deposit :: (Num a) => a -> a -> Maybe a
deposit value account = Just (account + value)

withdraw :: (Num a,Ord a) => a -> a -> Maybe a
withdraw value account = if (account < value)
                         then Nothing
                         else Just (account - value)

eligible :: (Num a, Ord a) => a -> Maybe Bool
eligible account = do
  account1 <- deposit 100 account
  account2 <- withdraw 200 account1
  account3 <- deposit 100 account2
  account4 <- withdraw 300 account3
  account5 <- deposit 1000 account4
  Just True

main = do
  print $ eligible 300 -- Just True
  print $ eligible 299 -- Nothing