example.hs

newtype EmployeeVisitor r = EmployeeVisitor { visit :: Employee -> r }
data Employee = PermanentEmployee | Contractor

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

class VisitableEmployee e where
  accept :: e -> EmployeeVisitor r -> r

instance VisitableEmployee Employee where
  accept e v = visit v e

main = do putStrLn (accept PermanentEmployee xmlvisitor)