As you noticed, this function is slow because it collects all the results before it starts printing them. To circumvent that problem, you need to interleave the collection of information and its printing.
A good way of doing that whilst keeping a compositional approach to solving the problem is to introduce a datatype reifying the structure of
walkDir's call graph. Instead of sequencing all
IO actions and getting a list of
FilePaths back, you'd build a tree describing the computation (
RTree for Rose Tree and
T for Transformer as it takes an
data RTreeT m a = Node a [m (RTreeT m a)]
You can now write
walkDir' describing your strategy to explore the directories on your filesystem: return the files present in the current directory immediately and then explore the subdirectories one after the other.
walkDir' :: FilePath -> IO (RTreeT IO [FilePath])
walkDir' r = do
contents <- fmap (r </>) . exceptLocal <$> getDirectoryContents r
(files, dirs) <- filesAndDirs contents
return $ Node files $ fmap walkDir' dirs
filesAndDirs partitions a list of
FilePath depending on whether they are files or directories (using
tagDirectories to perform that test).
tagDirectories :: [FilePath] -> IO [(FilePath, Bool)]
tagDirectories = mapM (\ x -> (x,) <$> doesDirectoryExist x)
filesAndDirs :: [FilePath] -> IO ([FilePath], [FilePath])
filesAndDirs c = bimap (fmap fst) . partition (not . snd) <$> tagDirectories c
where bimap f (a, b) = (f a, f b)
exceptLocal is the filter you had in your original code snippet:
exceptLocal :: [FilePath] -> [FilePath]
exceptLocal = filter ((&&) . (/=) "." <*> (/=) "..")
You now have an
RTreeT IO [FilePath] and you can described a strategy to print it which will interleave printing some of the content and running some of the remaining
printRTreeT :: Show a => RTreeT IO a -> IO ()
printRTreeT (Node a mts) = print a >> mapM_ (printRTreeT =<<) mts
Of course, this is a rather crude printing function (e.g. you will notice quite a few empty lists if you have empty subdirectories) but it gives you an idea of how to proceed from there on.
If this is still slow, you may want to play the same sort of trick on
filesAndDirs: rather than sequencing all tests in one go, you could want to have a structure allowing you to only deal with one
FilePath at a time.
ifwith a bind directly, you could just do
\e -> if e then walkDir x else return [x]. That's what the
donotation desugars to anyway. \$\endgroup\$
ifFunction x y z = if x then y else z. \$\endgroup\$