How do you write testable Go code using interfaces and mocks without a framework?

Go's implicit interface satisfaction means any struct with the right methods satisfies an interface — no registration or code generation required. This makes hand-written mocks straightforward, readable, and dependency-free.

// Interface in the consumer package (not the producer)
type EmailService interface {
    Send(ctx context.Context, to, subject, body string) error
}

// Production implementation
type SMTPEmailService struct { host string; port int }
func (s *SMTPEmailService) Send(ctx context.Context, to, subject, body string) error {
    // ... real SMTP logic
    return nil
}

// Hand-written mock for tests
type MockEmailService struct {
    Calls    []string // track what was called
    ReturnErr error   // control what error to return
}
func (m *MockEmailService) Send(ctx context.Context, to, subject, body string) error {
    m.Calls = append(m.Calls, to)
    return m.ReturnErr
}

// Functional mock — more flexible, inline in test
type EmailServiceFunc func(ctx context.Context, to, subject, body string) error
func (f EmailServiceFunc) Send(ctx context.Context, to, subject, body string) error {
    return f(ctx, to, subject, body)
}

// Test using the mock
func TestWelcomeEmail(t *testing.T) {
    mock := &MockEmailService{}
    svc  := NewUserService(nil, mock, slog.Default())

    err := svc.Register(context.Background(), RegisterRequest{
        Name: "Alice", Email: "alice@example.com",
    })
    if err != nil { t.Fatal(err) }

    if len(mock.Calls) != 1 {
        t.Errorf("expected 1 email sent, got %d", len(mock.Calls))
    }
    if mock.Calls[0] != "alice@example.com" {
        t.Errorf("email sent to wrong address: %s", mock.Calls[0])
    }
}

// Test error path
func TestWelcomeEmailFailure(t *testing.T) {
    mock := &MockEmailService{ReturnErr: errors.New("smtp down")}
    svc  := NewUserService(nil, mock, slog.Default())
    // Test that service handles email failure gracefully
}

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