Lazily prepend the line number to each line

lines.hs

import Control.Monad (unless)
import Prelude hiding (catch)
import System.IO (isEOF)

prependLineNums :: Int -> IO ()
prependLineNums n = isEOF >>= flip unless prependLineNum
  where prependLineNum = getLine >>= putStrLn . (++) (padTo 5 $ show n)
                                 >> prependLineNums (n+1)
        padTo c s = case c `compare` length s of
                      GT -> s ++ replicate (c - length s) ' '
                      _  -> take c s ++ " "
                  


main :: IO ()
main = prependLineNums 1