Flolac week of code 2: histogram

histogram.hs

module Histogram where
  import Data.List (unfoldr, foldl')

  histogram :: [Int] -> String
  histogram =  addFootter . unlines . graph . tally where
    addFootter s = s ++ "==========\n0123456789\n"

  tally :: [Int] -> [(Int, Int)]
  tally = foldl' accu ary where
    ary = zip [0..9] (repeat 0)
    tInc x t@(i, n) = if i == x then (i, n + 1) else t
    accu xs x = map (tInc x) xs

  graph :: [(Int, Int)] -> [String]
  graph xs = unfoldr row height where
    height = foldr (max . snd) 0 xs
    col n = map (\t -> if snd t >= n then '*' else ' ') xs
    row 0 = Nothing
    row h = Just(col h, h - 1)