How does Go handle errors, and what is the difference between %v and %w in fmt.Errorf?

// error is a built-in interface: type error interface { Error() string }

// Standard pattern
func parseAge(s string) (int, error) {
    age, err := strconv.Atoi(s)
    if err != nil {
        return 0, fmt.Errorf("parseAge: %w", err)  // %w wraps the error
    }
    if age < 0 || age > 150 {
        return 0, fmt.Errorf("parseAge: invalid age %d", age)  // %v or plain
    }
    return age, nil
}

// Sentinel errors — compare with errors.Is()
var ErrNotFound = errors.New("not found")

// %w wraps the error — errors.Is / errors.As can inspect the chain
wrapped := fmt.Errorf("lookupUser: %w", ErrNotFound)
fmt.Println(errors.Is(wrapped, ErrNotFound))  // true

// %v formats as string — errors.Is CANNOT find original error
notWrapped := fmt.Errorf("lookupUser: %v", ErrNotFound)
fmt.Println(errors.Is(notWrapped, ErrNotFound))  // false!

// Custom error type
type ValidationError struct{ Field, Msg string }
func (e *ValidationError) Error() string {
    return fmt.Sprintf("%s: %s", e.Field, e.Msg)
}

// errors.As — extract a specific error type from the chain
var ve *ValidationError
if errors.As(err, &ve) {
    fmt.Println("bad field:", ve.Field)
}