Question

Haskell新手在这里。我正在尝试使用http-enumerator通过HTTP上的XML查询服务。我能够连接并发送xml格式的请求，并接收xml格式的响应。

查询成功后，服务器会发回一个响应

我正在努力解决的是处理FAULT文档中指出的异常的正确方法。我正在尝试使用Either，但没有成功。

我在下面的内容中编译ghci：

import Network.HTTP.Enumerator 
import Network.HTTP.Types

import qualified Data.ByteString.Lazy as L 
import Data.ByteString.UTF8

import Text.XML.Light

hostname = "https://server..."

doPost username password token = do

    let q = QName "SYSTEM" Nothing Nothing

    let attribs = [Attr {attrKey = QName "user" Nothing Nothing, attrVal = username},
                   Attr {attrKey = QName "password" Nothing Nothing, attrVal = password},
                   Attr {attrKey = QName "token" Nothing Nothing, attrVal = token}]

    let doc = Element {elName=q, elAttribs=attribs, elContent= [], elLine=Nothing}

    req0 <- parseUrl hostname 

    let req = req0 { method = methodPost 
                  , requestHeaders = [("Content-Type", "text/xml")]   
                  , requestBody = RequestBodyBS $ fromString $ showTopElement doc
                  } 

    res <- withManager $ httpLbs req 

    let status = Network.HTTP.Enumerator.statusCode res
    let content = responseBody res

    -- this is where I would check for different fault codes using a case statement
    if content == "<FAULT/>"
        then Left "error"
        else Right content

然而，当我尝试在ghci中运行它时，我得到以下内容：

*Main> doPost "user" "password" ""

<interactive>:1:1:
    No instances for (Control.Failure.Failure
                        HttpException (Either a0),
                      Control.Monad.IO.Control.MonadControlIO (Either a0))
      arising from a use of `doPost'
    Possible fix:
      add instance declarations for
      (Control.Failure.Failure HttpException (Either a0),
       Control.Monad.IO.Control.MonadControlIO (Either a0))
    In the expression: doPost "user" "password" ""
    In an equation for `it': it = doPost "user" "password" ""

在这种情况下处理异常的最佳方法是什么？

提前致谢。尼尔

Answer 1

您需要在最后一个if之前加上“return $”。 parseUrl需要在monad中运行，monad是Failure HttpException的实例，例如IO或Maybe。 withManager需要一个MonadControlIO实例的monad，例如IO。

目前，您最后的if强制整个do - 块在Either String monad中运行，这就是您获得“无实例”异常的原因。如果您添加退货，则最终结果将类似于IO (Either String XML)。

如何使用http-enumerator处理异常

1 个答案: