// In package "storage"
type UserRepository interface {
    GetUser(id int) (*User, error)
    SaveUser(u *User) error
}

type PostgresRepo struct{}

func (p *PostgresRepo) GetUser(id int) (*User, error) { ... }
func (p *PostgresRepo) SaveUser(u *User) error { ... }

// In package "handler"
type UserGetter interface {
    GetUser(id int) (*User, error)
}

func HandleRequest(repo UserGetter) http.Handler {
    // Only needs GetUser, not entire repository
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        user, _ := repo.GetUser(123)
        // ...
    })
}

// Concrete type lives in storage package
// Handler depends on small interface, not full implementation

type DataService interface {
    Read() error
    Write() error
    Validate() error
    Transform() error
}

// Split by actual usage
type Reader interface { Read() error }
type Writer interface { Write() error }

// Compose where needed
type ReadWriter interface {
    Reader
    Writer
}