我尝试解析一个文件。它是一个 3d 点列表。我编写了这个函数来提取一行的第一个和第二个数字。现在我想检查从 String 到 Float(read function) 的转换是否成功。如果转换成功，我想返回 Point(Just Point2D)，而在尝试失败时返回 Nothing。问题是这个案例想要一个 IO 但得到一个简单的。代码是从这篇文章中抽象出来的：Haskell 中的异常处理

-- | reads out of a splitted line the first and second of the list and returns a list of Points2D
    pointExtraction 
        :: [String] 
        -> Maybe Point2D
    pointExtraction (x:y:tail) = do 
        let result = try ( evaluate (createPoint x y )) :: IO (Either SomeException Point2D)
        case result of 
            Left ex -> Nothing        
            Right point -> (Just point)

createPoint :: String -> String -> Point2D
createPoint x y = Point2D (read x) (read y)

-- | reads out of a splitted line the first and second of the list and returns a list of Points2D
    pointExtraction 
        :: [String] 
        -> Maybe Point2D
    pointExtraction (x:y:tail) = do 
        let result = try ( evaluate (createPoint x y )) :: IO (Either SomeException Point2D)
        case result of 
            Left ex -> Nothing        
            Right point -> (Just point)

createPoint :: String -> String -> Point2D
createPoint x y = Point2D (read x) (read y)

例外：

回答

-- | reads out of a splitted line the first and second of the list and returns a list of Points2D
pointExtraction 
    :: [String] 
    -> IO (Maybe Point2D)
pointExtraction (x:y:tail) = do 
    result <- try ( evaluate (createPoint x y )) :: IO (Either SomeException Point2D)
    case result of 
        Left ex -> pure Nothing        
        Right point -> pure (Just point)

createPoint :: String -> String -> Point2D
createPoint x y = Point2D (read x) (read y)

import Text.Read (readMaybe)

-- | reads out of a splitted line the first and second of the list and returns a list of Points2D
pointExtraction 
    :: [String] 
    -> Maybe Point2D
pointExtraction (x:y:tail) = createPoint x y

createPoint :: String -> String -> Maybe Point2D
createPoint strX strY = do
  x <- readMaybe strX
  y <- readMaybe strY
  pure (Point2D x y)