爲什麼QuickCheck放棄？

Question

我使用QuickCheck測試以下程序：雖然響應變化

{-# LANGUAGE TemplateHaskell #-} 

import Test.QuickCheck 
import Test.QuickCheck.All 

elementAt :: (Integral b) => [a] -> b -> a 
elementAt [x] _ = x 
elementAt (x:xs) 1 = x 
elementAt (x:xs) b = elementAt xs (b - 1) 

prop_elementAt xs b = length xs > 0 && b >= 0 && b < length xs ==> elementAt xs (b + 1) == xs !! b 

main = $(quickCheckAll) 

，我不斷收到消息

*** Gave up! Passed only x tests. 

這是不是我應該關心？或者測試輸入的性質決定了QuickCheck的運行時間？

Answer 1

的方式==>作品是第一個快速檢查將生成隨機值xs和b，然後檢查謂詞length xs > 0 && b >= 0 && b < length xs納才把它會檢查屬性的滿足性。

由於有多少測試用例會產生限制，可能會發生上述謂詞不滿足的情況。所以在生成足夠的有效測試用例（滿足謂詞）之前，quickcheck會放棄。

您應該將Arbitrary實例聲明爲newtype以僅生成滿足這些謂詞的測試用例。

{-# LANGUAGE TemplateHaskell #-} 

import Test.QuickCheck 
import Test.QuickCheck.All 

elementAt :: (Integral b) => [a] -> b -> a 
elementAt [x] _ = x 
elementAt (x:xs) 1 = x 
elementAt (x:xs) b = elementAt xs (b - 1) 

prop_elementAt (Foo xs b) = elementAt xs (b + 1) == xs !! b 

data Foo a b = Foo [a] b deriving (Show) 

instance (Integral b, Arbitrary a, Arbitrary b) => Arbitrary (Foo a b) where 
    arbitrary = do 
    as <- listOf1 arbitrary   -- length xs > 0 
    b <- choose (0,length as - 1)  -- b >= 0 and b < length xs 
    return (Foo as $ fromIntegral b) 

main = $(quickCheckAll)