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

[Katsutoshi Itoh]

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?