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copied!<p>I modified the older answer that Davorak linked to to use the new <code>pipes</code> library.</p> <p>It uses <code>StateP</code> to keep a queue of untraversed directories so that it can do a breadth first traversal. It uses <code>MaybeP</code> for exiting from the loop, as a convenience.</p> <pre><code>import Control.Monad import Control.Proxy import Control.Proxy.Trans.Maybe import Control.Proxy.Trans.State as S import Data.Sequence hiding (filter) import System.FilePath.Posix import System.Directory getUsefulContents :: FilePath -> IO [FilePath] getUsefulContents path = fmap (filter (`notElem` [".", ".."])) $ getDirectoryContents path traverseTree :: (Proxy p) => FilePath -> () -> Producer (MaybeP (StateP (Seq FilePath) p)) FilePath IO r traverseTree path () = do liftP $ S.modify (|> path) forever $ do x <- liftP $ S.gets viewl case x of EmptyL -> mzero file :< s -> do liftP $ S.put s respond file p <- lift $ doesDirectoryExist file when p $ do names <- lift $ getUsefulContents file let namesfull = map (file </>) names liftP $ forM_ namesfull $ \name -> S.modify (|> name) </code></pre> <p>This defines a breadth-first lazy producer of files. If you hook it up to a printing stage, it will print out the files as it traverses the tree:</p> <pre><code>main = runProxy $ evalStateK empty $ runMaybeK $ traverseTree "/tmp" >-> putStrLnD </code></pre> <p>Laziness means that if you only demand 3 files, it will only traverse the tree as much as necessary to generate three files, then it will stop:</p> <pre><code> main = runProxy $ evalStateK empty $ runMaybeK $ traverseTree "/tmp" >-> takeB_ 3 >-> putStrLnD </code></pre> <p>If you want to learn more about the <a href="http://hackage.haskell.org/package/pipes" rel="nofollow"><code>pipes</code> library</a>, then I recommend you read <a href="http://hackage.haskell.org/packages/archive/pipes/3.1.0/doc/html/Control-Proxy-Tutorial.html" rel="nofollow">the tutorial</a>.</p>

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