Christmas tree in Haskell IO

christmasTree :: Int -> IO ()
christmasTree n
    |n <= 2 = return ()
    |otherwise = putStrLn ( (topPart n) ++ (bottomPart n) )

addStar :: Int -> String
addStar n = [ '*' | _ <- [1..n]]

addSpace :: Int -> String
addSpace n = [ ' ' | _ <- [1..n]]

topPart n = maketree n 0
                where
                maketree 0 _ = ""
                maketree n m = ((addSpace (n-1)) ++ (addStar (m*2+1))++"\n") ++ maketree (n-1) (m+1)