golang-patterns

SKILL.md

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(id string) (*User, error) {
    user, err := db.FindUser(id)
    if err != nil {
        return nil, fmt.Errorf("get user %s: %w", id, err)
    }
    return user, nil
}

// Bad: Overly clever
func GetUser(id string) (*User, error) {
    return func() (*User, error) {
        if u, e := db.FindUser(id); e == nil {
            return u, nil
        } else {
            return nil, e
        }
    }()
}

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()
}

// Good: bytes.Buffer works with zero value
var buf bytes.Buffer
buf.WriteString("hello")

// Bad: Requires initialization
type BadCounter struct {
    counts map[string]int // nil map will panic
}

3. Accept Interfaces, Return Structs

Functions should accept interface parameters and return concrete types.

// Good: Accepts interface, returns concrete type
func ProcessData(r io.Reader) (*Result, error) {
    data, err := io.ReadAll(r)
    if err != nil {
        return nil, err
    }
    return &Result{Data: data}, nil
}

// Bad: Returns interface (hides implementation details unnecessarily)
func ProcessData(r io.Reader) (io.Reader, error) {
    // ...
}

Error Handling Patterns

Error Wrapping with Context

// Good: Wrap errors 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

// Define domain-specific errors
type ValidationError struct {
    Field   string
    Message string
}

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

// Sentinel errors for common cases
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) {
    // Check for specific error
    if errors.Is(err, sql.ErrNoRows) {
        log.Println("No records found")
        return
    }

    // Check for error type
    var validationErr *ValidationError
    if errors.As(err, &validationErr) {
        log.Printf("Validation error on field %s: %s",
            validationErr.Field, validationErr.Message)
        return
    }

    // Unknown error
    log.Printf("Unexpected error: %v", err)
}

Never Ignore Errors

// Bad: Ignoring error with blank identifier
result, _ := doSomething()

// Good: Handle or explicitly document why it's safe to ignore
result, err := doSomething()
if err != nil {
    return err
}

// Acceptable: When error truly doesn't matter (rare)
_ = writer.Close() // Best-effort cleanup, error logged elsewhere

Concurrency Patterns

Worker Pool

func WorkerPool(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

import "golang.org/x/sync/errgroup"

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 // Capture loop variables
        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

// Bad: Goroutine leak if context is cancelled
func leakyFetch(ctx context.Context, url string) <-chan []byte {
    ch := make(chan []byte)
    go func() {
        data, _ := fetch(url)
        ch <- data // Blocks forever if no receiver
    }()
    return ch
}

// 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

// Good: Single-method interfaces
type Reader interface {
    Read(p []byte) (n int, err error)
}

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

type Closer interface {
    Close() error
}

// Compose interfaces as needed
type ReadWriteCloser interface {
    Reader
    Writer
    Closer
}

Define Interfaces Where They're Used

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

// UserStore defines what this service needs
type UserStore interface {
    GetUser(id string) (*User, error)
    SaveUser(user *User) error
}

type Service struct {
    store UserStore
}

// Concrete implementation can be in another package
// It doesn't need to know about this interface

Optional Behavior with Type Assertions

type Flusher interface {
    Flush() error
}

func WriteAndFlush(w io.Writer, data []byte) error {
    if _, err := w.Write(data); err != nil {
        return err
    }

    // Flush if supported
    if f, ok := w.(Flusher); ok {
        return f.Flush()
    }
    return nil
}

Package Organization

Standard Project Layout

myproject/
├── cmd/
│   └── myapp/
│       └── main.go           # Entry point
├── internal/
│   ├── handler/              # HTTP handlers
│   ├── service/              # Business logic
│   ├── repository/           # Data access
│   └── config/               # Configuration
├── pkg/
│   └── client/               # Public API client
├── api/
│   └── v1/                   # API definitions (proto, OpenAPI)
├── testdata/                 # Test fixtures
├── go.mod
├── go.sum
└── Makefile

Package Naming

// Good: Short, lowercase, no underscores
package http
package json
package user

// Bad: Verbose, mixed case, or redundant
package httpHandler
package json_parser
package userService // Redundant 'Service' suffix

Avoid Package-Level State

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

func init() {
    db, _ = sql.Open("postgres", os.Getenv("DATABASE_URL"))
}

// 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, // default
        logger:  log.Default(),    // default
    }
    for _, opt := range opts {
        opt(s)
    }
    return s
}

// Usage
server := NewServer(":8080",
    WithTimeout(60*time.Second),
    WithLogger(customLogger),
)

Embedding for Composition

type Logger struct {
    prefix string
}

func (l *Logger) Log(msg string) {
    fmt.Printf("[%s] %s\n", l.prefix, msg)
}

type Server struct {
    *Logger // Embedding - Server gets Log method
    addr    string
}

func NewServer(addr string) *Server {
    return &Server{
        Logger: &Logger{prefix: "SERVER"},
        addr:   addr,
    }
}

// Usage
s := NewServer(":8080")
s.Log("Starting...") // Calls embedded Logger.Log

Memory and Performance

Preallocate Slices When Size is Known

// Bad: Grows slice multiple times
func processItems(items []Item) []Result {
    var results []Result
    for _, item := range items {
        results = append(results, process(item))
    }
    return results
}

// 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
}

Use sync.Pool for Frequent Allocations

var bufferPool = sync.Pool{
    New: func() interface{} {
        return new(bytes.Buffer)
    },
}

func ProcessRequest(data []byte) []byte {
    buf := bufferPool.Get().(*bytes.Buffer)
    defer func() {
        buf.Reset()
        bufferPool.Put(buf)
    }()

    buf.Write(data)
    // Process...
    return buf.Bytes()
}

Avoid String Concatenation in Loops

// Bad: Creates many string allocations
func join(parts []string) string {
    var result string
    for _, p := range parts {
        result += p + ","
    }
    return result
}

// Good: Single allocation with 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()
}

// Best: Use standard library
func join(parts []string) string {
    return strings.Join(parts, ",")
}

Go Tooling Integration

Essential Commands

# Build and run
go build ./...
go run ./cmd/myapp

# Testing
go test ./...
go test -race ./...
go test -cover ./...

# Static analysis
go vet ./...
staticcheck ./...
golangci-lint run

# Module management
go mod tidy
go mod verify

# Formatting
gofmt -w .
goimports -w .

Recommended Linter Configuration (.golangci.yml)

linters:
  enable:
    - errcheck
    - gosimple
    - govet
    - ineffassign
    - staticcheck
    - unused
    - gofmt
    - goimports
    - misspell
    - unconvert
    - unparam

linters-settings:
  errcheck:
    check-type-assertions: true
  govet:
    enable:
      - shadow

issues:
  exclude-use-default: false

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 between goroutines

Make the zero value useful
Types should work without explicit initialization

A little copying is better than a little dependency
Avoid unnecessary external dependencies

Clear is better than clever
Prioritize readability over cleverness

gofmt is no one's favorite but everyone's friend
Always format with gofmt/goimports

Return early
Handle errors first, keep happy path unindented

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 {
    // ...
}

// Bad: Mixing value and pointer receivers
type Counter struct{ n int }
func (c Counter) Value() int { return c.n }    // Value receiver
func (c *Counter) Increment() { c.n++ }        // Pointer receiver
// Pick one style and be consistent

Remember: Go code should be boring in the best way - predictable, consistent, and easy to understand. When in doubt, keep it simple.