Go Dependency Injection

DI in Go is a pattern, not a framework: pass dependencies to constructors, wire everything explicitly in main. Reach for a framework only when manual wiring measurably hurts.

1. Constructor Injection — the Default

// ✅ Good — dependencies are explicit parameters
type OrderService struct {
    repo     OrderRepository
    payments PaymentGateway
    logger   *slog.Logger
}

func NewOrderService(repo OrderRepository, payments PaymentGateway, logger *slog.Logger) *OrderService {
    return &OrderService{repo: repo, payments: payments, logger: logger}
}