Lib.hs

class Monad m => FileSystemMonad m where
  listDirectory' :: String -> m [String]
  readFile' :: String -> m String
  writeFile' :: String -> String -> m ()

instance FileSystemMonad IO where
  listDirectory' = listDirectory
  readFile' = readFile
  writeFile' = writeFile

instance FileSystemMonad (State [(String, String)]) where
  listDirectory' _ = fmap (map fst) get
  readFile' path = do
    fs <- get
    case lookup path fs of
      Just contents -> return contents
      Nothing -> error "no such file"
  writeFile' path contents = do
    fs <- get
    put ((path, contents) : fs)

copyDirectory :: FileSystemMonad m => String -> String -> m ()
copyDirectory src dest = do
  files <- listDirectory' src
  forM_ files $ \file ->
    copyFile file (dest ++ "/" ++ file)

copyFile :: FileSystemMonad m => String -> String -> m ()
copyFile fileSource fileDestination = do
  contents <- readFile' fileSource
  writeFile' fileDestination contents