Chapter 07: Validation

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
}

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).

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.

3.1 The Visual Signal (Airport Security)

Concept: Fail early, fail fast. Signal: Airport Security. You don't get on the plane (Database) if you have a knife (Invalid Data).

🎓 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.