为使用Maybe

Question

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

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

我知道Arbitrary的{​​{1}}实例可以写成如下：

Maybe

如何在不出现类型错误或类型错误的情况下为instance Arbitrary a => Arbitrary (Maybe a) where arbitrary = frequency [(1, return Nothing), (1, liftM Just arbitrary)] 实例编写以下内容：

Arbitrary

Answer 1

好吧，您可以使用相应的newtype构造函数替换该实例中的每个Maybe构造函数：

instance Arbitrary a => Arbitrary (WrapMaybe a) where
  arbitrary = frequency [ (1, return $ WrapMaybe Nothing)
                        , (1, fmap (WrapMaybe . Just) arbitrary) ]

但是，使用现有的Maybe实例对结果执行一次更简单：

instance Arbitrary a => Arbitrary (WrapMaybe a) where
  arbitrary = fmap WrapMaybe arbitrary

您也可以写WrapMaybe <$> arbitrary。但请不使用liftM，即obsolete！ （这些都是等价的。）