---

name: go-design-patterns description: > Idiomatic Go design patterns: functional options, builder, factory, strategy, middleware chain, pub/sub, and other patterns adapted for Go's type system. Use when: "design pattern", "functional options", "builder pattern", "factory pattern", "strategy pattern", "middleware chain", "option pattern", "how to structure this". Not for: interface design (go-interface-design), package layout (go-architecture-review), concurrency patterns (go-concurrency-review). user-invocable: true license: MIT compatibility: Designed for Claude Code or similar AI coding agents working on Go projects. Requires the Go toolchain. allowed-tools: Read Edit Write Glob Grep Bash(go:) Bash(gofmt:) metadata: author: eduardo-sl version: "1.2.1"

Go Design Patterns

Go favors composition over inheritance and simplicity over abstraction. These patterns are idiomatic Go — not Java patterns ported to Go.

Detailed reference material, loaded on demand:

• references/creation-patterns.md — functional options (full example), options vs config struct, constructors, factory.
• references/behavioral-patterns.md — strategy, middleware/decorator, result type, defer cleanup, sentinel vs zero values.

Read a reference file only when the summary below is not enough.

Pattern Selection

Need	Pattern	Reference
Constructor with many optional settings	Functional options	creation-patterns.md
Config loaded from file/env, mostly required fields	Config struct	creation-patterns.md
Enforce invariants at creation	Constructor returning error	creation-patterns.md
Pick implementation from runtime config	Factory returning interface	creation-patterns.md
Swap simple behavior at runtime	Strategy via function type	behavioral-patterns.md
Swap complex behavior at runtime	Strategy via interface	behavioral-patterns.md
Wrap cross-cutting concerns (log, cache, metrics)	Middleware / decorator	behavioral-patterns.md
Value-or-error in concurrent pipelines	Result[T] struct	behavioral-patterns.md

1. Functional Options (essentials)

type Option func(*Server)

func WithAddr(addr string) Option {
    return func(s *Server) { s.addr = addr }
}

func NewServer(opts ...Option) *Server {
    s := &Server{
        addr:        ":8080", // sensible defaults first
        readTimeout: 5 * time.Second,
        logger:      slog.Default(),
    }
    for _, opt := range opts {
        opt(s)
    }
    return s
}

srv := NewServer(WithAddr(":9090"))

Use when: many optional parameters with sensible defaults, API evolves over time (new options don't break callers), options need validation. Use a plain config struct instead when most fields are required or the configuration is deserialized from file/env.

2. Constructor Rules

• Every exported type with invariants needs a constructor.
• Validate required dependencies; return an error, don't panic:

// ✅ Good — constructor enforces invariants
func NewUserService(repo UserRepository, logger *slog.Logger) (*UserService, error) {
    if repo == nil {
        return nil, errors.New("user service: nil repository")
    }
    return &UserService{repo: repo, logger: logger}, nil
}

// ❌ Bad — struct literal with no validation
svc := &UserService{} // nil dependencies → panic at runtime

3. Factory

Return the interface, not a concrete type. The factory is the only place that knows about concrete implementations:

func NewStore(cfg Config) (Store, error) {
    switch cfg.StoreType {
    case "redis":
        return newRedisStore(cfg.RedisAddr)
    case "memory":
        return newMemoryStore(), nil
    default:
        return nil, fmt.Errorf("unknown store type: %s", cfg.StoreType)
    }
}

4. Middleware Chain

The standard HTTP composition pattern:

type Middleware func(http.Handler) http.Handler

func Chain(handler http.Handler, middlewares ...Middleware) http.Handler {
    for i := len(middlewares) - 1; i >= 0; i-- {
        handler = middlewares[i](handler)
    }
    return handler
}

handler := Chain(appHandler, Recoverer, RequestID, Logger, Auth)

The same shape works for any interface: stack decorators as cache → logging → metrics → actual repo (see references/behavioral-patterns.md).

5. Zero Values First

Prefer types whose zero value is useful (sync.Mutex, bytes.Buffer, nil slices). Reach for sentinel wrappers or pointers only when the zero value is ambiguous as an input (nil *float64 = "not configured").

Anti-Patterns to Avoid

// ❌ God interface — too many methods
type Service interface {
    GetUser(ctx context.Context, id string) (*User, error)
    CreateUser(ctx context.Context, u *User) error
    DeleteUser(ctx context.Context, id string) error
    ListOrders(ctx context.Context, userID string) ([]Order, error)
    // 20 more methods...
}
// → Split into focused interfaces: UserReader, UserWriter, OrderLister

// ❌ Premature abstraction — interface for one implementation
type UserCache interface {
    Get(key string) (*User, bool)
    Set(key string, user *User)
}
// If there's only ever one implementation, use the concrete type.
// Extract an interface when a second consumer or implementation appears.

// ❌ Java-style inheritance simulation
type BaseService struct{ /* ... */ }
type UserService struct{ BaseService } // embedding is NOT inheritance
// → Use composition: UserService has a dependency, not a parent.

Verification Checklist

1. Functional options used for types with optional configuration
2. Constructors validate required dependencies and return errors
3. Factory functions return interfaces, not concrete types
4. No god interfaces — each interface has 1-3 methods
5. Middleware follows func(http.Handler) http.Handler signature
6. Decorators wrap interfaces, not concrete types
7. defer used for all resource cleanup (files, connections, locks)
8. Zero values are meaningful — no unnecessary initialization
9. No premature abstractions — interfaces extracted only when needed
10. Composition used instead of embedding for code reuse