go-functional-options

The functional options pattern solves Go API configuration complexity by using an unexported options struct and exported With* constructors. Callers only configure what differs from the defaults, keeping call sites readable and extensible.

The functional options pattern in Golang uses an exported Option interface with an apply method and With* constructors to configure an unexported options struct. This allows callers to override specific defaults without passing multiple parameters to a constructor.

You create extensible Go APIs by defining an unexported options struct for defaults, an exported Option interface, and With* constructors. A base constructor then applies these functional options to the defaults, allowing callers to configure only what they need.

Functional options improve Golang library design by centralizing default management and avoiding brittle constructors with many parameters. They provide a clear, readable caller experience where developers only specify the configurations that differ from the defaults.

No, the functional options pattern does not require external dependencies in Go. It is implemented purely with standard language features: an unexported options struct, an exported Option interface with an apply method, and With* option constructors.

You should avoid functional options for API construction when a Go struct requires no optional parameters or has strictly mandatory fields. The pattern is designed specifically for scenarios with multiple optional configurations where defaults should be preserved.