golang-patterns

name	golang-patterns
description	Idiomatic Go patterns, best practices, and conventions for building robust, efficient, and maintainable Go applications. Use when writing, reviewing, or refactoring Go code.

Go Development Patterns

Idiomatic Go patterns and best practices for building robust, efficient, and maintainable applications.

When to Activate

• Writing new Go code
• Reviewing Go code
• Refactoring existing Go code
• Designing Go packages/modules

Core Principles

1. Simplicity and Clarity

Go favors simplicity over cleverness. Code should be obvious and easy to read.

// Good: Clear and direct
func GetUser(ctx context.Context, id string) (*User, error) {
    user, err := db.FindUser(ctx, id)
    if err != nil {
        return nil, fmt.Errorf("get user %s: %w", id, err)
    }
    return user, nil
}

2. Make the Zero Value Useful

Design types so their zero value is immediately usable without initialization.

// Good: Zero value is useful
type Counter struct {
    mu    sync.Mutex
    count int // zero value is 0, ready to use
}

func (c *Counter) Inc() {
    c.mu.Lock()
    c.count++
    c.mu.Unlock()
}

3. Accept Interfaces, Return Structs

Functions should accept interface parameters and return concrete types.

func ProcessData(r io.Reader) (*Result, error) {
    data, err := io.ReadAll(r)
    if err != nil {
        return nil, err
    }
    return &Result{Data: data}, nil
}

Error Handling Patterns

Error Wrapping with Context

func LoadConfig(path string) (*Config, error) {
    data, err := os.ReadFile(path)
    if err != nil {
        return nil, fmt.Errorf("load config %s: %w", path, err)
    }
    var cfg Config
    if err := json.Unmarshal(data, &cfg); err != nil {
        return nil, fmt.Errorf("parse config %s: %w", path, err)
    }
    return &cfg, nil
}

Custom Error Types

type ValidationError struct {
    Field   string
    Message string
}

func (e *ValidationError) Error() string {
    return fmt.Sprintf("validation failed on %s: %s", e.Field, e.Message)
}

var (
    ErrNotFound     = errors.New("resource not found")
    ErrUnauthorized = errors.New("unauthorized")
    ErrInvalidInput = errors.New("invalid input")
)

Error Checking with errors.Is and errors.As

func HandleError(err error) {
    if errors.Is(err, sql.ErrNoRows) {
        log.Println("No records found")
        return
    }
    var validationErr *ValidationError
    if errors.As(err, &validationErr) {
        log.Printf("Validation error on field %s: %s",
            validationErr.Field, validationErr.Message)
        return
    }
    log.Printf("unexpected error: %v", err)
}

Concurrency Patterns

Worker Pool

func WorkerPool(ctx context.Context, jobs <-chan Job, results chan<- Result, numWorkers int) {
    var wg sync.WaitGroup
    for i := 0; i < numWorkers; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
            for job := range jobs {
                results <- process(job)
            }
        }()
    }
    wg.Wait()
    close(results)
}

Context for Cancellation and Timeouts

func FetchWithTimeout(ctx context.Context, url string) ([]byte, error) {
    ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
    defer cancel()

    req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
    if err != nil {
        return nil, fmt.Errorf("create request: %w", err)
    }
    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        return nil, fmt.Errorf("fetch %s: %w", url, err)
    }
    defer resp.Body.Close()
    return io.ReadAll(resp.Body)
}

Graceful Shutdown

func GracefulShutdown(server *http.Server) {
    quit := make(chan os.Signal, 1)
    signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
    <-quit
    log.Println("Shutting down server...")

    ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
    defer cancel()

    if err := server.Shutdown(ctx); err != nil {
        log.Fatalf("server forced to shutdown: %v", err)
    }
    log.Println("Server exited")
}

errgroup for Coordinated Goroutines

func FetchAll(ctx context.Context, urls []string) ([][]byte, error) {
    g, ctx := errgroup.WithContext(ctx)
    results := make([][]byte, len(urls))

    for i, url := range urls {
        i, url := i, url
        g.Go(func() error {
            data, err := FetchWithTimeout(ctx, url)
            if err != nil {
                return err
            }
            results[i] = data
            return nil
        })
    }
    if err := g.Wait(); err != nil {
        return nil, err
    }
    return results, nil
}

Avoiding Goroutine Leaks

// Good: Properly handles cancellation
func safeFetch(ctx context.Context, url string) <-chan []byte {
    ch := make(chan []byte, 1) // Buffered channel
    go func() {
        data, err := fetch(url)
        if err != nil {
            return
        }
        select {
        case ch <- data:
        case <-ctx.Done():
        }
    }()
    return ch
}

Interface Design

Small, Focused Interfaces

type Reader interface {
    Read(p []byte) (n int, err error)
}

type Writer interface {
    Write(p []byte) (n int, err error)
}

// Compose interfaces as needed
type ReadWriter interface {
    Reader
    Writer
}

Define Interfaces Where They're Used

// In the consumer package, not the provider
package service

type UserStore interface {
    GetUser(ctx context.Context, id string) (*User, error)
    SaveUser(ctx context.Context, user *User) error
}

type Service struct {
    store UserStore
}

Package Organization

Avoid Package-Level State

// Bad: Global mutable state
var db *sql.DB

// Good: Dependency injection
type Server struct {
    db *sql.DB
}

func NewServer(db *sql.DB) *Server {
    return &Server{db: db}
}

Struct Design

Functional Options Pattern

type Server struct {
    addr    string
    timeout time.Duration
    logger  *log.Logger
}

type Option func(*Server)

func WithTimeout(d time.Duration) Option {
    return func(s *Server) { s.timeout = d }
}

func WithLogger(l *log.Logger) Option {
    return func(s *Server) { s.logger = l }
}

func NewServer(addr string, opts ...Option) *Server {
    s := &Server{
        addr:    addr,
        timeout: 30 * time.Second,
        logger:  log.Default(),
    }
    for _, opt := range opts {
        opt(s)
    }
    return s
}

Memory and Performance

Preallocate Slices When Size is Known

// Good: Single allocation
func processItems(items []Item) []Result {
    results := make([]Result, 0, len(items))
    for _, item := range items {
        results = append(results, process(item))
    }
    return results
}

Avoid String Concatenation in Loops

// Good: strings.Builder
func join(parts []string) string {
    var sb strings.Builder
    for i, p := range parts {
        if i > 0 {
            sb.WriteString(",")
        }
        sb.WriteString(p)
    }
    return sb.String()
}

Quick Reference: Go Idioms

Idiom	Description
Accept interfaces, return structs	Functions accept interface params, return concrete types
Errors are values	Treat errors as first-class values, not exceptions
Don't communicate by sharing memory	Use channels for coordination
Make the zero value useful	Types should work without explicit initialization
A little copying is better than a little dependency	Avoid unnecessary external deps
Clear is better than clever	Prioritize readability
Return early	Handle errors first, keep happy path unindented

Struct Tags

Add json/yaml tags for structs that are serialized or unmarshaled across package or process boundaries.

Validation and documentation tags are framework-dependent. Only add them when the current package already uses that framework's conventions; do not introduce framework-specific tags as a default project rule.

type Provider struct {
    Name    string `json:"name" yaml:"name"`
    Version string `json:"version" yaml:"version"`
    Port    int    `json:"port" yaml:"port"`
}

Anti-Patterns to Avoid

// Bad: Naked returns in long functions
func process() (result int, err error) {
    // ... 50 lines ...
    return // What is being returned?
}

// Bad: Using panic for control flow
func GetUser(id string) *User {
    user, err := db.Find(id)
    if err != nil {
        panic(err) // Don't do this
    }
    return user
}

// Bad: Passing context in struct
type Request struct {
    ctx context.Context // Context should be first param
    ID  string
}

// Good: Context as first parameter
func ProcessRequest(ctx context.Context, id string) error {
    // ...
    return nil
}