gistfile1.hs

import qualified Data.Map as Map
import qualified Data.Set as Set

type IntMap = Map.Map Int Int
type IntSet = Set.Set Int

extend :: IntMap -> IntSet -> IntMap -> Maybe (IntMap, IntSet)
extend inj codom map = Map.foldrWithKey f (Just (inj, codom)) map
  where f :: Int -> Int -> Maybe (IntMap, IntSet) -> Maybe (IntMap, IntSet)
        f i j (Just (inj, codom)) = if Set.member j codom
                                      then Nothing
                                      else Just ((Map.insert i j inj), (Set.insert j codom))
        f _ _ _ = Nothing