小编Mar*_* L.的帖子

我想Arbitrary为以下newtype 创建一个实例,以便将其用于QuickCheck:

newtype Wrapmaybe a = Wrapmaybe {getMaybe :: Maybe a} deriving (Eq, Show)

我知道Arbitrary实例Maybe可以写成如下:

instance Arbitrary a => Arbitrary (Maybe a) where
     arbitrary = frequency [(1, return Nothing), (1, liftM Just arbitrary)]

如何Arbitrary在不出现类型错误或类型错误的情况下为以下内容编写实例:

instance Arbitrary a => Arbitrary (Wrapmaybe Maybe a) where
   etc...