Better way to write deep maps in Haskell?

Question

I am parsing a text file that contains lines of numbers. I want to turn them into [[[Int]]]. (A list of cells inside of a list of rows inside of a list of boards.)

sudoku.txt

main.hs

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

splitBoards yields [[String]] and I want to use sudokus to yield [[[Int]]]. That requires a deep mapping that looks awful:

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

Is there a more Haskellian way to write the sudokus function?

For reference:

> 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]]]

user_67579 · Answer

You can wrap a value in a list by making use of pure :: Applicative f => a -> f a, hence we can write the \char -> read [char], as read . pure.

Next we can write the \row -> map (read . pure) row lambda expression as map (read . pure). So that means that we can rewrite the line to:

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

or we can avoid these brackets with:

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

Here it thus means that the (read . pure) is the mapping function of the "innermost" map.

Better way to write deep maps in Haskell?

1 Answers