How is the built-in error type defined and how do you implement custom errors?

type error interface { Error() string }

// Simple sentinel errors
var ErrNotFound   = errors.New("not found")
var ErrPermission = errors.New("permission denied")

// Structured custom error type
type ValidationError struct {
    Field   string
    Message string
}
func (e *ValidationError) Error() string {
    return fmt.Sprintf("validation failed on %s: %s", e.Field, e.Message)
}

// Function returning the error interface
func validateAge(age int) error {
    if age < 0 {
        return &ValidationError{Field: "age", Message: "must be non-negative"}
    }
    if age > 150 {
        return &ValidationError{Field: "age", Message: "unrealistically large"}
    }
    return nil  // untyped nil — truly nil interface
}

// Inspect the error type with errors.As
err := validateAge(-1)
var ve *ValidationError
if errors.As(err, &ve) {
    fmt.Println("Field:", ve.Field, "Message:", ve.Message)
}

// Wrapped errors (Go 1.13+) — %w preserves the chain
func openConfig(path string) error {
    if err := os.Open(path); err != nil {
        return fmt.Errorf("openConfig %s: %w", path, err)
    }
    return nil
}
err = openConfig("/missing/config.yaml")
if errors.Is(err, os.ErrNotExist) {
    fmt.Println("config file does not exist")
}