Tell me about the SOLID principles.

SOLID is a set of five principles of object-oriented design that help create flexible, extendable, and maintainable systems. In the context of Go, despite the absence of traditional OOP, these principles are applicable to interfaces and structs.

1. S — Single Responsibility Principle Each type or module should have one responsibility and one reason to change.

2. O — Open/Closed Principle Software entities should be open for extension but closed for modification.

3. L — Liskov Substitution Principle Objects in a program should be replaceable with instances of their subtypes without affecting correctness.

4. I — Interface Segregation Principle Clients should not depend on interfaces they do not use. Prefer many specific interfaces over one general-purpose interface.

5. D — Dependency Inversion Principle High-level modules should not depend on low-level modules. Both should depend on abstractions (interfaces).

Example in Go for Dependency Inversion:

// Abstraction
type Notifier interface {
    Notify(message string) error
}

// Concrete implementation
type EmailNotifier struct {}
func (e EmailNotifier) Notify(message string) error {
    // send email
    return nil
}

// Usage
func SendAlert(n Notifier, msg string) {
    n.Notify(msg)
}

Thus, SOLID helps write code that is easier to test, extend, and maintain.