Creates a vertical histogram of numbers

histogram.hs

import Data.List

histogram :: [Int] -> String
histogram xs = 
  let c = countNumbers xs
      h = foldr max 0 c
      l = "=========="
      n = "0123456789"
      g = unlines $ reverse $ transpose $ numbersToStars h c
  in g ++ l ++ "\n" ++ n

countN :: Int -> [Int] -> Int
countN n xs = length $ filter (== n) xs

countNumbers :: [Int] -> [Int]
countNumbers xs = map (\n -> countN n xs) [0..9]

repeatl :: Int -> x -> [x]
repeatl l w = take l $ repeat w

numbersToStars :: Int -> [Int] -> [String]
numbersToStars h (x:xs) = (repeatl x '*' ++ repeatl (h-x) ' ') :  numbersToStars h xs
numbersToStars _ [] = []

output.txt

         *
         *
         *
         *
   *     *
   **   **
 *********
==========
0123456789