unauthorized pipes solution to http://www.reddit.com/r/haskell/comments/2ejzst/streaming_tabseparated_logfile_analysis_with/ following Michael S's model

input.txt

server	user	timestamp	event
foo	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
foo	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
foo	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
foo	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
foo	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
foo	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
foo	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
foo	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
bar	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
bar	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
bar	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
bar	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
baz	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
baz	1	2014-10-10	{"tag": "Something else", "contents": {"foo": "bar"}}
baz	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
baz	1	2014-10-10	{"tag": "Something else", "contents": {"foo": "bar"}}
baz	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
baz	1	2014-10-10	{"tag": "Something else", "contents": {"foo": "bar"}}
baz	1	2014-10-10	{"tag": "Login", "contents": {"foo": "bar"}}
baz	1	2014-10-10	{"tag": "Something else", "contents": {"foo": "bar"}}

Main.hs

{-# LANGUAGE OverloadedStrings #-}
import Pipes
import Pipes.Group
import qualified Pipes.Prelude as P
import qualified Pipes.ByteString as PB
import qualified Pipes.Attoparsec as PA
import Data.Aeson
import Data.Aeson.Parser
import Data.Aeson.Types
import Data.Monoid
import Control.Monad.Trans.State.Strict
import Data.Text (Text)
import Lens.Family  -- from `lens-family`or Control.Lens from `lens`

import qualified Pipes.Internal as I -- for the missing fold'

main = runEffect $ toData PB.stdin >-> P.print
 where toData = concats . maps parseLogin . drops 1 . view PB.lines

parseLogin p = do  
   let divided = p ^. PB.break (== 9) . PB.splitAt 80 . to concat_bytes
   (server, rest) <- lift divided 
   good <- rest >-> P.drain -- drop any excess that comes before tab
   (me,p) <- lift $ runStateT (PA.parse json) $ skip_fields good
   case me of Just (Right a) -> 
                  case parseEither (withObject "" (.: "tag")) a of 
                    Right tag | tag == login -> yield (server, 1::Int)
                    _                        -> yield (server, 0)
              _                              -> return ()
   p >-> P.drain
 where login = "Login" :: Text
       concat_bytes = fold' (<>) mempty id
       skip_field = PB.drop 1 . PB.dropWhile (/= 9)
       skip_fields = skip_field . skip_field . PB.drop 1

-- sensible combinator:

fold' :: Monad m 
      => (x -> a -> x) -> x -> (x -> b) 
      -> Producer a m r -> m (b,r)
fold' step begin done p0 = loop p0 begin
     where
       loop p x = case p of
           I.Request v  _  -> I.closed v
           I.Respond a  fu -> loop (fu ()) $! step x a
           I.M          m  -> m >>= \p' -> loop p' x
           I.Pure    r     -> return (done x, r)