Haskell Iteratees Example

IterateesHomework.hs

module IterateesHomework where

import Prelude hiding (words, unwords, init)
import Data.Text hiding (reverse, map, take)
import Data.Enumerator hiding (map)
import Control.Applicative ((<$>))
import Data.List (sort)
import qualified Data.Enumerator.Binary as EB
import qualified Data.Enumerator.Text as ET
import qualified Data.Enumerator.List as EL
import qualified Data.Map as M

data Entry = Entry {
  date :: Text,
  host :: Text,
  program :: Text,
  message :: Text
  } deriving Show

type Histogram = M.Map Text Int

main = do
  let enumEntries = ET.enumFile "/var/log/syslog" $= EL.map parse
  s <- (run_ $ enumEntries $$ processEntry)
  print $ s

parse :: Text -> Entry
parse s = Entry d h p m
  where d1:d2:d3:h:p':ms = words s
        d = unwords [d1,d2,d3]
        p = init p'
        m = unwords ms

processEntry :: Monad m => Iteratee Entry m ([(Int, Text)], Int)
processEntry = (top10 <$> summary) `EL.zip` countLines

summary :: Monad m => Iteratee Entry m Histogram
summary = EL.fold op M.empty 
  where op :: Histogram -> Entry -> Histogram
        op m e = M.insertWith' (+) (program e) 1 m

countLines :: Monad m => Iteratee Entry m Int
countLines = EL.fold (flip $ const (+1)) 0

top10 :: Histogram -> [(Int, Text)]
top10 = take 10 . reverse . sort . map swap . M.toList 

swap (x,y) = (y, x) -- not in Data.Tuple