Haskell QuickCheck最小反例

Question

考虑一下distributivity law between reverse and ++的下列测试，

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

注意Int的列表

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

然而，对可等价物列表的测试，

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

第二次考试通过的原因是什么？

更新在使用main = quickCheck test2编译时，随后对不明确类型变量的错误提示了问题(如答案中所描述的)，

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

当您实际计算test2时，GHCi必须选择要使用的a类型。在没有更多信息的情况下，GHCi的扩展默认规则使其默认为()，对此法律是正确的。

Answer 2

> verboseCheck test2 

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

多态参数默认为()，当然所有这些值都是相等的。