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POListing all the contents of a directory by breadth-first order results in low efficiency
 

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    <p>I writed a Haskell module to list all the contents of a directory by breadth-first order. The below is the source code.</p> <pre><code>module DirElements (dirElem) where import System.Directory (getDirectoryContents, doesDirectoryExist) import System.FilePath ((&lt;/&gt;)) dirElem :: FilePath -&gt; IO [[FilePath]] dirElem dirPath = iterateM (not.null) (concatMapM getDirectoryContents') [dirPath] &gt;&gt;= return.tail getDirectoryContents' :: FilePath -&gt; IO [FilePath] getDirectoryContents' dirPath = do isDir &lt;- do doesDirectoryExist dirPath if isDir then dirContent else return [] where dirContent = do contents &lt;- getDirectoryContents dirPath return.(map (dirPath&lt;/&gt;)).tail.tail $ contents iterateM :: (Monad m) =&gt; (a -&gt; Bool) -&gt; (a -&gt; m a) -&gt; a -&gt; m [a] iterateM fb f x = do --Notice: Due to the the implementation of &gt;&gt;=, iterateM can't be writen like iterate which gives a infinite list and have type of iterateM :: (Monad m) =&gt; (a -&gt; Bool) -&gt; (a -&gt; m a) -&gt; a -&gt; m [a] if fb x then do tail &lt;- do {fx &lt;- f x; iterateM fb f fx} return (x:tail) else return [] concatMapM :: Monad m =&gt; (a -&gt; m[b]) -&gt; [a] -&gt; m[b] concatMapM f list = mapM f list &gt;&gt;= return.concat </code></pre> <p>It works correct but when performing on a large directory, it will "suspend" for a little while, and spring out all the results. </p> <p>After a research I find it is the same question with <code>sequence $ map return [1..]::[[Int]]</code> see <a href="https://stackoverflow.com/questions/14494648/why-the-haskell-sequence-function-cant-be-lazy-or-why-recursive-monadic-function">Why the Haskell sequence function can't be lazy or why recursive monadic functions can't be lazy</a></p>
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