go-functional-options

SKILL.md

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

Unexported options struct - holds all configuration

Exported Option interface - with unexported apply method

Option types - implement the interface

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

Aspect
Functional Options
Config Struct

Extensibility
Add new With* functions
Add new fields (may break)

Defaults
Built into constructor
Zero values or separate defaults

Caller experience
Only specify what differs
Must construct entire struct

Testability
Options are comparable
Struct comparison

Complexity
More boilerplate
Simpler setup

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:

Testability - Options can be compared in tests and mocks

Debuggability - Options can implement fmt.Stringer

Flexibility - Options can implement additional interfaces

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