Go (Golang) Style Guide Skill

This skill defines practical Go engineering conventions optimized for:

• humans reading and maintaining code
• coding agents generating or refactoring code reliably
• production readiness (correctness, testability, performance)

Use this skill whenever you are working with Go: new code, refactors, reviews, and architecture decisions.

---

TL;DR

• Design for testability first; inject dependencies and keep logic pure.
• Prefer Config in → concrete struct out; validate, default, and document important runtime knobs.
• Errors are contracts: use sentinels for durable branching; wrap the rest with %w or errors.Join.
• Keep packages reusable: no hidden globals, no default logging, no surprise side effects.
• Coverage is a signal, not proof; test edge cases and misuse paths, not just happy paths.
• Follow "accept interfaces, return structs"; consumers usually define interfaces, shared contract packages are a special case.
• Keep main.go thin; follow existing repo layout conventions rather than forcing one directory shape.
• Benchmark hot paths before claiming wins, and run concurrency code with -race.
• Maintain contracts such as function signatures, config shape, error behavior, and doc comments; they are as important as the code itself.

---

House Style Disclaimer

This is intentionally opinionated. It favors consistency and long-term maintainability over accommodating every Go style preference.

---

Compatibility

Examples assume modern Go and use standard-library features such as errors.Join and log/slog. Apply the guide within the constraints of the target repository's supported Go version.

---

Execution Protocol

Follow this workflow when using the skill for implementation work:

1. Inspect the repository first. Read existing package layout, constructors, tests, and error conventions before proposing new APIs or moving files.

2. Define the contract before coding. Decide the package boundary, config shape, concrete return type, sentinel errors, and context or shutdown expectations up front.

3. Write or update tests early when practical. Start with table-driven unit tests, add fuzz tests for parsing or other input-heavy code, and add benchmarks for performance-sensitive paths.

4. Implement the smallest maintainable change. Follow the repository's existing layout, keep main.go thin, and avoid introducing new abstractions without a clear boundary.

5. Run the finishing checks. Format with gofmt (and goimports if the repo uses it), run the relevant go test targets, run go test -race when concurrency is involved, and run benchmarks when claiming performance improvements.

6. Verify the human-facing contract. Make sure docs, comments, config defaults, and error behavior match the code you are shipping.

---

Quick Rules Table

Topic	Rule	Reference
Testability	Design for confidence, not coverage percentages; test edge and misuse cases	references/TESTING.md
Constructors	Config in → concrete struct out; validate + default in New; use Config.Validate() when config logic grows	references/CONFIG.md
Errors	Use sentinels for durable branching; wrap with %w or errors.Join; keep recover at app boundaries	references/ERRORS.md
Logging	Packages do not log by default; hot-path logging is a performance decision	references/LOGGING.md
Interfaces	"Accept interfaces, return structs"; consumers usually define interfaces	references/INTERFACES.md
Documentation	Write idiomatic godoc and durable comments; never add agent-context comments	references/DOCUMENTATION.md
Layout	Keep packages shallow, avoid junk drawers, and follow repo conventions	references/LAYOUT.md
Entry Points	main.go is wiring only	references/LAYOUT.md
Benchmarks	Benchmark hot paths; use b.ReportAllocs() and compare runs with benchstat	references/BENCHMARKS.md
Testing	Table-driven, stdlib-first, defensive against misuse, and fuzz-heavy where inputs are complex	references/TESTING.md
Concurrency	Every goroutine needs a shutdown path; use context.Context, -race, and jitter where needed	references/CONCURRENCY.md
Reviews	Use the checklist when reviewing Go changes	references/REVIEW-CHECKLIST.md

---

Common Pitfalls

• Returning interfaces by default instead of concrete types.
• Treating coverage percentages as proof of correctness.
• Logging in reusable packages instead of returning errors.
• Passing global app config through packages rather than local Config.
• Leaving critical runtime knobs on dangerous defaults.
• Forcing a house directory layout onto repos that already have clear conventions.
• Loading every reference document before you know which topic the task touches.
• Shipping changes that claim performance wins without benchmarks or concurrency safety without -race.

---

Core Principles

Testability is first-class

• Prefer designs that are easy to test without booting an entire application.
• Inject dependencies explicitly.
• Keep pure logic isolated.
• Test edge cases, invalid inputs, and misuse paths rather than only happy paths.

Config-driven construction

• Prefer Config in → struct out constructors.
• Validate at construction.
• Default explicitly.
• Make important runtime knobs visible and documented.

Errors are a contract

• Prefer sentinel errors for durable conditions callers need to branch on.
• Use %w (or errors.Join) so callers can use errors.Is/As.
• Prefer a small set of durable meanings plus contextual wrapping.

Benchmark what matters

• Add benchmarks for performance-sensitive code paths.
• Avoid "it's faster" claims without go test -bench.

Packages are reusable by default

• Keep packages domain-focused and individually testable.
• Avoid global state and hidden side effects.

Structure should reinforce intent

• Use package boundaries and entry points as architectural guardrails.
• Follow established repo conventions when they are clear.

---

Package Types

App package (orchestrator)

Owns:

• dependency wiring (DB, clients, loggers)
• lifecycle (start/stop)
• error policy (retry, ignore, crash)
• logging and metrics policy

Non-app packages (reusable units)

Rules:

• No direct logging (see references/LOGGING.md)
• Return errors, don't hide them
• Define a local Config/Opts contract
• Accept initialized dependencies (DB/client/etc); do not create them internally

---

Directory Structure

Services / apps

• cmd/<appname>/main.go for entrypoints
• Keep main.go thin: parse config, wire dependencies, call the app entrypoint.

This guide often prefers service repos that group packages under pkg/, with orchestration in something like pkg/app, but that is a house preference, not Go law.

Example:

cmd/myapp/main.go
pkg/...
pkg/app/...

If a repository already uses top-level packages, internal/, or a mixed shape with clear rules, follow the repository convention instead of forcing pkg/. Do not restructure an existing repository to introduce pkg/ unless you were explicitly asked to do that migration.

Libraries

• Packages at top-level directories, not nested under pkg/ or internal/.

• Avoid junk drawers (utils, common) unless they truly represent a domain.

---

Constructors and Config

• New(cfg Config) (*T, error) or Dial(cfg Config) (*T, error)
• Prefer passing Config by value; validate + default inside constructor
• For complex configs, move non-trivial validation into func (c *Config) Validate() error
• Return a concrete type by default
• Treat constructor-normalized config as immutable internal state
• Config is owned by the package, not the app
• Avoid init() for normal construction; it usually hides globals, ordering dependencies, or side effects better handled by explicit setup
• Follow "accept interfaces, return structs": inject boundary interfaces and return concrete types unless there is a clear multi-implementation boundary
• Make important runtime knobs explicit: timeouts, pool sizes, lifetimes, backoff/retry ceilings, and similar operational settings
• Use explicit runtime controls for degraded modes in critical services; do not expect callers to mutate Config after construction

See: references/CONFIG.md, references/INTERFACES.md

---

Logging

Logging is owned by the application.

If a package must log (rare async/network/runtime cases), inject *slog.Logger via Config, default to discard, keep structured logs, and prefer context-aware log methods when a real context.Context is already available.

Treat hot-path logging as a performance decision. Avoid chatty request-path Info logs, and if async logging is used, document buffering, backpressure, and drop behavior.

See: references/LOGGING.md

---

Errors

• Export var ErrX = errors.New("...") for stable, durable meanings callers may need to branch on
• Wrap with %w or use errors.Join so errors.Is works
• Don't use %s to wrap errors (it breaks unwrap semantics)
• Prefer a small set of sentinels plus contextual wrapping rather than a new sentinel for every failure path
• Keep recover at application boundaries or middleware, not in reusable packages
• Packages return errors; apps log them and decide whether to ignore, retry, or crash

See: references/ERRORS.md

---

Testing

• Prefer table-driven tests with clear, behavior-oriented case names.
• Coverage is a signal, not proof; confidence comes from meaningful assertions and edge cases.
• Test defensive behavior, not just the current happy path.
• Use go test -fuzz for parsers, decoders, and other input-heavy code.
• Use TestMain only for true package-wide lifecycle setup/teardown.
• Run -race in CI for concurrency-sensitive code.

See: references/TESTING.md

---

Documentation + Comments

• Public packages need package docs explaining purpose and main usage, using /** ... */ package comments.
• Exported identifiers get idiomatic doc comments, helpful links like [Config], and executable Example_ docs when teaching usage matters. Functions, methods, types, vars, consts, and fields use // doc comments.
• Comments explain why, contract, or intent; never narrate obvious code.
• No agent-context comments or self-referential TODO/FIXME notes.

See: references/DOCUMENTATION.md

---

Concurrency

• Every goroutine must have a clear shutdown path via context.Context.
• Long-lived services implement Close() or Stop() with concurrency synchronization to ensure graceful shutdown.
• Use sync.Mutex for complex state, sync/atomic for simple counters and flags.
• Prefer select with ctx.Done() for blocking operations.
• Use context-aware I/O APIs such as QueryContext and NewRequestWithContext.
• Use defer for cleanup in the scope that acquires a resource.
• Add jitter to recurring background work when synchronized schedules would create spikes.
• Graceful shutdown should fail readiness, drain in-flight work, stop listeners, and wait for tracked work to finish.
• If it's not tested with -race, assume it's not concurrency-safe.

See: references/CONCURRENCY.md

---

File Organization and Efficiency

Files follow idiomatic ordering: package docs, imports, consts/vars, sentinel errors, types, constructors, exported methods, unexported helpers. Avoid catch-all files like types.go or util.go.

Keep hot-path structs compact (field ordering for padding) but do not micro-optimize without benchmarks.

Use structure as an architectural guardrail. Package boundaries and entry points should make the intended placement of new behavior obvious.

Avoid init() in general. It is often a sign of hidden globals, implicit registration, or startup side effects that should be made explicit.

See: references/LAYOUT.md

---

Benchmarks

Add benchmarks for hot-path functions, serialization, concurrency primitives, and adapters in tight loops. Use b.ReportAllocs(), include realistic inputs, and compare alternatives when proposing changes.

Let the runner control b.N, run important numbers on a quiet machine, and use benchstat when comparing benchmark results.

See: references/BENCHMARKS.md

---

Interfaces + Implementations

Default to small consumer-defined interfaces and producer-owned concrete structs.

Use shared contract packages with subpackages (drivers/, backends/, etc.) as a special case when the package's primary purpose is to define a common boundary across multiple implementations.

See: references/INTERFACES.md

---

Reference Index

Use these supporting documents when deeper detail is needed:

• references/LOGGING.md Logging rules: default no-logging-in-packages guidance, exceptions, slog injection, and hot-path cost guidance.

• references/ERRORS.md Durable error contracts, wrapping rules, and errors.Is/As guidance.

• references/CONFIG.md Canonical Config struct patterns, constructor validation + defaults, and operational controls.

• references/INTERFACES.md Interface boundaries, consumer-defined defaults, and special-case driver patterns.

• references/DOCUMENTATION.md Package docs, idiomatic godoc, internal function comments, field docs, and durable comment rules.

• references/LAYOUT.md File organization, struct field efficiency, package naming guidance, and architectural guardrails.

• references/BENCHMARKS.md Benchmark expectations, templates, and result-comparison rules.

• references/TESTING.md Stdlib-first testing patterns, table-driven tests, helpers, defensive testing, and test file conventions.

• references/CONCURRENCY.md Goroutine lifecycle, context propagation, graceful shutdown, -race, jitter, and synchronization patterns.

• references/REVIEW-CHECKLIST.md PR review rubric for humans and coding agents.