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.

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.

Complete Implementation

package db

import "go.uber.org/zap"

// options holds all configuration for opening a connection.
type options struct {
    cache  bool
    logger *zap.Logger
}

// Option configures how we open the connection.
type Option interface {
    apply(*options)
}

// cacheOption implements Option for cache setting (simple type alias).
type cacheOption bool

func (c cacheOption) apply(opts *options) {
    opts.cache = bool(c)
}

// WithCache enables or disables caching.
func WithCache(c bool) Option {
    return cacheOption(c)
}

// loggerOption implements Option for logger setting (struct for pointers).
type loggerOption struct {
    Log *zap.Logger
}

func (l loggerOption) apply(opts *options) {
    opts.logger = l.Log
}

// WithLogger sets the logger for the connection.
func WithLogger(log *zap.Logger) Option {
    return loggerOption{Log: log}
}

// Open creates a connection.
func Open(addr string, opts ...Option) (*Connection, error) {
    // Start with defaults
    options := options{
        cache:  defaultCache,
        logger: zap.NewNop(),
    }

    // Apply all provided options
    for _, o := range opts {
        o.apply(&options)
    }

    // Use options.cache and options.logger...
    return &Connection{}, nil
}

Usage Examples

Without Functional Options (Bad)

// Caller must always provide all parameters, even defaults
db.Open(addr, db.DefaultCache, zap.NewNop())
db.Open(addr, db.DefaultCache, log)
db.Open(addr, false /* cache */, zap.NewNop())
db.Open(addr, false /* cache */, log)

With Functional Options (Good)

// Only provide options when needed
db.Open(addr)
db.Open(addr, db.WithLogger(log))
db.Open(addr, db.WithCache(false))
db.Open(
    addr,
    db.WithCache(false),
    db.WithLogger(log),
)

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.

Read references/OPTIONS-VS-STRUCTS.md when deciding between functional options and config structs, designing a config struct API with proper defaults, or evaluating the hybrid approach for complex constructors.

Why Not Closures?

An alternative implementation uses closures:

// Closure approach (not recommended)
type Option func(*options)

func WithCache(c bool) Option {
    return func(o *options) { o.cache = c }
}

The interface approach is preferred 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 when designing the Option interface or choosing between interface and closure approaches
• Naming conventions: See go-naming when naming With* constructors, option types, or the unexported options struct
• Function design: See go-functions when organizing constructors within a file or formatting variadic signatures
• Documentation: See go-documentation 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