Drop piece in Connect 4 column

connect4column.hs

numEmpty :: [Int] -> Int
numEmpty board = length $ filter (\x -> x == 0) board


addToColumn :: Int -> [Int] -> [Int]
addToColumn val board  = xs ++ [val] ++ ys
                        where
                            n = (numEmpty board)
                            xs = replicate (n - 1) 0
                            ys = snd (splitAt n board) 

-- simulate dropping a piece into a connect 4 column                      
main = do
    let board = [0,0,0,0,0,1,-1,1]
    print board
    print $ addToColumn 1 board