Walk through a complete Go OOP design: payment processing without inheritance.

// ── Interfaces ──────────────────────────────────────────────────

type PaymentProcessor interface {
    Process(amount float64, currency string) (txID string, err error)
}

type Refunder interface {
    Refund(txID string, amount float64) error
}

type FullPayment interface {
    PaymentProcessor
    Refunder
}

// ── Concrete implementations ────────────────────────────────────

type StripeProcessor struct{ apiKey string }

func NewStripe(key string) *StripeProcessor { return &StripeProcessor{apiKey: key} }
func (s *StripeProcessor) Process(amount float64, currency string) (string, error) {
    return "stripe_tx_" + uuid(), nil // call Stripe API
}
func (s *StripeProcessor) Refund(txID string, amount float64) error {
    return nil // call Stripe refund API
}

// Compile-time check
var _ FullPayment = (*StripeProcessor)(nil)

// ── Service: depends on narrow interfaces ───────────────────────

type OrderService struct {
    payments PaymentProcessor // only needs to charge
}

func (os *OrderService) PlaceOrder(amount float64) error {
    txID, err := os.payments.Process(amount, "USD")
    if err != nil { return fmt.Errorf("payment failed: %w", err) }
    log.Printf("order paid, tx=%s", txID)
    return nil
}

type RefundService struct {
    refunds Refunder // only needs to refund
}

func (rs *RefundService) Refund(txID string, amount float64) error {
    return rs.refunds.Refund(txID, amount)
}

// ── Wiring (main) ────────────────────────────────────────────────

stripe := NewStripe(os.Getenv("STRIPE_KEY"))
orderSvc  := &OrderService{payments: stripe}   // stripe as PaymentProcessor
refundSvc := &RefundService{refunds: stripe}   // stripe as Refunder

// ── Test mock ────────────────────────────────────────────────────

type MockPayment struct{ txID string; err error }
func (m *MockPayment) Process(float64, string) (string, error) { return m.txID, m.err }

func TestPlaceOrder(t *testing.T) {
    svc := &OrderService{payments: &MockPayment{txID: "mock_tx"}}
    if err := svc.PlaceOrder(99.99); err != nil {
        t.Fatal(err)
    }
}