functional_closure_set.hs

data Set a = S { insert :: a -> Set a
               , member :: a -> Bool
               , size :: Int
               }

emptySet = let insert xs n = makeSet (if n `elem` xs then xs else n : xs)
               member xs n = n `elem` xs
               size = length
               makeSet xs = S (insert xs) (member xs) (size xs)
            in makeSet []

main :: IO ()
main = do
  let s1 = emptySet
  let s2 = s1 `insert` 8
  let s3 = s2 `insert` 8
  let s4 = s3 `insert` 5
  print $ size s4