如何告诉QuickCheck只为参数生成有效的列表索引？

Question

假设我想为(!!)函数编写一些单元测试.

my_prop xs n = ...

我想将n限制为仅有效的索引,我知道我可以做类似的事情

my_prop xs n = (not.null) (drop n xs) ==> ...

但这使得绝大多数生成的案例都无效并被抛弃.有没有办法我可以设置,以便QuickCheck xs首先生成列表并使用其值只生成有效的情况n？

Answer 1

使用forAll,您可以指定一个生成器,n该生成器取决于先前的参数,例如

my_prop (NonEmpty xs) = forAll (choose (0, length xs - 1)) $ \n -> ...

Answer 2

你可以创建一个只生成有效索引的生成器并编写你的属性

import Test.QuickCheck
import Test.QuickCheck.Gen
import System.Random

indices :: [a] -> Gen Int
indices xs = MkGen $ \sg _ -> fst $ randomR (0, length xs - 1) sg

my_prop :: [Char] -> Property
my_prop xs = not (null xs) ==> forAll (indices xs) (\i -> xs !! i /= '0')

消除Int争论.

Answer 3

正如Daniel Wagner所建议的那样,一种可能性就是创建自己的数据类型并为其提供Arbitrary实例.

data ListAndIndex a = ListAndIndex [a] Int deriving (Show)

instance Arbitrary a => Arbitrary (ListAndIndex a) where
   arbitrary = do
     (NonEmpty xs) <- arbitrary
     n  <- elements [0..(length xs - 1)]
     return $ ListAndIndex xs n

NonEmpty来自自定义类型Test.QuickCheck.Modifiers用于生成非空列表.