How to Implement the Options Pattern (Functional Options) in Go

Implement the Options Pattern by defining a function type that modifies a configuration struct, then pass a variadic slice of these functions to your constructor.

type Config struct {
	Timeout time.Duration
	Verbose bool
}

type Option func(*Config)

func WithTimeout(d time.Duration) Option {
	return func(c *Config) { c.Timeout = d }
}

func WithVerbose() Option {
	return func(c *Config) { c.Verbose = true }
}

func NewClient(opts ...Option) *Client {
	cfg := &Config{Timeout: 5 * time.Second}
	for _, opt := range opts {
		opt(cfg)
	}
	return &Client{cfg: cfg}
}

The Options Pattern lets you customize a function's behavior by passing optional settings as arguments instead of adding many parameters. Think of it like ordering a pizza where you start with a base and add specific toppings only if you want them, keeping the main order simple.