golang-design-patterns

**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)

```go
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

```go
// 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 initialization, injectable
func NewUserRepository(db *sql.DB) *UserRepository {
    return &UserRepository{db: db}
}
```

### Enums: Start at 1

Zero values should represent invalid/unset state:

```go
type Status int

const (
    StatusUnknown Status = iota // 0 = invalid/unset
    StatusActive                // 1
    StatusInactive              // 2
    StatusSuspended             // 3
)
```

### Compile Regexp Once

```go
// Good — compiled once at package level
var emailRegex = regexp.MustCompile(`^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`)

func ValidateEmail(email string) bool {
    return emailRegex.MatchString(email)
}
```

### Use `//go:embed` for Static Assets

```go
import "embed"

//go:embed templates/*
var templateFS embed.FS

//go:embed version.txt
var version string
```

### Compile-Time Interface Checks

→ See `samber/cc-skills-golang@golang-structs-interfaces` for the `var _ Interface = (*Type)(nil)` pattern.

## Error Flow