Hi everyone, I'm currently trying to solve a problem in which I have to process a long list of files, more specifically I want to compute MD5 checksums for all files. I have code which lists me all the files and holds it in the following data structure: data DirTree = FileNode FilePath | DirNode FilePath [DirTree] I tried the following: -- calculates MD5 sums for all files in a dirtree addChecksums :: DirTree -> IO [(DirTree,MD5Digest)] addChecksums dir = addChecksums' [dir] where addChecksums' :: [DirTree] -> IO [(DirTree,MD5Digest)] addChecksums' [] = return [] addChecksums' (f@(FileNode fp):re) = do bytes <- BL.readFile fp rest <- addChecksums' re return ((f,md5 bytes):rest) addChecksums' ((DirNode fp filelist):re) = do efiles <- addChecksums' filelist rest <- addChecksums' re return $ efiles ++ rest This works fine, but only for a small number of files. If I try it on a big directory tree, the memory gets junked up and it aborts with an error message telling me that there are too many open files. So I guess, I have to sequentialize the code a little bit more. But at the same time, I want to keep it as functional as possible and I don't want to write C-like code. What would be the Haskell way to do something like this? Thanks for all the input, Michael