go-functional-options

To handle optional parameters in Go constructors, use the functional options pattern. It replaces brittle parameter lists by applying With* functions to an internal options struct, enabling flexible and evolving API configurations.

The functional options pattern in Go API design is a technique using an exported Option interface with an apply method. It modifies an unexported internal options struct through With* constructors to establish clear defaults and deterministic configuration.

You should use functional options for Go APIs when a constructor or public function has 3 or more optional parameters. It is specifically applicable for evolving configurations where long parameter lists would otherwise become brittle and hard to maintain.

To set default configurations in a Go constructor, establish clear defaults first, then apply exported Option values. Each option's apply method sequentially modifies the unexported internal options struct to compose the final configuration.

Yes, the functional options pattern works by maintaining an unexported internal options struct that holds configuration. It wraps this internal state with an exported Option interface, keeping the underlying struct hidden while allowing fluent API configuration.