Exercise: histogram and graph

FLOLAC02.hs

import Control.Arrow ((&&&))
import Data.String (unlines)
import Data.List (unfoldr,foldl')

histogram :: [Int] -> String
histogram = graph . histo

histo :: [Int] -> [Int]
histo = foldl' oplus [0,0,0,0,0,0,0,0,0,0]
  where oplus xs i = zipWith ($) (replicate i id ++ [(1+)] ++ repeat id) xs

graph :: [Int] -> String
graph = unlines . reverse . ("0123456789":) . ("==========":) . unfoldr step
  where step xs | all (<=0) xs = Nothing
                | otherwise    = Just . (map star &&& map decr) $ xs
        star n = if n > 0 then '*' else ' '
        decr n = if n > 0 then n-1 else 0