Lens.hs

data Flag = Flag {
    _flagName :: String
  , _flagDep :: [Int]
  }
  

-- If a flag with name 'fname' already exists in the input list,
-- add 'fdep' to it. Otherwise append a new Flag to the list with 'fdep'
-- being the only item.
addFlag :: [Flag] -> String -> Int -> [Flag]
addFlag flags fname fdep = undefined

Lol.hs

    data FlagDeps = FlagDeps {
    _flagName :: String
  , _flagDep :: [ExDeps]
  }
  deriving (Show)
    
    
    addToFlag :: [FlagDeps] -> String -> ExDeps -> [FlagDeps]
    addToFlag flags fname' deps' = runST $ do
      ref <- newSTRef False -- whether we inserted successfully
      go flags fname' deps' ref
      where
        go :: [FlagDeps] -> String -> ExDeps -> STRef s Bool -> ST s [FlagDeps]
        go [] fname deps ref = do
          v <- readSTRef ref
          if v then return []
               else return [FlagDeps fname [deps]]
        go (p@(FlagDeps n ds):fs) fname deps ref
          | n == fname = do
              writeSTRef ref True
              return $ (FlagDeps n (deps:ds)):fs
          | otherwise  = fmap (p:) $ go fs fname deps ref

setInsertL x = flip (foldr (\x' go b -> x' : go (b || x == x')) (\b -> if b then [] else [x])) False