尝试返回，而不是IO（前一个） [英] try is returning Either instead of IO (Either ex a)

问题描述

我正在尝试处理我的请求解析器中的异常：

  go bs = 
 case try $ parseRequest读者bs安全
左前 - > exceptionHandler作者ex 
右（请求，bs'） - > do 
 sendResponse writer =<<<应用程序请求
 go bs'
  

但是，当使用尝试：

 无法匹配预期类型'IO（E0（Request，ByteString） ）'
与实际类型'任一t0 t1'
在模式中：左ex 
在一种情况下：左ex  - > exceptionHandler作者ex 
在表达式中：
 case try $ parseRequest reader bs secure of {
 Left ex  - > exceptionHandler作者ex 
对（请求，bs'）
  - > do {sendResponse writer =<<应用请求; 
 go bs'}} 
  

IO（E0（请求，ByteString））正是我从尝试而得到的，因为它的类型是 try :: Exception e => ; IO a - > IO（Ea），而是我得到 ea 。

什么是我缺少？

解决方案

try 确实产生一个 IO（EEA）。您收到错误消息，因为您将 try 生成的值与 Left ex 具有类型



>要修复代码，您需要从 IO （使用<$ c $）中获取 c>>> = 或 < - do ），然后模式匹配。

I'm trying to handle exceptions from my request parser:

   go bs =                                                                 
       case try $ parseRequest reader bs secure of                             
         Left  ex             -> exceptionHandler writer ex                 
         Right (request, bs') -> do                                         
            sendResponse writer =<< app request                             
            go bs'                                                               

But I have an issue when using try:

Couldn't match expected type `IO (Either e0 (Request, ByteString))'
            with actual type `Either t0 t1'
In the pattern: Left ex
In a case alternative: Left ex -> exceptionHandler writer ex
In the expression:
  case try $ parseRequest reader bs secure of {
    Left ex -> exceptionHandler writer ex
    Right (request, bs')
      -> do { sendResponse writer =<< app request;
              go bs' } }

IO (Either e0 (Request, ByteString)) is exactly what I except to get from try because its type is try :: Exception e => IO a -> IO (Either e a), but instead I get Either e a.

What am I missing?

解决方案

try does produce an IO (Either e a). You're getting the error message because you're matching the value produced by try against the pattern Left ex, which has the type Either a b, not IO a.

To fix your code, you'd need to get the Either out of the IO (using >>= or <- inside do) and then pattern match against that.

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