If you mean typeclasses, Elm doesn't have them.
Regarding hardcoded messages, there's been at some point talk in the community regarding allowing library authors to have some sort of hook to compiler that would make it possible to customize specific error messages along with a package.
How do compiler errors look in Elm with generic functions involved, about which the compiler doesn't have any helpful hardcoded knowledge? (I.e. no "+" or strings involved).