go-idioms - Claude MCP Skill

# Go Idioms

Boring, explicit, race-safe Go. Accept interfaces, return structs.

## Error Handling

**Always wrap with context at package boundaries:**
```go
// Use %w to preserve error chain
return fmt.Errorf("fetching user %s: %w", userID, err)

// Check wrapped errors
if errors.Is(err, sql.ErrNoRows) { ... }

var paymentErr *PaymentError
if errors.As(err, &paymentErr) { ... }
```

Never: `%v` (loses type), raw errors from exported functions, generic context.

## Interface Design

**Define interfaces in consuming package, not provider:**
```go
// notification/sender.go (consumer defines interface)
type EmailSender interface {
    Send(ctx context.Context, to, subject, body string) error
}

// email/client.go (provider implements)
type Client struct { ... }
func (c *Client) Send(ctx context.Context, to, subject, body string) error { ... }
```

**Small interfaces (1-3 methods). Compose larger from smaller:**
```go
type Reader interface { Read(p []byte) (n int, err error) }
type Writer interface { Write(p []byte) (n int, err error) }
type ReadWriter interface { Reader; Writer }
```

**Compile-time verification:**
```go
var _ EmailSender = (*Client)(nil)
```

## Concurrency

**Always propagate context:**
```go
func FetchData(ctx context.Context) ([]byte, error) {
    select {
    case <-ctx.Done():
        return nil, ctx.Err()
    case result := <-dataChan:
        return result, nil
    }
}
```

**Bounded concurrency (semaphore):**
```go
sem := make(chan struct{}, 10)
for _, item := range items {
    sem <- struct{}{}
    go func(item Item) {
        defer func() { <-sem }()
        process(item)
    }(item)
}
```

**Race safety:** Always run `go test -race ./...`

## Package Design

```
internal/
  user/          # Domain: single purpose
    user.go
    service.go
    repository.go
  order/         # Another domain
  app/           # Dependency wiring
cmd/
  api/           # Entry points
```

**Rules:**
- Single purpose per package
- No generic names (utils, helpers, common)
- No circular dependencies
- Export only what's necessary

## Dependency Injection

```go
// Constructor accepts interfaces
func NewUserService(repo UserRepository, mailer EmailSender) *UserService {
    return &UserService{repo: repo, mailer: mailer}
}

// Wire in app/ package
func NewApp() *App {
    repo := postgres.NewUserRepo(db)
    mailer := sendgrid.NewClient(apiKey)
    userSvc := user.NewUserService(repo, mailer)
    return &App{UserService: userSvc}
}
```

## Deps Struct Pattern (5+ Parameters)

When constructor takes 5+ parameters, use a deps struct:

```go
// Group dependencies into named struct
type ServiceDeps struct {
    Repo     Repository
    Mailer   EmailSender
    Logger   Logger
    Config   *Config
    Cache    Cache
}

// Panic on nil - catches programming errors at construction
func NewService(deps ServiceDeps) *Service {
    if deps.Repo == nil {
        panic("NewService: Repo cannot be nil")
    }
    if deps.Mailer == nil {
        panic("NewService: Mailer cannot be nil")
    }
    // ... check all required deps
    return &Service{...}
}
```

**Benefits:**
- Named fields = self-documenting call sites
- Adding deps doesn't change signature
- Nil panics catch bugs at construction, not at runtime

**When changing old constructor to deps struct:**
- Update ALL call sites to struct literal pattern
- Check for tests expecting old nil-tolerance behavior

## Test Cleanup

Use `t.Cleanup()` for teardown, not `defer` with error suppression:

```go
// BAD - Error suppression hides failures
defer func() { _ = os.Chdir(orig) }()  // SA4017 warning

// GOOD - Proper cleanup with error handling
t.Cleanup(func() {
    if err := os.Chdir(orig); err != nil {
        t.Errorf("cleanup failed: %v", err)
    }
})
```

`t.Cleanup` runs after test completes (even on panic), integrates with test reporting.

## Anti-Patterns

- Goroutines without cancellation path (leaks)
- Monolithic interfaces (10+ methods)
- Framework-like inheritance patterns
- Reflection when explicit types work
- Global singletons for dependencies
- Generic everything (overuse of generics)
- `interface{}` / `any` without justification
- `defer` with error suppression in tests

## Embrace Boring

- Explicit error handling at each step
- Standard library first (`map`, `[]T`, `sort.Slice`)
- Table-driven tests
- Struct composition, not inheritance
- Clear, verbose code over clever code