what's wrong?

class AddGroup a where
    (+), (-) :: a -> a -> a
    negate   :: a -> a
    x - y = x + (negate y)

data Vector = Vector { dx :: Double
                     , dy :: Double
                     } deriving (Show, Eq)

instance AddGroup Vector where
    (Vector dx1 dy1) + (Vector dx2 dy2) = Vector (dx1 + dx2) (dy1 + dy2)
    negate (Vector dx dy) = Vector (-dx) (-dy)

main = do
    putStrLn $ show $ (Vector 1 2) + (Vector 3 4)

-- I get the following errors:
-- So I should write Prelude.+ for every Double addition??

{-

Geom.hs:4:14:
    Ambiguous occurrence `+'
    It could refer to either `Main.+', defined at Geom.hs:2:4
                          or `Prelude.+', imported from Prelude

Geom.hs:4:17:
    Ambiguous occurrence `negate'
    It could refer to either `Main.negate', defined at Geom.hs:3:4
                          or `Prelude.negate', imported from Prelude

Geom.hs:11:54:
    Ambiguous occurrence `+'
    It could refer to either `Main.+', defined at Geom.hs:2:4
                          or `Prelude.+', imported from Prelude

Geom.hs:11:66:
    Ambiguous occurrence `+'
    It could refer to either `Main.+', defined at Geom.hs:2:4
                          or `Prelude.+', imported from Prelude

Geom.hs:15:35:
    Ambiguous occurrence `+'
    It could refer to either `Main.+', defined at Geom.hs:2:4
                          or `Prelude.+', imported from Prelude
-}