haskell - Test.QuickCheck 未正确生成随机值

Question

我正在使用 Test.QuickCheck 生成随机数独谜题。

data Sudoku = Sudoku { getSudoku :: [[Maybe Int]] } deriving (Show, Eq)

rows :: Sudoku -> [[Maybe Int]]
rows (Sudoku rs) = rs

--B1
printSudoku :: Sudoku -> IO ()
printSudoku s = do
    putStrLn . unlines . map (map (maybe '.' (head . show))) $ rows s

--C1
cell :: Gen (Maybe Int)
cell = suchThatMaybe (frequency [(90, choose (0,0)),(10, choose(1,9))]) (/=0)

--C2
instance Arbitrary Sudoku where
    arbitrary = do
        rows <- sequence [ sequence [ cell | j <- [1..9] ] | i <- [1..9] ]
        return (Sudoku rows)

--C3
prop_Sudoku :: Sudoku -> Bool
prop_Sudoku = isSudoku

checkRandomSudoku :: IO [Bool]
checkRandomSudoku = do
    s <- sample' (arbitrary :: Gen Sudoku)
    return $ map isSudoku s

代码运行良好。然而，当我执行

a <- 样本'(任意::Gen Sudoku)

序列 $ map printSudoku a

它返回如下内容:

....5..3.
...4.....
...2.....
...5.....
.........
...33....
...5.....
...2.4...
.........

.........
.343.....
.........
......9.2
.........
45....5.1
.2......7
.7..88.34
.9....6..

....2..8.
.2121638.
.7.7...9.
4..45.6..
.....6.2.
..6.6....
53..9.6..
..9....7.
.47892...

.373411..
5...3282.
...45..9.
8989..18.
31.8113..
9..35.6..
4.685....
.4....39.
7..6.5.76

48.178.53
1.871.4.4
3165.17..
.1...7.59
.98126.51
6.6...775
9.4636952
.5..239..
372.....8

.34.73129
.5.8.27.1
344.34931
28.6.94.1
6327.3..8
3743.5496
93...7984
..82.8...
..3.54.93

273847853
5568.7465
832.73515
3766..6.7
.7.196256
1.96.9.3.
.7156.268
1615.196.
.392..633

731652284
863.8.768
31..5.5.6
961.5.467
1245.1159
5..275471
52.727759
6.656.849
99.72352.

这显然根本不是随机的。空单元格的分布最初非常高，然后慢慢减少。我是否使用了错误的功能或以错误的方式？谢谢

Answer 1

您看到此行为的原因是 QuickCheck 尝试逐渐增大的测试用例。这在 resizing 中有描述手册的部分。

在suchThatMaybe的情况下增加大小使其重试生成任意匹配您传递给它的谓词。您可以在源 http://hackage.haskell.org/package/QuickCheck-2.6/docs/src/Test-QuickCheck-Gen.html#suchThatMaybe 中看到.有趣的代码是:

try _ 0 = return Nothing
try k n = do x <- resize (2*k+n) gen
             if p x then return (Just x) else try (k+1) (n-1)

发生的事情是 suchThatMaybe重试 n次凡 n取决于传递给生成器的大小参数。

sample' tries the sizes [0,2..20] 一直传播到 suchThatMaybe .

您可以通过调用 resize 来覆盖增加的尺寸。在发电机上:

>>> a <- sample' $ (resize 2 arbitrary :: Gen Sudoku)
>>> sequence $ map printSudoku a
.....8.5.
.......4.
.........
4..25...5
......9..
....5....
........7
.....7...
...4.....

...5.....
.......4.
...6...6.
..14.....
...7...7.
....2....
.....6...
...4..572
.4.....6.

..6..8...
..4......
.........
......9..
839......
67..4....
.5.......
....5....
........3

4...1....
7..9.39..
....6....
...4.1...
.........
.........
..9....6.
.2.9...84
.....8...

.64......
..3.44...
4.......4
....1..8.
.9.......
34.......
.....6...
18.2..593
.4.7.....

.........
...8..6..
.2......5
...5.....
.2.......
........6
.3....13.
8.1.2..85
....5....

..7......
..67...5.
..6......
27....1.9
.9.......
78.....7.
......34.
.......2.
..81...81

3.1......
.........
.....6...
.........
.16.71...
.........
.2.......
.........
.....9.1.

..65..6.9
........5
..1.4....
....86...
.2..2..2.
.....9...
..6......
.........
...7..855

.......94
...14..8.
.....4...
...3....9
.........
.....5...
.5.......
45.....8.
..48.....

4........
......3..
5......4.
.4..6..2.
..3......
.........
..9......
6..9.....
....7....