Solution to the Ruler Problem in Haskll

Ruler.hs

ruler :: Int -> Int -> String
ruler h s = ruler' (2 ^ h * s + 1) h 1 [] []
  where
    outChar c r = if (c `mod` (2 ^ r)) == 1 then '|' else ' '
    ruler' _ 0 _ _ a = unlines $ reverse a
    ruler' w r c l a = let nl = (outChar c r) : l
                       in if c == w then ruler' w (r - 1) 1 [] (nl : a)
                          else ruler' w r (c + 1) nl a

-- to diplay results:
-- putStrLn $ ruler 6 2