“All about monads”中的MonadError部分 [英] MonadError section in "All about monads"

问题描述

它声称Error monad的定义为

  class（Monad m）=> Monaderror e m | m  - > e其中
 throwError :: e  - > m a 
 catchError :: m a  - > （e  - > m a） - > ma 
  

其中一个实例是Either e。

  instance MonadError（或e）其中
 throwError =左
（左e）`catchError`处理程序=处理程序e 
a`catchError` _ = a 
  

这是我不明白的地方。
MonadError类带有两个类型参数，（或e）取1，这个
实例化是如何工作的？这是因为函数依赖关系吗？我仍然不明白。

AND！我在GHCi中修改了这段代码（带-XFunctionalDependencies，-XMultiParamTypeClasses）没有编译！什么是这段代码，无论如何？

解决方案

这只是一个错字，实例应该是

  instance MonadError e（e）其中
 throwError = Left 
（Left e）`catchError`处理程序=处理程序e 
a`catchError` _ = a 
  

包含两个类型参数，如您所愿。

e 是monad， e 是相应的错误类型。

I'm now really confused about the Error monad in which "All about monads" describes.

It claims the definition of Error monad as

class (Monad m) => Monaderror e m | m -> e where
  throwError :: e -> m a
  catchError :: m a -> (e -> m a) -> m a

And one of the instance is Either e.

instance MonadError (Either e) where
  throwError = Left
  (Left e) `catchError` handler = handler e
  a        `catchError` _       = a

Here is what I don't understand. The MonadError class take two type parameters, and (Either e) takes one, how is this instantiation work? Is this because the functional dependencies? I still don't get it.

AND! I've runed this code in GHCi(with -XFunctionalDependencies, -XMultiParamTypeClasses) didn't compile! What's is this piece of code, anyway?

解决方案

It's just a typo, the instance should be

instance MonadError e (Either e) where
  throwError = Left
  (Left e) `catchError` handler = handler e
  a        `catchError` _       = a

with two type parameters, as you expected.

Either e is the monad, and e is the corresponding error type.

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