Haskell QuickCheck最小计数器示例

Question

考虑以下关于反向和++之间的分布律的测试,

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

注意列表Int是

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

然而,测试等于物品的列表,

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

是什么让第二次测试通过？

更新在编译时main = quickCheck test2,模糊类型变量的后续错误提示问题(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

当你真正评估时test2,GHCi必须选择一种类型a来使用.如果没有更多信息,GHCi的扩展默认规则将其默认为()违法,法律适用.

Answer 2

> verboseCheck test2 

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

多态参数默认为(),当然所有这些值都相等.