interfaces

interfaces define behavior, not data. implicit implementation — if your struct has the methods, it satisfies the interface automatically. no implements keyword.

interfaces define a set of methods a type must have. if your struct has those methods, it satisfies the interface — no declaration needed. this is called implicit implementation and it's one of go's best features.

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// java/ts — explicit
class Rectangle implements Shape { }
 
// go — implicit
// if Rectangle has Area() and Perimeter(), it satisfies Shape automatically
// no `implements Shape` anywhere

defining and implementing

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type Shape interface {
    Area() float64
    Perimeter() float64
}
 
type Rectangle struct { Length, Breadth float64 }
type Circle struct    { Radius float64 }
 
func (r *Rectangle) Area() float64      { return r.Length * r.Breadth }
func (r *Rectangle) Perimeter() float64 { return 2 * (r.Length + r.Breadth) }
func (c *Circle) Area() float64         { return math.Pi * math.Pow(c.Radius, 2) }
func (c *Circle) Perimeter() float64    { return 2 * math.Pi * c.Radius }
 
// one function works for any type that satisfies Shape
// add Triangle, Hexagon, whatever — this function never changes
func printShapeInfo(s Shape) {
    fmt.Printf("area: %.2f, perimeter: %.2f\n", s.Area(), s.Perimeter())
}
 
printShapeInfo(&Rectangle{Length: 4, Breadth: 5})
printShapeInfo(&Circle{Radius: 4})

if you add a new method to Shape without implementing it on Rectangle and Circle — compile error immediately. go enforces the full contract.

the stringer interface — built into fmt

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// defined in the fmt package
type Stringer interface {
    String() string
}

if your struct implements String() string, fmt.Println calls it automatically. fmt knows nothing about your struct — but because it has String(), fmt uses it.

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type ServerConfig struct {
    Host    string
    Port    int
    IsHTTPS bool
}
 
func (sc *ServerConfig) String() string {
    return fmt.Sprintf("%s:%d (https: %t)", sc.Host, sc.Port, sc.IsHTTPS)
}
 
sc := &ServerConfig{Host: "localhost", Port: 8080}
fmt.Println(sc) // localhost:8080 (https: false) — String() called automatically

interfaces as function parameters

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type Notifier interface {
    Send(message string) error
    Channel() string
}
 
type EmailNotifier struct{ recipient string }
type SMSNotifier   struct{ phoneNumber string }
type SlackNotifier struct{ webhook string }
 
func (e *EmailNotifier) Send(msg string) error {
    fmt.Printf("[email] to %s: %s\n", e.recipient, msg)
    return nil
}
func (e *EmailNotifier) Channel() string { return "email" }
 
func (s *SMSNotifier) Send(msg string) error {
    fmt.Printf("[sms] to %s: %s\n", s.phoneNumber, msg)
    return nil
}
func (s *SMSNotifier) Channel() string { return "sms" }
 
func (s *SlackNotifier) Send(msg string) error {
    fmt.Printf("[slack] to %s: %s\n", s.webhook, msg)
    return nil
}
func (s *SlackNotifier) Channel() string { return "slack" }
 
// works for email, sms, slack, or any future notifier you add
func notifyAll(notifiers []Notifier, msg string) {
    for _, n := range notifiers {
        n.Send(msg)
    }
}
 
notifiers := []Notifier{
    &EmailNotifier{recipient: "alice@example.com"},
    &SMSNotifier{phoneNumber: "+9779800000000"},
    &SlackNotifier{webhook: "https://hooks.slack.com/abc"},
}
notifyAll(notifiers, "system maintenance at midnight")

interface composition

interfaces can embed other interfaces — builds larger contracts from smaller ones.

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type Reader interface { Read() string }
type Writer interface { Write(data string) error }
type Closer interface { Close() }
 
// requires all methods from Reader + Writer + Closer
type ReadWriteCloser interface {
    Reader
    Writer
    Closer
}
 
type FileStorage   struct{ Name string }
type MemoryStorage struct{ data string }
 
// FileStorage implements all three — satisfies ReadWriteCloser
func (fs *FileStorage) Read() string           { return "reading from " + fs.Name }
func (fs *FileStorage) Write(data string) error { fmt.Println("writing to", fs.Name); return nil }
func (fs *FileStorage) Close()                  { fmt.Println("closing", fs.Name) }
 
// MemoryStorage also satisfies ReadWriteCloser
func (ms *MemoryStorage) Read() string           { return "reading from memory: " + ms.data }
func (ms *MemoryStorage) Write(data string) error { ms.data = data; return nil }
func (ms *MemoryStorage) Close()                  { ms.data = "" }
 
func useStorage(rwc ReadWriteCloser, data string) {
    rwc.Write(data)
    fmt.Println(rwc.Read())
    rwc.Close()
}
 
useStorage(&FileStorage{Name: "data.txt"}, "hello file")
useStorage(&MemoryStorage{}, "hello memory")

type assertion

get the concrete type back out of an interface value. always use comma ok — panics without it if type is wrong.

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var n Notifier = &EmailNotifier{recipient: "test@example.com"}
 
// safe — comma ok pattern
if email, ok := n.(*EmailNotifier); ok {
    fmt.Println("recipient:", email.recipient) // can access EmailNotifier fields
}
 
// unsafe — panics if wrong type
email := n.(*EmailNotifier) // panics if n is not *EmailNotifier

type switch

check multiple concrete types at once. use v := n.(type) to get the concrete value — lets you access type-specific fields.

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func describeNotifier(n Notifier) {
    switch v := n.(type) {
    case *EmailNotifier:
        fmt.Println("email to:", v.recipient)   // v is *EmailNotifier here
    case *SMSNotifier:
        fmt.Println("sms to:", v.phoneNumber)   // v is *SMSNotifier here
    case *SlackNotifier:
        fmt.Println("slack to:", v.webhook)     // v is *SlackNotifier here
    default:
        fmt.Println("unknown notifier")
    }
}
 
// without v — you can match but can't access type-specific fields
switch n.(type) {
case *EmailNotifier:
    fmt.Println("email") // can't access .recipient here
}

empty interface — interface / any

accepts any type. like typescript's any. use sparingly — you lose type safety.

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// any and interface{} are identical — any is just an alias (go 1.18+)
type APIResponse struct {
    Success bool        `json:"success"`
    Message string      `json:"message"`
    Data    interface{} `json:"data,omitempty"` // can hold Todo, []string, nil — anything
}
 
func respond(success bool, message string, data interface{}) *APIResponse {
    return &APIResponse{Success: success, Message: message, Data: data}
}
 
// same function handles all of these
respond(true,  "todo fetched",    &Todo{ID: "1", Title: "learn go"})
respond(true,  "routes fetched",  []string{"/users", "/posts"})
respond(false, "not found",       nil) // Data omitted — omitempty + nil

real world use case — dependency injection

the pattern you'd use in a real go backend:

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// instead of coupling handlers directly to gorm
type TodoRepository interface {
    GetAll() ([]Todo, error)
    Create(todo Todo) error
    Delete(id string) error
}
 
// real implementation — hits actual db
type GORMRepository struct{ db *gorm.DB }
func (r *GORMRepository) GetAll() ([]Todo, error) { /* real db query */ }
 
// test implementation — no db needed
type MockRepository struct{}
func (r *MockRepository) GetAll() ([]Todo, error) {
    return []Todo{{Title: "fake todo"}}, nil
}
 
// handler accepts the interface — works with gorm or mock
func TodoHandler(repo TodoRepository) gin.HandlerFunc {
    return func(c *gin.Context) {
        todos, err := repo.GetAll()
        // ...
    }
}
 
// swap implementations without changing the handler
TodoHandler(&GORMRepository{db: db})   // production
TodoHandler(&MockRepository{})          // testing

this is dependency injection — the interface is the contract, implementations are swappable.

what interfaces can and cannot contain

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// interfaces can only contain method signatures
type Valid interface {
    Do() error
    Describe() string
}
 
// interfaces can embed other interfaces
type Extended interface {
    Valid         // embeds Valid
    Extra() bool
}
 
// interfaces CANNOT contain fields
type Invalid interface {
    Name string  // compile error — fields not allowed in interfaces
    Do() error
}

gotchas

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// implicit implementation — you can satisfy someone else's interface
// without touching their code at all
// your struct + their interface = automatic satisfaction if methods match
 
// interface embedding in struct — rare but valid
type Service struct {
    Notifier        // embedded interface inside struct
    Name string
}
// methods are promoted — Service now has Send() and Channel()
// still need to assign a concrete implementation at runtime
 
// nil interface vs nil pointer — subtle trap
var e *EmailNotifier = nil  // nil pointer to EmailNotifier
var n Notifier = e          // n is not nil — it has type info (*EmailNotifier)
fmt.Println(n == nil)       // false — interface holds type + value, both needed for nil
// this causes unexpected behavior when returning errors from functions
// covered in topic 14 — error handling