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POWriting "wc -l" using Iteratee library - how to filter for newline?
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copied!<p>I am trying to come up with equivalent of "wc -l" using Haskell Iteratee library. Below is the code for "wc" (which just counts the words - similar to the code in iteratee example on hackage), and runs very fast:</p> <hr> <pre class="lang-hs prettyprint-override"><code>{-# LANGUAGE BangPatterns #-} import Data.Iteratee as I import Data.ListLike as LL import Data.Iteratee.IO import Data.ByteString length1 :: (Monad m, Num a, LL.ListLike s el) => Iteratee s m a length1 = liftI (step 0) where step !i (Chunk xs) = liftI (step $ i + fromIntegral (LL.length xs)) step !i stream = idone i stream {-# INLINE length1 #-} main = do i' <- enumFile 1024 "/usr/share/dict/words" (length1 :: (Monad m) => Iteratee ByteString m Int) result <- run i' print result {- Time measured on a linux x86 box: $ time ./test ## above haskell compiled code 4950996 real 0m0.013s user 0m0.004s sys 0m0.007s $ time wc -c /usr/share/dict/words 4950996 /usr/share/dict/words real 0m0.003s user 0m0.000s sys 0m0.002s -} </code></pre> <hr> <p>Now, how do you extend it to count the number of lines that too runs fast? I did a version using Prelude.filter to filter only "\n" to length but it is slower than linux "wc -l" because of too much memory, and gc (lazy evaluation, I guess). So, I wrote another version using Data.ListLike.filter but it won't compile because it doesn't type check - help here would be appreciated:</p> <hr> <pre class="lang-hs prettyprint-override"><code> {-# LANGUAGE BangPatterns #-} import Data.Iteratee as I import Data.ListLike as LL import Data.Iteratee.IO import Data.ByteString import Data.Char import Data.ByteString.Char8 (pack) numlines :: (Monad m, Num a, LL.ListLike s el) => Iteratee s m a numlines = liftI $ step 0 where step !i (Chunk xs) = liftI (step $i + fromIntegral (LL.length $ LL.filter (\x -> x == Data.ByteString.Char8.pack "\n") xs)) step !i stream = idone i stream {-# INLINE numlines #-} main = do i' <- enumFile 1024 "/usr/share/dict/words" (numlines :: (Monad m) => Iteratee ByteString m Int) result <- run i' print result </code></pre>

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