String haskell-需要整数的字符串

我有以下代码

data Ops = Sum Integer | Div Integer | None deriving (Read)

main = do
    ans <- getLine
print $ case read ans of
    Sum n -> sum n
    Div n -> div n

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我知道我不能在这里插入字符串，因此如何实现错误条件

更好的选择是使用readMaybe from Text。阅读：

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这使您可以将错误处理与处理输入的方式分开，将其提取到一个纯函数中，远离IO

更好的选择是使用readMaybe from Text。阅读：

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这使您可以将错误处理与处理输入的方式分开，如果无法使用类似于模块的文本，则将错误处理与输入分离成一个纯函数。这里阅读的是如何定义readMaybe，仅从Prelude函数的角度：

data Foo = Sum Int Int | Div Int Int deriving (Show,Read)

readMaybe str =
  case reads str of
    [] -> Nothing
    ((a,_):_) -> Just a

main = do
  str <- getLine
  case readMaybe str of
    Nothing         -> putStrLn "bad input"
    Just (Sum a b)  -> print $ a + b
    Just (Div a b)  -> print $ div a b

---

如果您不能使用像文本这样的模块，请阅读这里的内容，了解如何定义readMaybe，只使用前奏功能：

data Foo = Sum Int Int | Div Int Int deriving (Show,Read)

readMaybe str =
  case reads str of
    [] -> Nothing
    ((a,_):_) -> Just a

main = do
  str <- getLine
  case readMaybe str of
    Nothing         -> putStrLn "bad input"
    Just (Sum a b)  -> print $ a + b
    Just (Div a b)  -> print $ div a b

data Foo = Sum Int Int | Div Int Int deriving (Show,Read)

readMaybe str =
  case reads str of
    [] -> Nothing
    ((a,_):_) -> Just a

main = do
  str <- getLine
  case readMaybe str of
    Nothing         -> putStrLn "bad input"
    Just (Sum a b)  -> print $ a + b
    Just (Div a b)  -> print $ div a b