import System.Random

-- generate one random integer
randInt :: Int -> Int -> IO Int
randInt min max = do
  gen <- newStdGen
  return $ fst $ randomR (min,max) gen

-- generate n random integers
randInts :: Int -> Int -> Int -> IO [Int]
randInts n min max = do
  let vals1 = [randInt min max | i <- [1..n]]
  vals <- sequence vals1
  return vals

-- convert a list of values into a histogram (one string per value)
histogram :: (Enum a, Eq a, Ord a) => [a] -> [String]
histogram vals = bars
  where counts = [length (filter (==i) vals) | i <- [(minimum vals)..(maximum vals)]]
        bars = [take n (repeat 'X') | n <- counts]

-- print histogram of randomly-generated values
printHisto :: IO ()
printHisto = do
  vals <- randInts 1000 1 20
  let bars = histogram vals
  mapM_ putStrLn bars