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

Prefer Config Struct when: Fewer than 3 options, options rarely change, all options usually specified together, or internal APIs only.

Why Not Closures?

The interface approach is preferred over closure-only options because:

1. Testability - Options can be compared in tests and mocks
2. Debuggability - Options can implement fmt.Stringer
3. Flexibility - Options can implement additional interfaces
4. Visibility - Option types are visible in documentation

Quick Reference

// 1. Unexported options struct with defaults
type options struct {
    field1 Type1
    field2 Type2
}

// 2. Exported Option interface, unexported method
type Option interface {
    apply(*options)
}

// 3. Option type + apply + With* constructor
type field1Option Type1

func (o field1Option) apply(opts *options) { opts.field1 = Type1(o) }
func WithField1(v Type1) Option            { return field1Option(v) }

// 4. Constructor applies options over defaults
func New(required string, opts ...Option) (*Thing, error) {
    o := options{field1: defaultField1, field2: defaultField2}
    for _, opt := range opts {
        opt.apply(&o)
    }
    // ...
}

Checklist

• options struct is unexported
• Option interface has unexported apply method
• Each option has a With* constructor
• Defaults are set before applying options
• Required parameters are separate from ...Option

Related Skills

• Interface design: See [go-interfaces](../go-interfaces/SKILL.md) when designing the Option interface or choosing between interface and closure approaches
• Naming conventions: See [go-naming](../go-naming/SKILL.md) when naming With* constructors, option types, or the unexported options struct
• Function design: See [go-functions](../go-functions/SKILL.md) when organizing constructors within a file or formatting variadic signatures
• Documentation: See [go-documentation](../go-documentation/SKILL.md) when documenting Option types, With* functions, or constructor behavior

External Resources

• Self-referential functions and the design of options - Rob Pike
• Functional options for friendly APIs - Dave Cheney