需要一個返回列表中的位置的基準點

data Sudoku = Sudoku [[Maybe Int]] 

blank :: Sudoku -> Pos 
blank (Sudoku rs) = [(y,x) | (r,y) <- rs `zip` [0..8], (c,x) <- r `zip` [0..8], c == Nothing] !! 0

空白返回的第一個位置是Nothing。如果它不是Nothing，我想返回一些合理的，而不是錯誤！我怎麼能這樣做？需要一個返回列表中的位置的基準點

輸出：

Main> blank example 
(0,2) 

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

（SUD包含什麼都沒有）

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 ] 
     ]

來源

2014-10-09 Josu24

可能您可以爲輸入添加預期輸出的示例？ – Sibi 2014-10-09 12:27:05

如何返回'Maybe Pos'呢？ – MathematicalOrchid 2014-10-09 12:33:54

剛剛添加的信息。不，任務說我應該返回一個Pos .. – Josu24 2014-10-09 12:45:53

(!! 0)是head，但head只適用於非空列表，所以你需要檢查這是否是案例：

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)

這當然是非常不明智的;您應該返回包含在Maybe中的位置（即Just (y,x)或Nothing），以便函數的類型更改爲blank :: Sudoku -> Maybe Pos。然後，你從Data.Maybe使用內置函數listToMaybe：

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

使

blankMaybe :: Sudoku -> Maybe Pos 
blankMaybe (Sudoku rs) = listToMaybe [(y,x) | (r,y) <- rs `zip` [0..8] 
              , (c,x) <- r `zip` [0..8] 
              , c == Nothing]

或者你可以使用take 1代替head，與blankHead :: Sudoku -> [Pos]; blankHead (Sudoku rs) = take 1 [(y,x) | ..... ]。但使用Maybe是最好的選擇，因爲這種類型完全符合我們的意圖。

來源

2014-10-09 12:54:05

需要一個返回列表中的位置的基準點

回答

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