定义一个函数时不使用任何语言扩展，但必须使用语言扩展来声明其类型？
我定义了一个自定义错误类型，我想将其与错误单子一起使用。举个例子，这里有一个虚拟的：
newtype CustomError = CustomError String
instance Error CustomError where
strMsg = CustomError
我想定义一个像 throwError 一样工作的 throwCustomError 函数，但它不只是抛出给定的字符串，而是使用它创建一个 CustomError，然后抛出 that。这个定义有效：
-- | Throws a 'CustomError' containing the given error message.
throwCustomError msg = throwError $ CustomError msg
但是，我想添加一个类型声明，主要是为了让 Haddock 在生成的文档中包含函数的描述。如果我在 GHCI 中使用 :t ，它会告诉我这个函数的类型是 MonadError CustomError m =>字符串 -> m a，这对我来说很有意义，但是如果我写
throwCustomError :: MonadError CustomError m => String -> m a
GHC 会抱怨“非变量类型参数”，并告诉我必须使用 -XFlexibleContexts 来允许它。当我可以定义这种类型的函数而无需使用任何语言扩展时，为什么我必须使用语言扩展来声明这种类型的函数？有没有办法在不使用语言扩展的情况下声明该函数的类型？
另外，我最初尝试将函数定义为
throwCustomError = throwError . CustomError
但 GHC 告诉我“没有因使用 throwError 而产生的 (MonadError CustomError m0) 实例”。我不太明白为什么这个定义与另一个定义不等价；据我所知，它们的含义相同。
退一步：我是否应该定义这个函数？或者当我想抛出错误时，我应该只写 throwError $ strMsg "foo" 吗？ （目前我的代码中有 throwError "foo" ，所以我需要以任何方式更改这些行。）
I've defined a custom error type that I want to use with the Error monad. For the sake of example, here's a dummy one:
newtype CustomError = CustomError String
instance Error CustomError where
strMsg = CustomError
I'd like to define a throwCustomError function that works like throwError, but instead of just throwing the string it's given, it uses it to create a CustomError and then throws that. This definition works:
-- | Throws a 'CustomError' containing the given error message.
throwCustomError msg = throwError $ CustomError msg
However, I'd like to add a type declaration, mainly so that Haddock will include the function's description in the generated documentation. If I use :t in GHCI, it tells me that the type of this function is MonadError CustomError m => String -> m a, and that makes sense to me, but if I write
throwCustomError :: MonadError CustomError m => String -> m a
GHC complains about a "non-variable type argument" and tells me I have to use -XFlexibleContexts to permit it. Why do I have to use a language extension to declare a function of this type, when I can define a function of this type without using any language extensions? Is there a way to declare this function's type without using language extensions?
On a separate note, I originally tried defining the function as
throwCustomError = throwError . CustomError
but GHC tells me "No instance for (MonadError CustomError m0) arising from a use of throwError". I don't quite understand why this definition isn't equivalent to the other one; they both mean the same thing as far as I can see.
Taking a step back: should I even bother defining this function? Or should I just write throwError $ strMsg "foo" when I want to throw an error? (Currently I have throwError "foo" throughout my code, so I need to change those lines either way.)
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由于您使用的是
Errormonad，因此您已经依赖于扩展，尤其是MultiParamTypeClasses和FunctionalDependency，所以我不确定尝试保持代码无扩展是有意义的。有些扩展可能会产生一些潜在的令人讨厌的副作用（例如
IncoherentInstances），并且将这些扩展的使用保持在最低限度当然是有必要的，但是像FlexibleContexts这样的扩展是完全无害。Since you're using the
Errormonad, you're already relying on extensions, most notablyMultiParamTypeClassesandFunctionalDependencies, so I'm not sure that there is any point in trying to keep your code extension-free.There are some extensions which have some potentially nasty side-effects (e.g.
IncoherentInstances), and there's certainly a point to keeping usage of those to a minimum, but extensions likeFlexibleContextsare completely harmless.