go-functional-options

Optional parameters in a Go API constructor are handled using the functional options pattern, which decouples configuration from the constructor by applying With* functions to a defaultable Open function.

The functional options pattern in Go API design is a technique using an unexported options struct and an exported Option interface with an unexported apply method to provide flexible, scalable configuration.

You should use functional options for Go constructors when an API requires three or more optional parameters, or when you need future extensibility without breaking existing callers.

Configurable Go constructors are created by defining With* functions that compose configuration, then passing them to an exported Open function that applies them over an unexported default options struct.

Designing flexible Go APIs for future extensibility is best achieved with the functional options pattern, allowing clean caller experience by adding new With* constructors without modifying existing function signatures.