为什么 quickCheck 创建单位列表

Question

为了乐趣和收益，我尝试了 QuickCheck 测试论文中的以下内容。

prop_revApp xs ys = reverse (xs ++ ys) == reverse xs ++ reverse ys

它通过了，尽管它不应该通过。我运行 verboseCheck 并且我看到它只检查单位列表，即：

Passed:
[(),(),(),(),(),(),(),(),(),(),(),(),(),()]

我想知道这是为什么。

我知道我可以通过定义属性的类型来修复它，但想知道这是否有必要，或者我遗漏了什么。

Answer 1

prop_revApp 函数非常通用：

*Main> :t prop_revApp
prop_revApp :: Eq a => [a] -> [a] -> Bool

如果您只是在 GHCi 中加载代码，并且运行它，是的，确实属性通过了：

*Main> quickCheck prop_revApp
+++ OK, passed 100 tests.

这是因为 GHCi 带有一组首选默认值。为方便起见，它会尽量使用最简单的类型。

它并没有比 () 简单多少，并且由于 () 有一个 Eq 实例，它选择了那个。

另一方面，如果您实际上尝试编写和编译某些属性，则代码无法编译：

import Test.Framework (defaultMain, testGroup)
import Test.Framework.Providers.QuickCheck2 (testProperty)

import Test.QuickCheck

main :: IO ()
main = defaultMain tests

prop_revApp xs ys = reverse (xs ++ ys) == reverse xs ++ reverse ys

tests = [
        testGroup "Example" [
                testProperty "prop_revApp" prop_revApp
           ]
      ]

如果您尝试运行使用 stack test 这些测试，您将收到编译器错误：

test\Spec.hs:11:17: error:
    * Ambiguous type variable `a0' arising from a use of `testProperty'
      prevents the constraint `(Arbitrary a0)' from being solved.
      Probable fix: use a type annotation to specify what `a0' should be.
      These potential instances exist:
        instance (Arbitrary a, Arbitrary b) => Arbitrary (Either a b)
          -- Defined in `Test.QuickCheck.Arbitrary'
        instance Arbitrary Ordering
          -- Defined in `Test.QuickCheck.Arbitrary'
        instance Arbitrary Integer
          -- Defined in `Test.QuickCheck.Arbitrary'
        ...plus 19 others
        ...plus 61 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    * In the expression: testProperty "prop_revApp" prop_revApp
      In the second argument of `testGroup', namely
        `[testProperty "prop_revApp" prop_revApp]'
      In the expression:
        testGroup "Example" [testProperty "prop_revApp" prop_revApp]
   |
11 |                 testProperty "prop_revApp" prop_revApp
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

您必须给属性一个更具体的类型；例如

tests = [
        testGroup "Example" [
                testProperty "prop_revApp" (prop_revApp :: [Int] -> [Int] -> Bool)
           ]
      ]

现在测试编译了，但是失败了：

$ stack test
Q56101904-0.1.0.0: test (suite: Q56101904-test)

Example:
  prop_revApp: [Failed]
*** Failed! Falsifiable (after 3 tests and 3 shrinks):
[1]
[0]
(used seed -7398729956129639050)

         Properties  Total
 Passed  0           0
 Failed  1           1
 Total   1           1

Q56101904-0.1.0.0: Test suite Q56101904-test failed
Test suite failure for package Q56101904-0.1.0.0
    Q56101904-test:  exited with: ExitFailure 1
Logs printed to console

为什么 quickCheck 创建单位列表

why does quickCheck create lists of units

haskell

quickcheck