demonstrate a bug

bug.hs

{-# LANGUAGE OverloadedStrings #-}
import Data.ByteString.Char8 ()
import Data.Monoid (mappend)
import qualified Blaze.ByteString.Builder as B
import Data.Conduit.Blaze (builderToByteString)

import Data.Conduit
import qualified Data.Conduit.List as CL

main :: IO ()
main = do
    r <- CL.sourceList ["1", "2"]
           $$ CL.map B.fromByteString
           =$ CL.map (`mappend` B.flush)
           =$ builderToByteString
           =$ CL.consume
    print r 

-- ["1","","2",""]
-- It seems `builderToByteString` add an empty string implicitly.