Cute script for drawing attention to errors in scrolling log output

escroll.hs

#!/usr/bin/env runhaskell
import Data.List
import Control.Concurrent
import System.IO

takeBlock :: [String] -> (String, [String])
takeBlock (firstLine:moreLines) = (block, restLines) where
  block = intercalate "\n" (firstLine:indLines)
  (indLines, restLines) = span (isPrefixOf " ") moreLines

data PrintOrSleep = Print String
                  | Sleep Int

processBlock :: String -> [PrintOrSleep] -> [PrintOrSleep]
processBlock block acts = Print block : restActs where
  restActs = if "ERROR" `isInfixOf` block
    then Sleep 5000000 : acts
    else acts

makeBlocks :: [String] -> [String]
makeBlocks [] = []
makeBlocks lines = block : makeBlocks remLines where
  (block, remLines) = takeBlock lines

processStream :: String -> [PrintOrSleep]
processStream = foldr processBlock [] . makeBlocks . lines

actOn :: PrintOrSleep -> IO()
actOn (Sleep usecs) = threadDelay usecs
actOn (Print block) = putStrLn block

main :: IO ()
main = do
  hSetBuffering stdout NoBuffering
  input <- getContents
  mapM_ actOn $ processStream input

Basically just do

yourprogram | ./escroll.hs

mostly it'll just echo the output of your program to the screen, but on any line containing "ERROR", it'll pause for five seconds, then scramble to catch up.

Indented lines are assumed to be part of the line above them -- ie for tracebacks and the like.