haskell - 测试和应用

Question

我想在自定义类型上测试应用实例:

module SumMonad where

  import Test.QuickCheck (Arbitrary, arbitrary, elements, Gen)
  import Test.QuickCheck.Checkers (quickBatch, eq, (=-=), EqProp)
  import Test.QuickCheck.Classes (applicative)

  data Sum a b =
      First a
    | Second b
    deriving (Eq, Show)

  instance Functor (Sum a) where
    fmap _ (First x) = First x
    fmap f (Second y) = Second (f y)

  instance Applicative (Sum a) where
    pure = Second
    First x <*> _ = First x
    _ <*> First x = First x 
    Second f <*> Second x = Second (f x)

  instance Arbitrary a => Arbitrary (Sum a b) where
    arbitrary = do
      a <- arbitrary
      b <- arbitrary
      return ???(What should I write here)

  instance (Eq a, Eq b)  => EqProp (Sum a b) where (=-=) = eq 

如您所见，三个问号突出显示，我不知道那里有什么？

Answer 1

您需要在 First 之间随机选择和 Second ，然后生成合适的第一/第二分量。

使用 oneOf 可以快速完成有限数量的选择之间的选择。 .

 instance (Arbitrary a, Arbitrary b) => Arbitrary (Sum a b) where
    arbitrary = oneOf [ First <$> arbitrary, Second <$> arbitrary ]

以上等价于以下较低级别的实例，您一开始可能会觉得更容易理解。

 instance (Arbitrary a, Arbitrary b) => Arbitrary (Sum a b) where
    arbitrary = do
       which <- arbitrary   -- a random Bool
       if which
         then do
            a <- arbitrary
            return (First a)
         else do
            b <- arbitrary
            return (Second b)