在 Haskell 中编写深度映射的更好方法？

Question

我正在解析一个包含数字行的文本文件。我想把它们变成[[[Int]]]. （板列表内的行列表内的单元格列表。）

数独.txt

530070000
600195000
098000060
800060003
400803001
700020006
060000280
000419005
000080079

400700500
300800001
700900008
060030010
090070050
080010790
002006900
005000030
000004200
...

主文件

parseTextExample :: IO ()
parseTextExample = do
  handle <- openFile "sudoku.txt" ReadMode
  contents <- hGetContents handle
  let splitBoards = splitWhen (== "") $ lines contents
  let sudokus = map (\board -> map (\row -> map (\char -> read [char] :: Int) row) board) splitBoards
  print sudokus
  hClose handle

splitBoardsyields [[String]]，我想用sudokusyield [[[Int]]]。这需要一个看起来很糟糕的深度映射：

map (\board -> map (\row -> map (\char -> read [char] :: Int) row) board) splitBoards

是否有更 Haskellian 的方式来编写sudokus函数？

---

以供参考：

> print splitBoards
>[["530070000","600195000","098000060","800060003","400803001","700020006","060000280","000419005","000080079"],["400700500","300800001","700900008","060030010","090070050","080010790","002006900","005000030","000004200"]]

> print sudokus
>
[[[5,3,0,0,7,0,0,0,0],[6,0,0,1,9,5,0,0,0],[0,9,8,0,0,0,0,6,0],[8,0,0,0,6,0,0,0,3],[4,0,0,8,0,3,0,0,1],[7,0,0,0,2,0,0,0,6],[0,6,0,0,0,0,2,8,0],[0,0,0,4,1,9,0,0,5],[0,0,0,0,8,0,0,7,9]],[[4,0,0,7,0,0,5,0,0],[3,0,0,8,0,0,0,0,1],[7,0,0,9,0,0,0,0,8],[0,6,0,0,3,0,0,1,0],[0,9,0,0,7,0,0,5,0],[0,8,0,0,1,0,7,9,0],[0,0,2,0,0,6,9,0,0],[0,0,5,0,0,0,0,3,0],[0,0,0,0,0,4,2,0,0]]]

Answer 1

您可以通过使用 将值包装在列表中pure :: Applicative f => a -> f a，因此我们可以将\char -> read [char],写为read . pure。

接下来我们可以将\row -> map (read . pure) rowlambda 表达式写为map (read . pure). 所以这意味着我们可以将该行重写为：

let sudokus = map (map (map (read . pure))) splitBoards :: [[[Int]]]

或者我们可以避免这些括号：

let sudokus = (map . map . map) (read . pure) splitBoards :: [[[Int]]]

因此，这里的意思是(read . pure)是“最内层”的映射函数map。