Overview

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

📖 go-interfaces