Clean code

Clean Code

These are mandatory engineering standards for any code you write, change, or review while this skill is active. They are language-agnostic: apply them in whatever language the project uses, respecting that language's idioms.

If there is tension between speed and cleanliness, choose the clean solution unless there is a demonstrated operational emergency. If a shortcut is unavoidable, contain it, make it obvious, and leave the code safe to clean up later.

For detailed rationale, examples, and design heuristics, read reference.md — consult it when a rule needs interpretation or when you face a non-obvious design decision.

Core Rules

1. Leave the code cleaner

Every change must improve at least one of: clarity, naming, simplicity, cohesion, testability, error handling, duplication removal, or separation of concerns. Do not add code that is merely functional — add code that is understandable.

2. Optimize for readers

• Write code for the next engineer, not for impressiveness.
• Prefer straightforward code over clever code.
• Keep control flow obvious.
• If code is hard to explain, treat that as a design problem.

3. Use strong names

• Names must reveal intent. Use domain language consistently.
• Prefer specific names over generic ones. Avoid data, info, thing, stuff, manager, helper, util, misc, handle, temp, obj unless they are genuinely correct in the domain or an external protocol.
• Boolean names read like predicates (active?, isActive, hasItems, retryable?), following the language's convention.
• Collection names are plural.
• Function names describe the action performed, not vague activity (calculate_invoice_total, not do_invoice).

4. Keep methods small and focused

• A method should do one thing at one level of abstraction.
• Do not mix business rules, formatting, persistence, external calls, and error recovery in one method. Extract helpers when a method grows multiple phases.
• Prefer zero, one, or two parameters; three should trigger scrutiny; more usually means a missing value object or domain object.
• Do not use boolean parameters to switch behavior when separate methods would express intent more clearly.

5. Keep abstraction levels separate

• Do not mix high-level policy with low-level mechanics in one method.
• Keep orchestration separate from implementation detail, and domain logic separate from I/O, framework glue, and serialization.

6. Make side effects explicit

• Avoid methods that both answer a question and perform an action.
• Side effects must be visible in naming and placement; avoid hidden mutation.
• Prefer explicit dependencies over implicit globals.

7. Control conditionals

• Prefer guard clauses over deep nesting; keep branching shallow.
• Extract complex predicates into intention-revealing names.
• Avoid repeated conditional logic across multiple places.
• Prefer positive conditions where they read more clearly.

8. Treat errors as design

• Fail fast on invalid assumptions; do not swallow errors silently.
• Raise or return errors with useful, diagnostic context.
• Keep the happy path readable.
• Do not blur validation errors, domain errors, and infrastructure failures without reason.

9. Keep modules and classes cohesive

• Each unit should have one clear reason to change. Keep public surfaces small.
• Avoid god objects, god modules, and catch-all utility namespaces.
• Prefer composition over tangled inheritance.

10. Remove duplication at the right level

• Remove duplicated logic, decisions, business rules, and meaningful literals.
• Do not invent premature or unstable abstractions just to merge superficially similar lines. Deduplicate at the right level of meaning.

11. Work test-first

• Default to test-driven development. Write or update a failing test first when changing behavior, fixing a bug, or adding a feature.
• Test behavior, not incidental implementation details.
• Tests must be readable, deterministic, independent of order, and named by the expected outcome. Keep setup small and intention-revealing.
• Add regression coverage for bugs before fixing them whenever practical.
• If strict test-first is genuinely blocked (missing harness, legacy constraints), say so explicitly and add the nearest useful coverage immediately, leaving the area more testable than before.

12. Refactor while you work

• Improve weak names, break up long methods, and remove dead code in the area you touch.
• Keep tests passing while refactoring.
• If a full cleanup is out of scope, improve the nearest meaningful boundary and leave a clear note.

13. Respect language idioms — without hiding intent

• Use the target language's idioms only when they remain obvious to a competent engineer in that language.
• Prefer small, expressive units over procedural scripts with shared mutable state.
• Avoid metaprogramming, magic, or DSLs when plain code would be clearer at both the call site and the definition site.
• Isolate external systems (databases, APIs, queues, frameworks) behind clear boundaries; keep vendor-specific knowledge from leaking everywhere.

14. Definition of done

A task is done only when:

• the implementation is correct
• the design is understandable and the names are clear
• the touched code is cleaner than before
• relevant tests exist and pass (test-first unless a real constraint was noted)
• no obvious duplication or dead code remains in the changed area