如果f :: Float -> Float -> Bool
f x y = x*y <= 0那么该函数可以扩展为重载的函数 选项:
A:f :: Num a => a -> a -> Bool
f x y = x*y <= 0; B:f :: Num a => a -> a -> a
f x y = x*y <= 0; C:f :: Ord a => a -> a -> Bool
f x y = x*y <= 0; D:f :: Ord a => a -> a -> a
f x y = x*y <= 0