gistfile1.hs

newtype Visitor e r = Visitor { visit :: e -> r }

data Employee = PermanentEmployee | Contractor

xmlvisitor :: Visitor Employee String
xmlvisitor = Visitor { visit = visitEmp }
  where visitEmp PermanentEmployee = "<employee />"
        visitEmp Contractor        = "<contractor />"

class Visitee e where
  accept :: e -> Visitor e r -> r

instance Visitee Employee where
  accept e v = visit v e

main = do putStrLn (accept PermanentEmployee xmlvisitor)