Your call to mkMessage creates an instance of the RenderMessage typeclass, which is the core of Yesod’s i18n. It is defined as:

class RenderMessage master message where
    renderMessage :: master
                  -> [Text] -- ^ languages
                  -> message
                  -> Text

Notice that there are two parameters to the RenderMessage class: the master site and the message type. In theory, we could skip the master type here, but that would mean that every site would need to have the same set of translations for each message type. When it comes to shared libraries like forms, that would not be a workable solution.

The renderMessage function takes a parameter for each of the class’s type parameters: master and message. The extra parameter is a list of languages the user will accept, in descending order of priority. The method then returns a user-ready Text that can be displayed.