[web-devel] [Yesod] how to write PersistFIeld instance?

I'd like to introduce serializable data type like this: data Role = Teacher | Student deriving (Read, Show, Eq, Ord) share2 mkPersist (mkMigrate "migrateAll") [$persist| User ident String : : role Role Maybe Update : |] instance PersistField Role where ??? Would you teach me how to write this instance?