go

# Go ## Overview Go programming patterns including concurrency, error handling, and idiomatic Go code. --- ## Basic Patterns ### Structs and Methods ```go package main import ( "encoding/json" "fmt" "time" ) // Struct definition type User struct { ID string `json:"id"` Email string `json:"email"` Name string `json:"name"` CreatedAt time.Time `json:"created_at"` metadata map[string]interface{} // unexported (private) } // Constructor function func NewUser(email, name string) *User { return &User{ ID: generateID(), Email: email, Name: name, CreatedAt: time.Now(), metadata: make(map[string]interface{}), } } // Value receiver (for read-only) func (u User) FullName() string { return u.Name } // Pointer receiver (for mutations or large structs) func (u *User) SetMetadata(key string, value interface{}) { u.metadata[key] = value } // Embedding (composition) type Admin struct { User // Embedded struct Permissions []string } func (a *Admin) HasPermission(perm string) bool { for _, p := range a.Permissions { if p == perm { return true } } return false } ``` ### Interfaces ```go // Interface definition type Repository interface { Find(id string) (*User, error) FindAll() ([]*User, error) Create(user *User) error Update(user *User) error Delete(id string) error } // Interface implementation (implicit) type MemoryRepository struct { users map[string]*User } func NewMemoryRepository() *MemoryRepository { return &MemoryRepository{ users: make(map[string]*User), } } func (r *MemoryRepository) Find(id string) (*User, error) { user, ok := r.users[id] if !ok { return nil, ErrNotFound } return user, nil } func (r *MemoryRepository) Create(user *User) error { r.users[user.ID] = user return nil } // Compile-time interface check var _ Repository = (*MemoryRepository)(nil) // Empty interface (any type) func PrintAny(v interface{}) { fmt.Printf("%v\n", v) } // Type assertion func ProcessValue(v interface{}) { switch val := v.(type) { case string: fmt.Println("String:", val) case int: fmt.Println("Int:", val) case *User: fmt.Println("User:", val.Name) default: fmt.Println("Unknown type") } } ``` --- ## Error Handling ```go import ( "errors" "fmt" ) // Sentinel errors var ( ErrNotFound = errors.New("not found") ErrBadRequest = errors.New("bad request") ) // Custom error type type ValidationError struct { Field string Message string } func (e *ValidationError) Error() string { return fmt.Sprintf("validation error on %s: %s", e.Field, e.Message) } // Error wrapping func GetUser(id string) (*User, error) { user, err := repository.Find(id) if err != nil { return nil, fmt.Errorf("getting user %s: %w", id, err) } return user, nil } // Error checking func ProcessUser(id string) error { user, err := GetUser(id) if err != nil { if errors.Is(err, ErrNotFound) { return fmt.Errorf("user not found: %s", id) } var validationErr *ValidationError if errors.As(err, &validationErr) { return fmt.Errorf("validation failed: %s", validationErr.Field) } return err } // Process user... return nil } // Multi-error handling type MultiError struct { Errors []error } func (m *MultiError) Error() string { var msgs []string for _, err := range m.Errors { msgs = append(msgs, err.Error()) } return strings.Join(msgs, "; ") } func (m *MultiError) Add(err error) { if err != nil { m.Errors = append(m.Errors, err) } } func (m *MultiError) HasErrors() bool { return len(m.Errors) > 0 } ``` --- ## Concurrency ### Goroutines and Channels ```go // Basic goroutine func main() { go func() { fmt.Println("Hello from goroutine") }() time.Sleep(100 * time.Millisecond) } // Channel basics func worker(jobs <-chan int, results chan<- int) { for job := range jobs { results <- job * 2 } } func main() { jobs := make(chan int, 100) results := make(chan int, 100) // Start workers for w := 0; w < 3; w++ { go worker(jobs, results) } // Send jobs for j := 0; j < 9; j++ { jobs <- j } close(jobs) // Collect results for r := 0; r < 9; r++ { fmt.Println(<-results) } } // Select for multiple channels func main() { ch1 := make(chan string) ch2 := make(chan string) go func() { time.Sleep(100 * time.Millisecond) ch1 <- "one" }() go func() { time.Sleep(200 * time.Millisecond) ch2 <- "two" }() for i := 0; i < 2; i++ { select { case msg1 := <-ch1: fmt.Println("Received:", msg1) case msg2 := <-ch2: fmt.Println("Received:", msg2) case <-time.After(500 * time.Millisecond): fmt.Println("Timeout") } } } ``` ### Concurrency Patterns ```go import ( "context" "sync" ) // Worker pool type WorkerPool struct { numWorkers int jobs chan func() wg sync.WaitGroup } func NewWorkerPool(numWorkers int) *WorkerPool { pool := &WorkerPool{ numWorkers: numWorkers, jobs: make(chan func(), numWorkers*2), } pool.Start() return pool } func (p *WorkerPool) Start() { for i := 0; i < p.numWorkers; i++ { go func() { for job := range p.jobs { job() p.wg.Done() } }() } } func (p *WorkerPool) Submit(job func()) { p.wg.Add(1) p.jobs <- job } func (p *WorkerPool) Wait() { p.wg.Wait() } func (p *WorkerPool) Close() { close(p.jobs) } // Fan-out, fan-in func FanOut(ctx context.Context, input <-chan int, workers int) []<-chan int { outputs := make([]<-chan int, workers) for i := 0; i < workers; i++ { outputs[i] = worker(ctx, input) } return outputs } func FanIn(ctx context.Context, channels ...<-chan int) <-chan int { var wg sync.WaitGroup merged := make(chan int) output := func(c <-chan int) { defer wg.Done() for v := range c { select { case merged <- v: case <-ctx.Done(): return } } } wg.Add(len(channels)) for _, c := range channels { go output(c) } go func() { wg.Wait() close(merged) }() return merged } // Rate limiter type RateLimiter struct { ticker *time.Ticker tokens chan struct{} } func NewRateLimiter(rate int, burst int) *RateLimiter { rl := &RateLimiter{ ticker: time.NewTicker(time.Second / time.Duration(rate)), tokens: make(chan struct{}, burst), } // Fill initial burst for i := 0; i < burst; i++ { rl.tokens <- struct{}{} } // Refill tokens go func() { for range rl.ticker.C { select { case rl.tokens <- struct{}{}: default: } } }() return rl } func (rl *RateLimiter) Wait(ctx context.Context) error { select { case <-rl.tokens: return nil case <-ctx.Done(): return ctx.Err() } } ``` --- ## Context ```go import ( "context" "time" ) // Context with timeout func FetchWithTimeout(url string) ([]byte, error) { ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second) defer cancel() req, err := http.NewRequestWithContext(ctx, "GET", url, nil) if err != nil { return nil, err } resp, err := http.DefaultClient.Do(req) if err != nil { return nil, err } defer resp.Body.Close() return io.ReadAll(resp.Body) } // Context with values type contextKey string const userIDKey contextKey = "userID" func WithUserID(ctx context.Context, userID string) context.Context { return context.WithValue(ctx, userIDKey, userID) } func GetUserID(ctx context.Context) (string, bool) { userID, ok := ctx.Value(userIDKey).(string) return userID, ok } // Passing context through layers func Handler(w http.ResponseWriter, r *http.Request) { ctx := r.Context() ctx = WithUserID(ctx, r.Header.Get("X-User-ID")) result, err := ProcessRequest(ctx) if err != nil { http.Error(w, err.Error(), http.StatusInternalServerError) return } json.NewEncoder(w).Encode(result) } func ProcessRequest(ctx context.Context) (*Result, error) { // Check for cancellation select { case <-ctx.Done(): return nil, ctx.Err() default: } userID, ok := GetUserID(ctx) if !ok { return nil, errors.New("user ID not found in context") } return fetchData(ctx, userID) } ``` --- ## Generics (Go 1.18+) ```go // Generic function func Map[T, U any](items []T, fn func(T) U) []U { result := make([]U, len(items)) for i, item := range items { result[i] = fn(item) } return result } func Filter[T any](items []T, predicate func(T) bool) []T { var result []T for _, item := range items { if predicate(item) { result = append(result, item) } } return result } func Reduce[T, U any](items []T, initial U, fn func(U, T) U) U { result := initial for _, item := range items { result = fn(result, item) } return result } // Generic type constraint type Number interface { ~int | ~int32 | ~int64 | ~float32 | ~float64 } func Sum[T Number](items []T) T { var sum T for _, item := range items { sum += item } return sum } // Generic struct type Stack[T any] struct { items []T } func (s *Stack[T]) Push(item T) { s.items = append(s.items, item) } func (s *Stack[T]) Pop() (T, bool) { if len(s.items) == 0 { var zero T return zero, false } item := s.items[len(s.items)-1] s.items = s.items[:len(s.items)-1] return item, true } // Usage