go-functional-options

The functional options pattern handles optional parameters in Go constructors by passing With* functions to a base constructor, allowing you to add new configuration incrementally without altering existing call sites.

The functional options pattern in Go API design uses an unexported options struct and an exported Option interface with an unexported apply method, enabling flexible configuration composition through With* constructors while maintaining safe defaults.

You should use functional options in Go when building public APIs with three or more parameters where extensibility matters, allowing you to add options over time without breaking existing callers or relying on long parameter lists.

To set default values using the functional options pattern, define an unexported options struct initialized with sensible defaults, then apply With* constructors in order within the constructor function to override defaults and validate the final configuration.

Yes, you can add new configuration options to a Go API without breaking changes by introducing additional With* constructors that implement the Option interface, keeping existing calls unchanged while extending the API's behavior.