TODO comments are acceptable
• Javadoc for public APIs is valuable; Javadoc for internal code is noise
• Commented-out code should be deleted -- version control remembers it
• Journal comments (changelog in the file) are obsolete -- use git log
• Vertical openness: separate concepts with blank lines
• Vertical density: related code should appear close together
• Variables should be declared close to their usage
• Instance variables should be declared at the top of the class

Code applications:

Context	Pattern	Example
Explaining "what"	Replace with better name	Rename // check if eligible to isEligible()
Explaining "why"	Keep as comment	// RFC 7231 requires this header for proxies
Commented-out code	Delete it	Trust version control to remember
File organization	Newspaper metaphor	High-level functions at top, details below
Related code	Group vertically	Keep caller near callee in the same file
Team formatting	Agree on rules once	Use automated formatters (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 force the caller to check immediately, cluttering the happy path with error-checking logic. Exceptions let you separate the happy path from error handling, making both easier to read. Returning null forces every caller to add null checks, and a single missing check produces a NullPointerException far from the source.

Key insights:

• Write your try-catch block first -- it defines a transaction boundary
• Use unchecked exceptions -- checked exceptions violate the Open/Closed Principle
• Create informative exception messages: include the operation that failed and the context
• Define exception classes in terms of the caller's needs, not the type of failure
• The Special Case Pattern: return a special-case object instead of null (e.g., empty list, guest user)
• Don't return null -- return empty collections, Optional, or throw
• Don't pass null -- no reasonable behavior exists for null arguments
• Wrap third-party APIs to translate their exceptions into your domain

Code applications:

Context	Pattern	Example
Null returns	Return 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 vendor API, translates exceptions
Null arguments	Fail fast with assertion	Objects.requireNonNull(user, "user must not be null")
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. They must be clean, readable, and maintained 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 serve as executable documentation, showing exactly how the system is intended to behave. They provide a safety net for refactoring and a regression check for every change. Without tests, every modification is a potential bug. With dirty tests, every modification requires fighting through incomprehensible test code.

Key insights:

• The Three Laws of TDD: (1) write a failing test first, (2) write only enough test to fail, (3) write only enough code to pass
• One concept per test -- not necessarily one assert, but one logical assertion
• Tests should be readable: use the Build-Operate-Check pattern (Arrange-Act-Assert)
• F.I.R.S.T. principles: Fast, Independent, Repeatable, Self-validating, Timely
• Test names should describe the scenario and expected behavior
• Test code deserves the same refactoring attention as production code
• Domain-specific testing language: build helper functions that read like a DSL

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	Extract builder/factory	aUser().withRole(ADMIN).build()
Multiple scenarios	Parameterized tests	One test method, multiple input/output pairs
Flaky tests	Remove external dependencies	Mock time, network, file system
Test readability	Domain-specific helpers	assertThatInvoice(inv).isPaidInFull()

See: references/testing-principles.md

6. Code Smells and Heuristics

Core concept: Code smells are surface indicators of deeper design problems. Learn to recognize them quickly and apply targeted refactorings. Not every smell requires immediate action, but ignoring them accumulates technical debt.

Why it works: Smells are heuristics -- they point toward likely problems without requiring deep analysis. A developer who can quickly identify "this function has too many arguments" or "this class has feature envy" can make targeted improvements instead of vague "cleanup" efforts.

Key insights:

• Comments: inappropriate information, obsolete comments, redundant comments that repeat the code
• Functions: too many arguments, output arguments, flag arguments, dead functions never called
• General: obvious duplication, code at wrong level of abstraction, feature envy (method uses another class more than its own), magic numbers
• Names: names at wrong abstraction level, names that don't describe side effects, ambiguous short names
• Tests: insufficient tests, skipped tests, untested boundary conditions, no failure-path tests
• Apply the Boy Scout Rule: leave code cleaner than you found it
• Refactor in small, tested steps -- never refactor and add features simultaneously

Code applications:

Context	Pattern	Example
Duplication	Extract shared logic	Common validation → validateEmail() helper
Long parameter list	Introduce parameter object	SearchCriteria groups related params
Feature envy	Move method to 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	Use full, descriptive names; IDEs autocomplete
"Clever" one-liners	Impressive to write, impossible to debug	Expand into readable steps with clear names
Comments instead of refactoring	Comments rot; code is the truth	Extract well-named function instead of commenting
Catching generic exceptions	Swallows bugs along with expected errors	Catch specific exceptions, let unexpected ones 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 performance	Write clean code first, optimize measured bottlenecks
God classes	One class, 2000 lines, does everything	Apply SRP -- split by responsibility
Refactoring without tests	No safety net to catch regressions	Write characterization tests before refactoring
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	Use Optional, empty collections, or Null Object pattern

Quick Diagnostic

Audit any codebase:

Question	If No	Action
Can you understand each function without reading its body?	Names don't reveal intent	Rename functions to describe what they do
Are all functions under 20 lines?	Functions do too many things	Extract sub-operations into named helpers
Are there zero commented-out code blocks?	Dead code creating confusion	Delete them -- version control has history
Is error handling separate from business logic?	Try-catch blocks cluttering main flow	Extract error handling; use exceptions not return codes
Does every class have a single responsibility?	Classes accumulate unrelated duties	Split into focused classes with clear names
Is there a test for every public method?	No safety net for changes	Add tests before making further changes
Are test names descriptive of behavior?	Tests are hard to understand when they fail	Rename to shouldDoXWhenY pattern
Is duplication below 3 occurrences?	Copy-paste spreading bugs	Extract into shared function or module
Are magic numbers replaced with named constants?	Intent is hidden behind raw values	Extract constants with descriptive names
Can you run all tests in under 10 seconds?	Slow tests discourage running them	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

This skill is 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") is a software engineer, instructor, and author who has been programming since 1970. He is a co-author of the Agile Manifesto and the founder of Uncle Bob Consulting LLC and Clean Coders. His books -- Clean Code (2008), The Clean Coder (2011), Clean Architecture (2017), and Clean Agile (2019) -- have shaped how an entire generation of developers think about code quality, professional responsibility, and software design. Martin is known for his uncompromising stance that developers are professionals who must take responsibility for the quality of their work, and that the only way to go fast is to go well.