/golang-design-patterns

name: golang-design-patterns
description: "Idiomatic Golang design patterns — functional options, constructors, error flow and cascading, resource management and lifecycle, graceful shutdown, resilience, architecture, dependency injection, data handling, streaming, and more. Apply when explicitly choosing between architectural patterns, implementing functional options, designing constructor APIs, setting up graceful shutdown, applying resilience patterns, or asking which idiomatic Go pattern fits a specific problem."
user-invocable: true
license: MIT
compatibility: Designed for Claude Code or similar AI coding agents, and for projects using Golang.
metadata:
  author: samber
  version: "1.1.5"
  openclaw:
    emoji: "🏗"
    homepage: https://github.com/samber/cc-skills-golang
    requires:
      bins:
        - go
    install: []
allowed-tools: Read Edit Write Glob Grep Bash(go:*) Bash(golangci-lint:*) Bash(git:*) Agent AskUserQuestion

**Persona:** You are a Go architect who values simplicity and explicitness. You apply patterns only when they solve a real problem — not to demonstrate sophistication — and you push back on premature abstraction.

**Modes:**

• **Design mode** — creating new APIs, packages, or application structure: ask the developer about their architecture preference before proposing patterns; favor the smallest pattern that satisfies the requirement.
• **Review mode** — auditing existing code for design issues: scan for `init()` abuse, unbounded resources, missing timeouts, and implicit global state; report findings before suggesting refactors.

> **Community default.** A company skill that explicitly supersedes `samber/cc-skills-golang@golang-design-patterns` skill takes precedence.

Go Design Patterns & Idioms

Idiomatic Go patterns for production-ready code. For error handling details see the `samber/cc-skills-golang@golang-error-handling` skill; for context propagation see `samber/cc-skills-golang@golang-context` skill; for struct/interface design see `samber/cc-skills-golang@golang-structs-interfaces` skill.

Best Practices Summary

1. Constructors SHOULD use **functional options** — they scale better as APIs evolve (one function per option, no breaking changes) 2. Functional options MUST **return an error** if validation can fail — catch bad config at construction, not at runtime 3. **Avoid `init()`** — runs implicitly, cannot return errors, makes testing unpredictable. Use explicit constructors 4. Enums SHOULD **start at 1** (or Unknown sentinel at 0) — Go's zero value silently passes as the first enum member 5. Error cases MUST be **handled first** with early return — keep happy path flat 6. **Panic is for bugs, not expected errors** — callers can handle returned errors; panics crash the process 7. **`defer Close()` immediately after opening** — later code changes can accidentally skip cleanup 8. **`runtime.AddCleanup`** over `runtime.SetFinalizer` — finalizers are unpredictable and can resurrect objects 9. Every external call SHOULD **have a timeout** — a slow upstream hangs your goroutine indefinitely 10. **Limit everything** (pool sizes, queue depths, buffers) — unbounded resources grow until they crash 11. Retry logic MUST **check context cancellation** between attempts 12. **Use `strings.Builder`** for concatenation in loops → see `samber/cc-skills-golang@golang-code-style` 13. string vs []byte: **use `[]byte` for mutation and I/O**, `string` for display and keys — conversions allocate 14. Iterators (Go 1.23+): **use for lazy evaluation** — avoid loading everything into memory 15. **Stream large transfers** — loading millions of rows causes OOM; stream keeps memory constant 16. `//go:embed` for **static assets** — embeds at compile time, eliminates runtime file I/O errors 17. **Use `crypto/rand`** for keys/tokens — `math/rand` is predictable → see `samber/cc-skills-golang@golang-security` 18. Regexp MUST be **compiled once at package level** — compilation is O(n) and allocates 19. Compile-time interface checks: **`var _ Interface = (*Type)(nil)`** 20. **A little recode > a big dependency** — each dep adds attack surface and maintenance burden 21. **Design for testability** — accept interfaces, inject dependencies

Constructor Patterns: Functional Options vs Builder

Functional Options (Preferred)

type Server struct {
    addr         string
    readTimeout  time.Duration
    writeTimeout time.Duration
    maxConns     int
}

type Option func(*Server)

func WithReadTimeout(d time.Duration) Option {
    return func(s *Server) { s.readTimeout = d }
}

func WithWriteTimeout(d time.Duration) Option {
    return func(s *Server) { s.writeTimeout = d }
}

func WithMaxConns(n int) Option {
    return func(s *Server) { s.maxConns = n }
}

func NewServer(addr string, opts ...Option) *Server {
    // Default options
    s := &Server{
        addr:         addr,
        readTimeout:  5 * time.Second,
        writeTimeout: 10 * time.Second,
        maxConns:     100,
    }
    for _, opt := range opts {
        opt(s)
    }
    return s
}

// Usage
srv := NewServer(":8080",
    WithReadTimeout(30*time.Second),
    WithMaxConns(500),
)

Constructors SHOULD use **functional options** — they scale better with API evolution and require less code. Use builder pattern only if you need complex validation between configuration steps.

Constructors & Initialization

Avoid `init()` and Mutable Globals

`init()` runs implicitly, makes testing harder, and creates hidden dependencies:

• Multiple `init()` functions run in declaration order, across files in **filename alphabetical order** — fragile
• Cannot return errors — failures must panic or `log.Fatal`
• Runs before `main()` and tests — side effects make tests unpredictable

// Bad — hidden global state
var db *sql.DB

func init() {
    var err error
    db, err = sql.Open("postgres", os.Getenv("DATABASE_URL"))
    if err != nil {
        log.Fatal(err)
    }
}

// Good — explicit ini