`error` is a built-in interface type in Go:

type error interface {
    Error() string
}

Any type that implements `Error() string` can be used as an error.

• Standardize how functions return failure information.
• Allow custom error types while still matching `error`.

type MyError struct{ Msg string }
 
func (e MyError) Error() string { return e.Msg }
 
func f() error {
    return MyError{Msg: "something went wrong"}
}

• `nil` error means success.
• Be careful: an interface can be non-nil even if it holds a typed nil (advanced pitfall).

• interface /ˈɪntərfeɪs/: giao diện
• implement /ˈɪmplɪment/: triển khai
• pitfall /ˈpɪtfɔːl/: bẫy thường gặp