How to Use the Functional Options Pattern in Go

The Functional Options Pattern in Go uses variadic functions accepting a slice of option functions to configure a struct without creating many constructors. Define an option type as a function that modifies a struct, then pass these functions to your constructor to apply settings selectively.

type Option func(*Config)

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

func NewConfig(opts ...Option) *Config {
    c := &Config{Timeout: 5 * time.Second}
    for _, o := range opts {
        o(c)
    }
    return c
}

// Usage:
// cfg := NewConfig(WithTimeout(10 * time.Second))

The Functional Options Pattern in Go lets you build complex objects by passing small, reusable functions that tweak specific settings. Instead of writing a separate constructor for every possible combination of features, you just pick the options you need and pass them in. It's like ordering a pizza where you start with a base and then add toppings one by one.