Haskell中的Either和Except在用法上有什么区别？ [英] What are the difference in usage between Either and Except in Haskell?

问题描述

我有一个可以从Haskell中的多个参数创建的类，该类需要对这些参数进行一些复杂的验证。当前我有类似的东西

I have a class that can be created from several arguments in Haskell which requires some complex validation of those arguments. Currently I have something like

makeAThingExcept :: String -> String -> ... String -> Except ThingError AThing
makeAThingExcept s1 s2 ... = do
    unless (s1CheckPasses s1) (throwError (BadS1 s1))
    ...

data ThingError = BadS1 String ...

instance Show ThingError where
        show (BadS1 s) = "Bad S1: " ++ s

makeAThing :: String -> String -> ... String -> AThing
makeAThing s1 s2 ... = case runExcept (makeAThingExcept s1 s2 ...) of
        Right thing  -> thing
        Left err -> error (show err)

通过使用更具体的类型，是否还有更好的方法比 String 作为 makeAThingExcept 的参数，是否有理由更喜欢 Except 在以上的情况下？ 除与两者 c之外的功能和习语之间有什么区别？

Setting aside whether there is a better way to do this by using more specific types than String as arguments to makeAThingExcept, is there a reason to prefer Except over Either in a case like this? What are the differences here between the capabilities and idiom of Except vs Either?

推荐答案

如注释中所述，很容易在除与& 要么。即使在运行时表示形式也是一样。

As noted in the comments, it's easy to convert between Except & Either. The runtime representation is the same, even.

我总是选择使用 Ething 。它在图书馆中无处不在。我很少看到例外。

I would always choose to use Either. It's ubiquitous in libraries. I very rarely see Except.

除了是一个 ExceptT的特例，您将在库中看到 。如果您发现自己使用 Reader SomeType（任意一个）或 IO（任意一个）或 Monad m => m（或者e a），那么您可能需要考虑 ExceptT 。最好不要担心，直到 都容易使用，直到不再使用。

Except is a special case of ExceptT, which you will see in libraries. If you find yourself writing a lot of functions with Reader SomeType (Either e a) or IO (Either e a) or Monad m => m (Either e a), then you might want to consider ExceptT. It's fine not to worry about it until then - Either is easier to use until it isn't.

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