Pascal triangle row generator in Haskell

pascal.hs

-- Gubatron's method
-- n=3 [1, 2, 1]
-- copy the list and append a 0 on the left of the first
-- and append a 0 at the end of the second
--      [0, 1, 2, 1]
--      [1, 2, 1, 0]
-- add them up!
-- n=4  [1, 3, 3, 1]
--
-- append 0s to both sides and add them up
-- n=4  [1, 3, 3, 1]
--      [0, 1, 3, 3, 1]
--      [1, 3, 3, 1, 0]
-- n=5  [1, 4, 6, 4, 1]
-- and so on

-- add two lists, for clarity
addLists :: Num c => [c] -> [c] -> [c]
addLists l1 l2 = zipWith (+) l1 l2

pascal :: (Eq a1, Num a1, Num a2) => a1 -> [a2]
pascal 1 = [ 1 ]
pascal n =
 let prev = pascal(n-1)
     zero_prev = [0] ++ prev
     prev_zero = prev ++ [0]
 in
 addLists zero_prev prev_zero
 
-- [1,2,3] -> "1 2 3"
listToString = unwords. map show

-- mapM_ -> map    monadic    so no weird IO errors are triggered
printTriangle n = mapM_ putStrLn (map listToString (map pascal [1..n]))

main = do
    input <- getLine
    printTriangle . (read :: String -> Int) $ input