Go — Opinionated Coding Standards

Execution Order

When implementing or changing Go code, run this sequence unless project tooling defines a different workflow:

1. Detect project commands in this order:
   • make targets (if Makefile exists)
   • mage targets (if magefile.go exists)
   • raw Go commands
2. Run checks in this order: fmt -> lint -> test -> build

Default fallback commands when project tooling is missing:

• Format: golangci-lint fmt
• Lint: golangci-lint run
• Test: go test ./...
• Build: go build ./...

Project Structure

Choose structure based on project type:

Libraries / small tools — flat structure:

mylib/
  mylib.go
  mylib_test.go
  helpers.go

Services / microservices — standard layout:

cmd/
  server/main.go
  worker/main.go
internal/
  handler/
  service/
  repository/
  model/
go.mod

Rules:

• Use internal/ for private packages.
• cmd/ contains only main.go entrypoints. Keep them thin — parse config, wire dependencies, call run().
• Group by domain, not by technical layer, when the service grows beyond a few packages.
• One package = one responsibility. If a package name needs "and" to describe it, split it.

Code Style & Naming

• Format code with golangci-lint fmt (runs gofumpt + goimports via config).
• Prefer descriptive names over short names, especially for exported symbols and function parameters.
  • Acceptable short names: ctx, err, t, i, n for well-established Go idioms.
  • Bad: func Process(d []byte, f string) — Good: func Process(data []byte, filename string)
• Exported names must be self-documenting. Comment every exported type, function, and constant.
• Unexported helpers can be terser but must still be clear in context.
• Avoid stuttering: user.User is fine, user.UserService is not — use user.Service.
• Acronyms are all-caps: HTTPClient, ID, URL. Not HttpClient, Id, Url.
• Struct tags use snake_case for JSON, YAML, XML, and BSON.

Constants — No iota

Never use iota. It is implicit, fragile to reordering, and produces opaque values that are hard to grep, log, and debug. Always use explicit string-typed constants.

// Bad — iota: implicit values, break on reorder, meaningless in logs
const (
    assertJSON assertType = iota
    assertYAML
    assertHTML
    assertFile
)

// Good — explicit string constants
const (
    assertJSON assertType = "json"
    assertYAML assertType = "yaml"
    assertHTML assertType = "html"
    assertFile assertType = "file"
)

Do not use iota for new code in this skill.

Interfaces

• Keep interfaces small: 1-2 methods. Compose larger interfaces from small ones.
• Accept interfaces, return concrete structs.
• Define interfaces at the consumer side, not the implementation side.
• Name single-method interfaces with the -er suffix: Reader, Writer, Closer, Validator.

// Defined where it's used, not where it's implemented
type OrderStore interface {
    GetOrder(ctx context.Context, id string) (*Order, error)
}

type OrderService struct {
    store OrderStore // depends on interface
}

Error Handling

Use sentinel errors + wrapping with fmt.Errorf and %w.

var (
    ErrNotFound  = errors.New("not found")
    ErrConflict  = errors.New("conflict")
    ErrForbidden = errors.New("forbidden")
)

func (s *Service) GetUser(ctx context.Context, id string) (*User, error) {
    user, err := s.repo.Find(ctx, id)
    if err != nil {
        return nil, fmt.Errorf("get user %s: %w", id, err)
    }
    return user, nil
}

• Always wrap errors with context about what operation failed.
• Never discard errors silently. If intentionally ignoring, assign to _ with a comment.
• No pkg/errors or third-party error packages. Stdlib is sufficient.
• Return errors; do not panic. Reserve panic for truly unrecoverable programmer errors.
• Error messages are lowercase, no punctuation: "get user: not found".

Testing

Use stdlib testing + testastic. Test the public API through external _test packages. Use same-package tests only for complex internals that are not meaningfully reachable through the public surface. Use // given: / // when: / // then: comments in every test case. Hand-roll mocks, no additional mocking frameworks.

Choose the right testing approach based on project type:

• Libraries / packages — unit tests. Test the public API through _test package suffix. See references/unit-testing.md.
• Services / microservices — integration tests. Test components working together (handlers, DB, external APIs). See references/integration-testing.md.

Test Data Isolation

• Each test/subtest must own its test data (actual, expected, fixtures).
• Do not share package-level test payload constants/variables across tests.
• Do not reuse the same testdata/... fixture path across multiple tests.
• Fixture content must be unique per test case; avoid copy-paste fixtures with identical bytes.
• If two tests need similar data, create separate fixtures and change at least one scenario-relevant field so intent stays explicit.
• Repetition is acceptable only in non-executable documentation comments/examples.

Linting

Use golangci-lint v2. Run golangci-lint run for linting and golangci-lint fmt for formatting.

For the full reference config, enabled/disabled linter rationale, and rules, see references/linting.md.

Key rules:

• All code must pass golangci-lint run before committing.
• No //nolint without a comment explaining why.
• Test files have relaxed rules (funlen, dupl, mnd, wrapcheck, err113, gocognit, gosec excluded).

Context

• ctx context.Context is always the first parameter. No exceptions.
• Never store context.Context in a struct. Pass it through function calls.
• Use context.WithTimeout or context.WithCancel to scope work.
• Respect context cancellation: check ctx.Err() or select on ctx.Done() in long operations.

Concurrency

• Do not start goroutines without a clear ownership and shutdown strategy.
• Use errgroup.Group for concurrent operations that can fail.
• Channels for communication, mutexes for state protection. Do not mix unnecessarily.
• Always handle goroutine lifecycle: context.Context for cancellation, sync.WaitGroup or errgroup for joining.
• Prefer sync.Once over init() for lazy initialization.
• Never fire-and-forget goroutines in production code.
• Buffer channels only with a clear reason and known capacity.

g, ctx := errgroup.WithContext(ctx)
for _, item := range items {
    g.Go(func() error {
        return process(ctx, item)
    })
}
if err := g.Wait(); err != nil {
    return fmt.Errorf("process items: %w", err)
}

Dependency Injection

• Constructor-based injection. No global mutable state. No DI frameworks.
• Constructors return concrete types, accept interfaces.
• Wire dependencies in main() or a dedicated run() function in cmd/.

func NewOrderService(store OrderStore) *OrderService {
    return &OrderService{store: store}
}

Options Pattern

Use the functional options pattern for configurable constructors or functions with optional parameters. Do not use config structs with zero-value ambiguity.

// Option type is an unexported function that modifies config.
type Option func(*serverConfig)

// serverConfig holds all configurable fields with sensible defaults.
type serverConfig struct {
    port         int
    readTimeout  time.Duration
    writeTimeout time.Duration
}

// Exported option constructors — one per configurable field.
func WithPort(port int) Option {
    return func(c *serverConfig) {
        c.port = port
    }
}

func WithReadTimeout(timeout time.Duration) Option {
    return func(c *serverConfig) {
        c.readTimeout = timeout
    }
}

// Constructor applies defaults, then options.
func NewServer(opts ...Option) *Server {
    cfg := serverConfig{
        port:         8080,
        readTimeout:  5 * time.Second,
        writeTimeout: 10 * time.Second,
    }
    for _, opt := range opts {
        opt(&cfg)
    }

    return &Server{cfg: cfg}
}

// Usage: clean, readable, self-documenting.
srv := NewServer(
    WithPort(9090),
    WithReadTimeout(10 * time.Second),
)

Rules:

• Config struct is unexported. Options are the public API.
• Set sensible defaults in the constructor, not in each option.
• Each option constructor is a With* function returning Option.
• Option constructors return the Option type (not the concrete closure) — this is a valid exception to "return concrete types".
• Use this pattern when a function has more than 2-3 optional parameters. Do not use it for required parameters — those stay as regular arguments.

Logging

Use log/slog with context-aware logging. Configure the default logger once in main() via slog.SetDefault(). Always log with context using slog.InfoContext(ctx, ...), slog.ErrorContext(ctx, ...), etc. Do not pass *slog.Logger as a parameter — ctx already carries correlation IDs, trace IDs, and other request-scoped data.

For setup patterns, handler configuration, and context attribute extraction, see references/logging.md.

No init(), No Globals

• No init() functions. They make testing hard and hide side effects.
• No mutable global state. Pass dependencies explicitly.
• Package-level var is acceptable only for sentinel errors, constants, and sync.Once.

Dependency Management

• Use Go modules (go.mod). Run go mod tidy before committing.
• Pin direct dependency versions. Do not use latest.
• Review go.sum changes in PRs.
• Avoid vendoring unless required by build environment constraints.
• Minimize external dependencies. Prefer stdlib when reasonable.

Ask Before Proceeding

Ask the user before:

• Adding a new third-party dependency.
• Introducing or switching framework-level choices (router, ORM, logging stack, queue client).
• Using same-package tests for internals as the primary strategy in a package.
• Choosing integration test runtime strategy (containers, docker-compose, external shared services).

Done Criteria

A Go change is done when all are true:

• Formatting passes (golangci-lint fmt or project equivalent).
• Lint passes (golangci-lint run or project equivalent).
• Relevant tests pass (go test ./... or project equivalent).
• Build passes (go build ./... or project equivalent for changed binaries/packages).
• No unresolved placeholders, partial implementations, or undocumented exceptions to rules.