haskell - 如何将此函数从 ExceptT 转换为 Haskell 中的 Except？

Question

我对 Except 很困惑，特别是因为网络上没有好的教程。我不知道如何将此函数从 ExceptT 转换为 Except:

data Error = Empty deriving (Show)

badFunction :: ExceptT Error IO ()
badFunction = throwError Empty

main :: IO ()
main = do
    caught_result <- runExceptT badFunction
    case caught_result of
      Left _ -> putStrLn "caught some error"
      Right _ -> putStrLn "no errors were caught"

Answer 1

badFunction 应该是 ExceptT Error IO () 的原因是因为 runExceptT 的类型为 runExceptT :: ExceptT e m a -> m (Either e a) .由于您的 main 类型为 main::IO ()，这意味着 runExceptT badFunction 需要为 IO …，因此 m ExceptT e m a中的应该是IO。

但你本身并不需要这个，你的 badFunction 不做任何 IO，所以你可以将它定义为一个 Except:

badFunction :: <b>Except</b> Error ()
badFunction = throwE Empty

然后可以在IO()中使用runIdentity从identity中获取值，然后使用pure将结果“包装”到 IO 中:

main :: IO ()
main = do
    caught_result <- pure (<b>runIdentity</b> (runExceptT badFunction))
    case caught_result of
      Left _ -> putStrLn "caught some error"
      Right _ -> putStrLn "no errors were caught"

但是我们可以在 main 中使用 let … 子句并删除 pure:

main :: IO ()
main = do
    let caught_result = <b>runIdentity</b> (runExceptT badFunction)
    case caught_result of
      Left _ -> putStrLn "caught some error"
      Right _ -> putStrLn "no errors were caught"

作为@JonPurdy says , runIdentity 和 runExceptT 的组合是 runExcept :: Except e a -> Either e a ，所以我们可以将其重写为:

main :: IO ()
main = do
    let caught_result = <b>runExcept</b> badFunction
    case caught_result of
      Left _ -> putStrLn "caught some error"
      Right _ -> putStrLn "no errors were caught"