Print the Sierpinski and Pascal triangle in Haskell

sierpinski.hs

instance Num a => Num [a] where               -- (1)

   (f:fs) + (g:gs) = f+g : fs+gs               -- (2)
   fs + [] = fs                                -- (3a)
   [] + gs = gs                                -- (3b)

   (f:fs) * (g:gs) = f*g : [f]*gs + fs*(g:gs)  -- (4)
   _ * _ = []                                  -- (5)

   abs           = undefined   -- I can't think of a sensible definition
   signum        = map signum
   fromInteger n = [fromInteger n]
   negate        = map (\x -> -x)

pascal = map ([1,1]^) [0..]
sierpinski = map (map (\a -> a `mod` 2)) pascal

printTri n t = zipWith (\a b -> a++show b) paddings t'
  where t' = take n t
        paddings = map (flip replicate ' ') offset
        offset = map (maxwidth `div` 2 -) widths
        widths = map (flip div 2.length.show) t'
        maxwidth = length.show.last $ t'

-- todo: why is the offset still off?
printPascal = mapM_ (putStrLn.show) (printTri 20 pascal)
printSierpinski = mapM_ (putStrLn.show) (printTri 20 sierpinski)