breadth-first traversal of directory tree is not lazy
- by user855443
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?
