listDirectoryDeep.hs

import qualified System.Directory as Directory
import qualified System.Environment as Environment
import qualified System.FilePath as FilePath

listDirectoryDeep :: FilePath -> IO [FilePath]
listDirectoryDeep directory = do
  entries <- listDirectoryShallow directory
  foldMap listEntry entries
  where
  listEntry entry = do
    isDirectory <- Directory.doesDirectoryExist entry
    isSymLink <- Directory.pathIsSymbolicLink entry
    if not isDirectory || isSymLink then
      pure [entry]
    else
      listDirectoryDeep entry

listDirectoryShallow :: FilePath -> IO [FilePath]
listDirectoryShallow directory = do
  entries <- Directory.listDirectory directory
  pure $ fmap (FilePath.combine directory) entries