How can a single Go type implement multiple interfaces simultaneously?

A type can implement any number of interfaces simultaneously — all it needs is the right set of methods. There is no limit and no explicit declaration. This enables a type to play many roles in different contexts.

// Several independent interfaces
type Saver    interface { Save()   error }
type Loader   interface { Load()   error }
type Deleter  interface { Delete() error }
type Stringer interface { String() string }

// One type, many roles
type UserRepo struct{ db *sql.DB }

func (r *UserRepo) Save()   error  { /* INSERT */ return nil }
func (r *UserRepo) Load()   error  { /* SELECT */ return nil }
func (r *UserRepo) Delete() error  { /* DELETE */ return nil }
func (r *UserRepo) String() string { return "UserRepo" }

// Use through different interface lenses
repo := &UserRepo{db: db}

var s  Saver   = repo  // *UserRepo satisfies Saver
var l  Loader  = repo  // *UserRepo satisfies Loader
var d  Deleter = repo  // *UserRepo satisfies Deleter
var st fmt.Stringer = repo // satisfies fmt.Stringer too

// Composed interface — all at once
type CRUD interface {
    Saver
    Loader
    Deleter
}
var crud CRUD = repo   // *UserRepo satisfies CRUD

// Passing to functions that only need part of the capability
func backup(s Saver)   { s.Save() }  // only needs Save
func restore(l Loader) { l.Load() }  // only needs Load
backup(repo)
restore(repo)

// Verify all at compile time
var _ Saver    = (*UserRepo)(nil)
var _ Loader   = (*UserRepo)(nil)
var _ Deleter  = (*UserRepo)(nil)
var _ CRUD     = (*UserRepo)(nil)

Passing repo to backup(s Saver) exposes only the Save method — the function cannot call Load or Delete even though the concrete type has them. This is the principle of minimal interface exposure: functions accept only the capability they need, reducing coupling and making code easier to understand and test.

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