The problem there is that you lose the guarantee that the parser validated the string value. A caller can just say: // This is returning an error for some

The problem there is that you lose the guarantee that the parser validated the string value. A caller can just say: // This is returning an error for some reason, so let's do it directly. // username, err := parsers.NewUsername(raw) username := parsers.Username(raw) You also get implicit conversions in ways you probably don't want: var u Username u = "<hello>" // Implicitly converts from string to Username