person.hs

import Data.Maybe

data Person = Person { name :: String
                     , partner :: Partner
                     } deriving Eq
-- パートナー
type Partner = Maybe Person

instance Show Person where
    show (Person n Nothing) = n
    show (Person n p)       = n ++ "{" ++ (name $ fromJust p) ++ "}"

-- 誕生
born n = Person n Nothing

-- 結婚
marry p1 p2 =  (p1 { partner = Just p2 }, p2 { partner = Just p1 })

-- 離婚
divorce (p1,p2) = (p1 { partner = Nothing }, p2 { partner = Nothing })

main = do let tarou  = born "Tarou"
              hanako = born "Hanako"
              m = marry tarou hanako
              d = divorce m
          print m  -- (Tarou{Hanako},Hanako{Tarou})
          print d  -- (Tarou,Hanako)