Haskell QuickCheck 最小计数器示例

Question

考虑以下针对 distributivity law between reverse and ++、

的测试

import Test.QuickCheck

test :: [Int] -> [Int] -> Bool
test xs ys = reverse (xs ++ ys) == reverse xs ++ reverse ys

test2 :: (Eq a) => [a] -> [a] -> Bool
test2 xs ys = reverse (xs ++ ys) == reverse xs ++ reverse ys

Int 的列表请注意

*Main> quickCheck test
*** Failed! Falsifiable (after 5 tests and 3 shrinks):    
[1]
[0]

然而，对于可等式项目列表的测试，

*Main> quickCheck test2
+++ OK, passed 100 tests.

第二次测试通过的原因是什么？

Update 在使用 main = quickCheck test2 编译时，随后出现的模糊类型变量错误提示了问题（如答案中所述），

No instance for (Eq a0) arising from a use of `test2'
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)

Answer 1

当您实际评估 test2 时，GHCi 必须选择一个类型 a 来使用。在没有更多信息的情况下，GHCi 的扩展默认规则使其默认为 ()，法律适用。

Answer 2

> verboseCheck test2 

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

多态参数默认为()，当然这样的值都是相等的

Haskell QuickCheck 最小计数器示例

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