Question

我有一个小程序，它读入文件并将数据处理成自定义数据类型。正在读取的文件包含如下所示的数据行：

cat    10    20    dog
hamster    12    2    wombat
monkey    1    9    zebra
lion    30    60    rhino
...

我处理文件的程序如下所示：

main :: IO ()
    main = do
    contents <- readFile "myfile.xyz"
    let process = clean contents
    let f = processFile process
    print f

clean :: String -> [[String]]
clean x = Prelude.map words $ lines x

processFile :: [[String]] -> [(XyzData)]
processFile [[a,b,c,d]] = [(XyzData a (read b :: Int) (read c :: Int) d)]
processFile (x:xs) = processFile xs 

data XyzData = XyzData { animal1 :: String, 
                         number1 :: Int,
                         number2 :: Int,
                         animal2 :: String
                       } deriving (Show)

我的问题在于processFile功能。目前，此功能仅捕获文件的最后一行并将其打印到屏幕上。我对如何使用元组实现递归而不是在此函数中使用带有列表的append感到困惑。谁能告诉我如何修复我的功能和/或更好地实现这个功能？该程序的打印输出应为：

[XyzData {animal1 = "cat", number1 = 10, number2 = 20, animal2 = "dog"},
[XyzData {animal1 = "hampster", number1 = 12, number2 = 2, animal2 = "wombat"},
[XyzData {animal1 = "monkey", number1 = 1, number2 = 9, animal2 = "zebra"},
[XyzData {animal1 = "lion", number1 = 30, number2 = 60, animal2 = "rhino"}]

感谢您的时间。

Answer 1

你可能打算

processFile :: [[String]] -> [(XyzData)]
processFile ([a,b,c,d]:xs) = (XyzData a (read b :: Int) (read c :: Int) d) : processFile xs
processFile (x:xs) = processFile xs 
processFile [] = []

您的第一个模式[[a,b,c,d]]仅匹配包含一个元素的列表，这是一个包含四个元素的列表。

具有递归元组函数的Haskell难度

1 个答案: