test.hs

import Data.Time.Calendar
import Control.Monad
import Data.List

data Person = P { p_id   :: Int
                , name   :: String
                , gender :: Int
                , age    :: Int
                } deriving (Show, Eq)

data Group = G { g_id  :: Int
               , gname :: String
               } deriving Show

data Assginment = A { a_id   :: Int
                    , a_p_id :: Int
                    , a_g_id :: Int
                    , date   :: Day
                    } deriving Show

persons = [ P 1 "Tarou"   1 10
          , P 2 "Hanako"  2 20
          , P 3 "Jirou"   1 30
          , P 4 "Saburou" 1 40]

groups = [ G 1 "Matu", G 2 "Take", G 3 "Ume"]

assignments = [ A 1 1 1 (fromGregorian 2010 4 1)
              , A 2 2 1 (fromGregorian 2009 9 1)
              , A 3 3 2 (fromGregorian 2010 4 1)
              , A 4 1 2 (fromGregorian 2010 5 1)]

main = print $ nub $ do 
         p <- persons
         a <- assignments
         guard (p_id p == a_p_id a && 
                case toGregorian (date a) of 
                  (y,_,_) -> y == 2010)
         return p