Merge arrays

array.hs

import Control.Monad
import Data.Array.IO
import Data.Array.Base
import Data.Char (ord)
import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as C8

addToIOArray :: IOUArray Int Int -> Int -> IO ()
addToIOArray arr idx = do
  val <- unsafeRead arr idx
  unsafeWrite arr idx $ val + 1

fillIOArray :: IOUArray Int Int -> IO ()
fillIOArray arr = readLine >>= addList
  where
    readLine = parseLine <$> B.getLine
    parseLine = tail . map parseInt . B.split (fromIntegral $ ord ' ')
    parseInt str = case C8.readInt str of
                     Nothing -> error $ "not an integer: " ++ show str
                     Just (int, _) -> int
    addList = mapM_ (addToIOArray arr)

printPair :: Show a => (a, Int) -> IO ()
printPair (a, b) = replicateM_ b $ print a

main :: IO ()
main = do
    k <- read <$> getLine
    arr <- newArray (0,100) 0 :: IO (IOUArray Int Int)
    replicateM k $ fillIOArray arr
    getAssocs arr >>= mapM_ printPair