I try to traverse the diretory tree. A naive depth-first traversal seems not to produce the data in a lazy fashion and runs out of memory. I next tried a breadth first approach, which shows the same problem - it uses all the memory available and then crashes.

the code i have is:

getFilePathBreadtFirst :: FilePath -> IO [FilePath]
getFilePathBreadtFirst fp = do
  fileinfo <- getInfo fp 
  res :: [FilePath]  <- if isReadableDirectory fileinfo
          then do
                children  <- getChildren fp 
                lower    <-  mapM getFilePathBreadtFirst children  
                return (children ++  concat lower)
               return (children ++  concat ()
          else return [fp]        -- should only return the files? 
  return res 

getChildren :: FilePath -> IO [FilePath]
getChildren path = do 
          names <- getUsefulContents path
          let namesfull = map (path </>) names
          return namesfull

testBF fn = do  -- crashes for /home/frank, does not go to swap 
  fps <- getFilePathBreadtFirst fn
  putStrLn $ unlines fps

I think all the code is either linear or tail recursive, and I would expect that the listing of filenames starts immediately, but in fact it does not. Where is the error in my code and my thinking? where have I lost lazy evaluation?