Chapter 21: Validation

21.1 Errors as Values

In Java or Python, if validation fails, you Throw an Exception. The program explodes, and you hope someone catches the debris. In Go, an error is just a value, like an integer or a string. We pass it back politely.

Comparison: Error Handling

Python (Exception):

def check(price):
    if price < 0:
        raise ValueError("Invalid Price") # KABOOM!

Go (Values):

func check(price int) error {
    if price < 0 {
        return errors.New("Invalid Price") # Polite Return
    }
    return nil # No error
}

21.2 Guard Clauses (The "Quality Gate" Pattern)

We handle errors immediately at the top of the function.

if title == "" {
    return errors.New("missing title")
}
// ... Continue ..

Anatomy of errors.New

1. errors: Standard library package.
2. New: Constructor function.
3. Returns: An interface called error. (It's basically just an object with an Error() string method).

21.3 Why "No Exceptions"?

Go believes exceptions hide control flow. By forcing you to check if err != nil, your code becomes:

• Explicit: You see exactly where things can go wrong.
• Reliable: You handle the error right there, instead of bubbling it up 10 layers.

🎓 Knowledge Check: Why doesn't Go use "Exceptions" (try/catch)?

Answer: Go prefers Errors as Values. Exceptions hide control flow (you don't know where they might explode). Returning an error forces you to handle it explicitly (if err != nil), making code safer and more readable.