List 需要一个basecase来返回列表中的位置

（sud不包含任何内容）

（！！0）

是

头

，但是

头

只对非空列表有效，因此您需要检查是否是这种情况：

所以

Prelude> :m +Data.Maybe Prelude Data.Maybe> listToMaybe [1..5] Just 1 Prelude Data.Maybe> listToMaybe [] Nothing

blankMaybe:：数独->Maybe Pos
blankMaybe（数独rs）=listToMaybe[（y，x）|（r，y）可能您可以为输入添加一个预期输出的示例？不如返回
Maybe Pos
？只是添加了mor信息。不，任务说我应该返回一个Pos。那么您收到的作业是错误的。您可以返回
（-1，-1）
正如@WillNess所建议的，但是如果你被哈斯克尔教授而没有被教授，这可能是
的工作，那么你的老师并没有真正做好他们的工作。另外，
某物！！0
通常被写成
头某物
。
Main> blank example
(0,2)

Main> blank sud
*** Exception: Prelude.(!!): index too large

example =
    Sudoku
      [ [Just 3,  Just 6,  Nothing, Nothing, Just 7,  Just 0,  Just 2,  Nothing, Nothing]
      , [Nothing, Just 5,  Nothing, Nothing, Nothing, Nothing, Just 1,  Just 8,  Nothing]
      , [Nothing, Nothing, Just 9,  Just 2,  Nothing, Just 4,  Just 7,  Nothing, Nothing]
      , [Nothing, Nothing, Nothing, Nothing, Just 1,  Just 3,  Nothing, Just 2,  Just 8 ]
      , [Just 4,  Nothing, Nothing, Just 5,  Nothing, Just 2,  Nothing, Nothing, Just 9 ]
      , [Just 2,  Just 7,  Nothing, Just 4,  Just 6,  Nothing, Nothing, Nothing, Nothing]
      , [Nothing, Nothing, Just 5,  Just 3,  Nothing, Just 8,  Just 9,  Nothing, Nothing]
      , [Nothing, Just 8,  Just 3,  Nothing, Nothing, Nothing, Nothing, Just 6,  Nothing]
      , [Nothing, Nothing, Just 7,  Just 6,  Just 9,  Nothing, Nothing, Just 4,  Just 3 ]
      ]

blank :: Sudoku -> Pos
blank (Sudoku rs) = case [(y,x) | (r,y) <- rs `zip` [0..8]
                                , (c,x) <- r `zip` [0..8]
                                , c == Nothing] 
                    of (pos:_) -> pos; _ -> (-1, -1)

Prelude> :m +Data.Maybe
Prelude Data.Maybe> listToMaybe [1..5]
Just 1
Prelude Data.Maybe> listToMaybe []
Nothing
blankMaybe :: Sudoku -> Maybe Pos
blankMaybe (Sudoku rs) = listToMaybe [(y,x) | (r,y) <- rs `zip` [0..8]
                                            , (c,x) <- r `zip` [0..8]
                                            , c == Nothing]