Go Functional Options

Functional Options Pattern

Functional options is a pattern where you declare an opaque Option type that records information in an internal struct. The constructor accepts a variadic number of these options and applies them to configure the result.

Resource Routing

• references/OPTIONS-VS-STRUCTS.md - Read when choosing between config structs and functional options, implementing the full interface-based option pattern, or evaluating hybrid constructor APIs.

When to Use

Use functional options when:

• 3+ optional arguments on constructors or public APIs
• Extensible APIs that may gain new options over time
• Clean caller experience is important (no need to pass defaults)

The Pattern

Core Components

1. Unexported options struct - holds all configuration
2. Exported Option interface - with unexported apply method
3. Option types - implement the interface
4. With* constructors - create options

Option Interface

type Option interface {
    apply(*options)
}

The unexported apply method ensures only options from this package can be used.

Comparison: Functional Options vs Config Struct