without-concatMapAccumC.hs

#!/usr/bin/env stack
-- stack script --resolver lts-8.12
{-# LANGUAGE OverloadedStrings #-}
import Conduit
import Control.Monad
import Text.Regex (matchRegex,mkRegex,Regex)
import qualified Data.Text as T


loghead = mkRegex "^([0-9]{4}-[0-9]{2}-[0-9]{2} [0-9]{2}:[0-9]{2}:[0-9]{2},[0-9]{3} )"
-- "2015-01-25 00:04:18,840"

logMerge :: Monad m => Regex -> ConduitM String String m ()
logMerge logregex =
  loop ""
  where
    loop accum = do
      mline <- await
      case mline of
        Nothing -> unless (null accum) (yield accum)
        Just line ->
          case matchRegex logregex line of
            Just _ -> do
              yield accum
              loop line
            Nothing
              | null accum -> loop accum
              | otherwise -> loop $ accum ++ "<br>" ++ line


runMerge :: String -> String -> IO ()
runMerge infile outfile =
    runConduitRes
  $ sourceFile infile
 .| decodeUtf8C
 .| linesUnboundedC
 .| mapC T.unpack
 .| logMerge loghead
 .| mapC T.pack
 .| unlinesC
 .| encodeUtf8C
 .| sinkFile outfile

main :: IO ()
main = runMerge "infile.txt" "outfile.txt"