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我正在尝试从文件中读取两个自己的数据类型“BoardEdge”的列表。当我尝试运行代码时，出现异常：

“Main.hs：Prelude.read：没有解析”

正如我怀疑我在负责验证输入（validateInput）的函数上得到这个。当我在 ghci 中尝试插入两个 BoardEdge“对象”时，它运行良好并给出了 True。

谁能给我建议我做错了什么以及如何解决问题？

数据类型：

data Field = Empty | Black | Yellow deriving (Eq, Ord, Enum, Show, Read)

data BoardEdge = BoardEdge { colRow :: [[(Field, Int)]]} deriving (Read, Eq, Ord, Show)

主文件

    main :: IO()
    main = do
      args <- getArgs
      input <- loadInput args
      putStrLn "Puzzle input loaded:"
      putStrLn input
      let parsedInput = parseInput input
      if (validateInput parsedInput)
        then putStrLn "Input is valid."
        else error "Input invalid!"

    -- asks for path and reads input file
    loadInput :: [String] -> IO String
    loadInput [] =  getPath >>= readFile where
      getPath = do
        putStrLn "Provide path to puzzle input file:"
        getLine
    loadDefinition (a:_) = readFile a


    -- get valid data from input file
    parseInput :: String -> (B.BoardEdge,B.BoardEdge)
    parseInput d = parseInput' $ lines d where
      parseInput' (columns: rows :_) =
        (read columns, read rows)

Board.hs 中的验证函数导入合格为 B：

    validateInput :: (B.BoardEdge,B.BoardEdge) -> Bool
    validateInput (columns, rows) = rowColEq where
      rowColEq = countBlocks columns == countBlocks rows


    -- function that counts total quantity of colored blocks
    countBlocks :: (B.BoardEdge)-> Int
    countBlocks (B.BoardEdge colRow) = countBlocks' $ concat colRow where
      countBlocks' [] = 0
      countBlocks' (x:xs) = snd x + countBlocks' xs

我的输入文件是这样的：

    [[(Black,2),(Yellow,2),(Black,1)],[(Black,2),(Yellow,1),(Black,3)]]
    [[(Black,5)],[(Black,2),(Black,1)],[(Black,2),(Black,2)],[(Black,1),(Black,2)]]
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1 回答 1

1

将您自己的代码片段和 Fyodor 的评论粘合在一起：

Prelude> data Field = Empty | Black | Yellow deriving (Eq, Ord, Enum, Show, Read)
Prelude> data BoardEdge = BoardEdge { colRow :: [[(Field, Int)]]} deriving (Read, Eq, Ord, Show)
Prelude> let edge1 = BoardEdge [[(Black,2),(Yellow,2),(Black,1)],[(Black,2),(Yellow,1),(Black,3)]]
Prelude> show edge1
"BoardEdge {colRow = [[(Black,2),(Yellow,2),(Black,1)],[(Black,2),(Yellow,1),(Black,3)]]}"
Prelude> let correctInput = it -- ^^ the above

所以现在我们知道read期望什么，show产生什么。这与您用作输入的内容相比如何？

Prelude> let myInput =     "[[(Black,2),(Yellow,2),(Black,1)],[(Black,2),(Yellow,1),(Black,3)]]"
Prelude> correctInput == myInput
False

Or less snarky: If you want default read instances to parse your input then your input must be correct Haskell code with data constructors and all. In this case using BoardEdge was needed.

于 2019-05-16T18:19:33.657 回答