Construct a histogram of a million random numbers

gistfile1.hs

-- hw3.hs
module Main where

import System.Random
import Data.Vector.Unboxed hiding (forM_, take, unfoldr, map)
import Data.Vector.Unboxed.Mutable hiding (take)
import Data.List
import Control.Monad.ST (runST)
import Control.Monad (forM_)
import Prelude hiding (read)

main :: IO ()
main = do
    seed <- newStdGen
    let rs = randomlist 1000000 seed
    print $ histogram $ map (`mod` 10) rs

-- Exercise 3 - Histogram
histogram :: [Int] -> Vector Int
histogram l = runST $ do
  vec <- new 10
  set vec (0 :: Int)
  forM_ l $ \i -> do
  	val <- read vec i
  	write vec i (val + 1) 
  freeze vec

randomlist :: Int -> StdGen -> [Int]
randomlist n = take n . unfoldr (Just . random)

-- Exercise 1 - Hopscotch
-- skips :: [a] -> [[a]]
-- skips l = [l]

-- -- Exercise 2 - Local maxima
-- localMaxima :: [Integer] -> [Integer]
-- localMaxima = undefined