Haskell monad 和不需要字符串的失败
我有以下 monad 转换器来处理 Haskell 中的错误。
instance (Monad m, Error e) => Monad (EitherT e m) where
return = EitherT . return . return
m >>= k = EitherT $ do
a <- runEitherT m
case a of
Left l -> return (Left l)
Right r -> runEitherT (k r)
fail = EitherT . return . Left . strMsg
它工作得相当好，因为我可以使用自定义类实例化 Error ，并且有一种非常灵活的方法来处理错误。
不过， fail 有点傻，因为它是 String -> 类型。无论是 T e m，还是 String 限制都可能是一种产生错误的恼人方式。我最终得到了很多：
instance Error BazError where
strMsg "foo" = FooError -- oh look we have no error context
strMsg "bar" = BarError -- isn't that nice
我想做的是创建一个新函数，例如 fail，其类型为 a ->; e 以便我可以删除 (Error e) 限制。当 monad 堆栈变大时，fail 特别方便，就像当我最终遇到
EitherT BazError (StateT [BazWarning] IO) Foo
Is有办法创建一个与 fail 具有相同行为但限制较少的函数 时类型？或者 fail 是使用深度 haskell 黑魔法实现的吗？
I have the following monad transformer for dealing with errors in Haskell.
instance (Monad m, Error e) => Monad (EitherT e m) where
return = EitherT . return . return
m >>= k = EitherT $ do
a <- runEitherT m
case a of
Left l -> return (Left l)
Right r -> runEitherT (k r)
fail = EitherT . return . Left . strMsg
It works fairly well, as I can instantiate Error with a custom class and have a pretty flexible means by which to handle errors.
fail is a bit silly, though, because it is type String -> EitherT e m, and the String restriction can be an annoying way to create errors. I end up with a whole lot of:
instance Error BazError where
strMsg "foo" = FooError -- oh look we have no error context
strMsg "bar" = BarError -- isn't that nice
What I'd like to do is create a new function, like fail, that is of type a -> e so that I can remove the (Error e) restriction. fail is especially convenient when the monad stack gets large, like when I end up with
EitherT BazError (StateT [BazWarning] IO) Foo
Is there a way to create a function that has the same behavior as fail with a less restrictive type? Or is fail implemented using deep haskell dark magic?
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好吧，如果 do 块中出现模式匹配失败，则调用
fail，例如如果您有Just x <- Something和something> 的结果是Nothing。除此之外，fail是一个普通函数。对于
strMsg "foo" = FooError等问题，throwError是否为您的用例提供了更好的界面？Well,
failis called if you have a pattern-match failure in a do block, like if you haveJust x <- somethingandsomething's result isNothing. Apart from that,failis an ordinary function.For the problem with
strMsg "foo" = FooErroretc, doesthrowErroroffer a nicer interface for your use case?本文可能有用：http: //blog.ezyang.com/2011/08/8-ways-to-report-errors-in-haskell-revisited/
你的 EitherT 已经在标准库中并被调用错误T。请参阅文档： http ://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Monad-Error.html#t:ErrorT
fail现在是一个历史好奇心，并可考虑设计缺陷，以及缺少 Functor a => Monad 是一个约束。在do表示法中处理失败的模式匹配只是一个有争议的功能。是
fail最常见的替代品，但还有更多可用的替代品（请参阅文章）。This article may be useful: http://blog.ezyang.com/2011/08/8-ways-to-report-errors-in-haskell-revisited/
Your EitherT is already in the standard library and called
ErrorT. See the documentation: http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Monad-Error.html#t:ErrorTfailis a historical curiosity now, and may considered a design flaw, along with lack ofFunctor a => Monad aconstraint. It is only a controversial feature to handle failed pattern matches indonotation.is the most common replacement for
fail, but more are available (see the article).