Go Interfaces

Master Go's interface system for creating flexible, decoupled code through implicit implementation and composition patterns.

Basic Interfaces

Defining and implementing interfaces:

package main

import "fmt"

// Define interface
type Writer interface {
    Write(p []byte) (n int, err error)
}

// Implement interface (implicit)
type ConsoleWriter struct{}

func (cw ConsoleWriter) Write(p []byte) (n int, err error) {
    fmt.Print(string(p))
    return len(p), nil
}

func main() {
    var w Writer = ConsoleWriter{}
    w.Write([]byte("Hello, World!\n"))
}

Multiple methods in interface:

type Reader interface {
    Read(p []byte) (n int, err error)
}

type ReadWriter interface {
    Read(p []byte) (n int, err error)
    Write(p []byte) (n int, err error)
}

// Implement ReadWriter
type File struct {
    name string
}

func (f *File) Read(p []byte) (n int, err error) {
    // Implementation
    return 0, nil
}

func (f *File) Write(p []byte) (n int, err error) {
    // Implementation
    return len(p), nil
}

Empty Interface

Using interface{} (any in Go 1.18+):

// Accepts any type
func printValue(v interface{}) {
    fmt.Println(v)
}

// Modern syntax (Go 1.18+)
func printAny(v any) {
    fmt.Println(v)
}

func main() {
    printValue(42)
    printValue("hello")
    printValue(true)

    printAny(3.14)
}

Type assertions:

func processValue(v interface{}) {
    // Type assertion
    if str, ok := v.(string); ok {
        fmt.Println("String:", str)
    }

    // Type switch
    switch val := v.(type) {
    case int:
        fmt.Println("Integer:", val)
    case string:
        fmt.Println("String:", val)
    case bool:
        fmt.Println("Boolean:", val)
    default:
        fmt.Println("Unknown type")
    }
}

Interface Composition

Embedding interfaces:

type Reader interface {
    Read(p []byte) (n int, err error)
}

type Writer interface {
    Write(p []byte) (n int, err error)
}

type Closer interface {
    Close() error
}

// Compose interfaces
type ReadWriter interface {
    Reader
    Writer
}

type ReadWriteCloser interface {
    Reader
    Writer
    Closer
}

// Standard library example
import "io"

func useReadWriteCloser(rwc io.ReadWriteCloser) {
    // Can call Read, Write, and Close
    rwc.Write([]byte("data"))
    rwc.Close()
}

Common Interfaces

Standard library interfaces:

// Stringer interface
type Stringer interface {
    String() string
}

type Person struct {
    Name string
    Age  int
}

func (p Person) String() string {
    return fmt.Sprintf("%s (%d years old)", p.Name, p.Age)
}

// error interface
type error interface {
    Error() string
}

type MyError struct {
    Message string
}

func (e MyError) Error() string {
    return e.Message
}

// sort.Interface
type Interface interface {
    Len() int
    Less(i, j int) bool
    Swap(i, j int)
}

type ByAge []Person

func (a ByAge) Len() int           { return len(a) }
func (a ByAge) Less(i, j int) bool { return a[i].Age < a[j].Age }
func (a ByAge) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

Interface Design Patterns

Small interfaces:

// Good: small, focused interfaces
type Getter interface {
    Get(key string) (value string, exists bool)
}

type Setter interface {
    Set(key, value string)
}

type Deleter interface {
    Delete(key string)
}

// Compose as needed
type Cache interface {
    Getter
    Setter
    Deleter
}

Accept interfaces, return structs:

// Accept interface parameter
func processReader(r io.Reader) error {
    data, err := io.ReadAll(r)
    if err != nil {
        return err
    }
    fmt.Println(string(data))
    return nil
}

// Return concrete type
func newConfig() *Config {
    return &Config{
        Host: "localhost",
        Port: 8080,
    }
}

type Config struct {
    Host string
    Port int
}

Nil Interfaces

Understanding nil interfaces:

func checkNil() {
    var i interface{}
    fmt.Println(i == nil) // true

    var p *Person
    i = p
    fmt.Println(i == nil) // false! (type is set, value is nil)

    // Proper nil check
    v, ok := i.(*Person)
    fmt.Println(v == nil, ok) // true, true
}

Interface Satisfaction

Checking interface implementation:

// Compile-time check
var _ io.Writer = (*MyWriter)(nil)
var _ io.Reader = (*MyReader)(nil)

type MyWriter struct{}

func (w *MyWriter) Write(p []byte) (n int, err error) {
    return len(p), nil
}

// If MyWriter doesn't implement Writer, compilation fails

Duck Typing

Implicit interface satisfaction:

// No explicit "implements" keyword needed
type Duck interface {
    Quack()
    Walk()
}

type RealDuck struct{}

func (d RealDuck) Quack() {
    fmt.Println("Quack!")
}

func (d RealDuck) Walk() {
    fmt.Println("Waddle waddle")
}

type Robot struct{}

func (r Robot) Quack() {
    fmt.Println("Beep boop quack")
}

func (r Robot) Walk() {
    fmt.Println("*mechanical walking sounds*")
}

func makeDuckDoThings(d Duck) {
    d.Quack()
    d.Walk()
}

func main() {
    makeDuckDoThings(RealDuck{})
    makeDuckDoThings(Robot{})
}

Polymorphism

Using interfaces for polymorphism:

type Shape interface {
    Area() float64
    Perimeter() float64
}

type Rectangle struct {
    Width, Height float64
}

func (r Rectangle) Area() float64 {
    return r.Width * r.Height
}

func (r Rectangle) Perimeter() float64 {
    return 2 * (r.Width + r.Height)
}

type Circle struct {
    Radius float64
}

func (c Circle) Area() float64 {
    return 3.14159 * c.Radius * c.Radius
}

func (c Circle) Perimeter() float64 {
    return 2 * 3.14159 * c.Radius
}

func printShapeInfo(s Shape) {
    fmt.Printf("Area: %.2f, Perimeter: %.2f\n",
        s.Area(), s.Perimeter())
}

func main() {
    shapes := []Shape{
        Rectangle{Width: 10, Height: 5},
        Circle{Radius: 7},
    }

    for _, shape := range shapes {
        printShapeInfo(shape)
    }
}

Dependency Injection

Using interfaces for testability:

// Define interface for dependency
type UserRepository interface {
    GetUser(id int) (*User, error)
    SaveUser(user *User) error
}

// Production implementation
type PostgresUserRepo struct {
    db *sql.DB
}

func (r *PostgresUserRepo) GetUser(id int) (*User, error) {
    // Database query
    return &User{}, nil
}

func (r *PostgresUserRepo) SaveUser(user *User) error {
    // Database insert/update
    return nil
}

// Test implementation
type MockUserRepo struct {
    users map[int]*User
}

func (m *MockUserRepo) GetUser(id int) (*User, error) {
    user, exists := m.users[id]
    if !exists {
        return nil, errors.New("user not found")
    }
    return user, nil
}

func (m *MockUserRepo) SaveUser(user *User) error {
    m.users[user.ID] = user
    return nil
}

// Service depends on interface, not concrete type
type UserService struct {
    repo UserRepository
}

func (s *UserService) GetUserName(id int) (string, error) {
    user, err := s.repo.GetUser(id)
    if err != nil {
        return "", err
    }
    return user.Name, nil
}

type User struct {
    ID   int
    Name string
}

Builder Pattern with Interfaces

Fluent interface pattern:

type QueryBuilder interface {
    Select(fields ...string) QueryBuilder
    From(table string) QueryBuilder
    Where(condition string) QueryBuilder
    Build() string
}

type sqlQueryBuilder struct {
    selectFields []string
    fromTable    string
    whereClause  string
}

func NewQueryBuilder() QueryBuilder {
    return &sqlQueryBuilder{}
}

func (b *sqlQueryBuilder) Select(fields ...string) QueryBuilder {
    b.selectFields = fields
    return b
}

func (b *sqlQueryBuilder) From(table string) QueryBuilder {
    b.fromTable = table
    return b
}

func (b *sqlQueryBuilder) Where(condition string) QueryBuilder {
    b.whereClause = condition
    return b
}

func (b *sqlQueryBuilder) Build() string {
    query := "SELECT " + strings.Join(b.selectFields, ", ")
    query += " FROM " + b.fromTable
    if b.whereClause != "" {
        query += " WHERE " + b.whereClause
    }
    return query
}

func main() {
    query := NewQueryBuilder().
        Select("id", "name", "email").
        From("users").
        Where("age > 18").
        Build()

    fmt.Println(query)
}

Strategy Pattern

Implementing strategy pattern:

type PaymentStrategy interface {
    Pay(amount float64) error
}

type CreditCardPayment struct {
    CardNumber string
}

func (c *CreditCardPayment) Pay(amount float64) error {
    fmt.Printf("Paying %.2f with credit card %s\n",
        amount, c.CardNumber)
    return nil
}

type PayPalPayment struct {
    Email string
}

func (p *PayPalPayment) Pay(amount float64) error {
    fmt.Printf("Paying %.2f via PayPal to %s\n",
        amount, p.Email)
    return nil
}

type ShoppingCart struct {
    paymentMethod PaymentStrategy
}

func (cart *ShoppingCart) SetPaymentMethod(pm PaymentStrategy) {
    cart.paymentMethod = pm
}

func (cart *ShoppingCart) Checkout(amount float64) error {
    return cart.paymentMethod.Pay(amount)
}

func main() {
    cart := &ShoppingCart{}

    cart.SetPaymentMethod(&CreditCardPayment{CardNumber: "1234-5678"})
    cart.Checkout(100.00)

    cart.SetPaymentMethod(&PayPalPayment{Email: "user@example.com"})
    cart.Checkout(50.00)
}

Adapter Pattern

Adapting interfaces:

// Third-party logger
type ThirdPartyLogger struct{}

func (t *ThirdPartyLogger) LogMessage(msg string, level int) {
    fmt.Printf("[Level %d] %s\n", level, msg)
}

// Our application interface
type Logger interface {
    Info(msg string)
    Error(msg string)
}

// Adapter
type LoggerAdapter struct {
    thirdParty *ThirdPartyLogger
}

func (a *LoggerAdapter) Info(msg string) {
    a.thirdParty.LogMessage(msg, 0)
}

func (a *LoggerAdapter) Error(msg string) {
    a.thirdParty.LogMessage(msg, 2)
}

func useLogger(logger Logger) {
    logger.Info("Application started")
    logger.Error("An error occurred")
}

func main() {
    adapter := &LoggerAdapter{
        thirdParty: &ThirdPartyLogger{},
    }
    useLogger(adapter)
}

When to Use This Skill

Use go-interfaces when you need to:

• Define contracts for behavior without implementation
• Enable polymorphism and code reuse
• Create testable code with dependency injection
• Implement design patterns (strategy, adapter, etc.)
• Build plugin systems or extensible architectures
• Decouple components in large applications
• Mock dependencies in tests
• Follow SOLID principles in Go
• Create flexible, maintainable APIs
• Support multiple implementations of same behavior

Best Practices

• Keep interfaces small and focused (1-3 methods)
• Accept interfaces, return concrete types
• Define interfaces where they're used, not implemented
• Use interface composition for complex interfaces
• Don't use empty interface unless absolutely necessary
• Verify interface implementation at compile time
• Document expected behavior in interface comments
• Prefer many small interfaces over large ones
• Use standard library interfaces when applicable
• Name interfaces with -er suffix (Reader, Writer, etc.)

Common Pitfalls

• Making interfaces too large or generic
• Defining unused interfaces "just in case"
• Returning interfaces instead of concrete types
• Not checking for nil interface values properly
• Over-abstracting simple code
• Forgetting that interfaces are satisfied implicitly
• Using empty interface excessively
• Not documenting interface contracts
• Creating interfaces for single implementation
• Confusing nil value vs nil interface

Resources

• Effective Go - Interfaces
• Go Blog - Laws of Reflection
• Interface Documentation
• Go by Example - Interfaces
• Accept Interfaces, Return Structs