“Prelude.read: no parse”，自己的数据类型

Question

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

“Main.hs: Prelude.read: 无解析”

我怀疑我在负责验证输入 (validateInput) 的函数上得到了这个。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。

谁能告诉我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) 

Main.hs

    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 中的验证函数导入限定为 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

而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)]]

Answer 1

将您自己的代码 sn-ps 和 Fyodor 的注释粘合在一起：

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，这就是show 产生的。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

或者不那么狡猾：如果您希望默认的 read 实例解析您的输入，那么您的输入必须是正确的带有数据构造函数的 Haskell 代码等等。在这种情况下，需要使用 BoardEdge。