类型与Data.PSQueue不匹配 [英] Type mismatch with Data.PSQueue

问题描述

最终，我想实现一个简单的内存消息队列，它将消息存储为密钥元组，例如{Dst-IP，Dst-Port {CreationTime，MessageList}}特定目标IP地址和目标端口的所有未来消息应附加到MessageList中。

关于Data.PSQueue的调查（或者Data.Map）。在上面的Dst-IP示例中，Dst-Port可能是关键，而CreationTime可能是重点。 （我不知道实际的MessageList可能是什么，但不知何故，我必须开始看到）。

从PSQ开始，我甚至没有超过最初的障碍类型。根据我如何修改下面的代码片段，我得到了各种类似的错误（无论是查找函数还是打印函数），都与

 无法匹配预期类型`IO t0'
，其实际类型为`PSQ.PSQ k0 p0  - >也许p0'
  

或类似。我怎样才能克服这个最初的问题？
有什么比Data.PSQueue更适合我的需求吗？

  { - ＃LANGUAGE OverloadedStrings＃ - } 
 
导入Control.Monad 
导入Control.Monad.State.Strict 
导入System.CPUTime 
 
导入限定的Data.PSQueue作为PSQ 
 --import Language.Haskell.Pretty 
 
 main = do 
 time<  -  getCPUTime 
 let q = PSQ.singletonmsgtime 
r<  -  PSQ .lookupmsg
 print（r）
  

解决方案

你已经写了 r < - PSQ.lookupmsg。 < - 是从注释块中提取一次值的语法。您应该使用 let r = ... ，这是绑定纯数值的语法。

您也忘记了队列参数本身。这就是错误信息告诉你的：< - 的右边必须是 IO a 对于某些 a ，但是它是一个从 PSQ kp 到查找结果（ Maybe p ）。

在这两个修正后，修正的行是 let r = PSQ.lookupmsgq 。

也许你想要的是 State monad 作为状态的 PSQ 例如 StateT PSQ IO 。这可以让你重写你的代码片段，如下所示：

  main :: IO（）
 main = flip runStateT PSQ。 empty $ do 
 time<  -  liftIO getCPUTime 
 modify $ PSQ.insertmsgtime 
r<  -  gets $ PSQ.lookupmsg
 liftIO。打印$ r 
  

如果您打算编写并发程序，最好的解决方案可能是 MVar 或包含 PSQ 的TVAR 。

您可能也对 fingertree-psqueue a>包，基于手指树的优先搜索队列的实现。我没有使用它或您正在考虑的 PSQueue 软件包，但它似乎更多积极维护。

Eventually I want to achieve a simple in memory message queue that stores messages as key-tuples e.g. {Dst-IP, Dst-Port {CreationTime, MessageList}} where all future messages for a specific Destinations IP address and Destination Port should be appended to MessageList.

I thought about invetigating Data.PSQueue (or maybe Data.Map). In the example above the Dst-IP, Dst-Port could be the key whilst CreationTime could be the priority. (I have no idea yet what the actual MessageList could be, but somehow I have to start and see).

Starting with PSQs I do not even get past the initial hurdle of types. Depending on how I modify the below snippet, I get various errors (either with the lookup function or with the print function) that all resemble

Couldn't match expected type `IO t0'
                with actual type `PSQ.PSQ k0 p0 -> Maybe p0'

or similar. How can I get past this initial problem? Is there anything that fits my requirements better than Data.PSQueue?

{-# LANGUAGE OverloadedStrings #-}

import Control.Monad
import Control.Monad.State.Strict
import System.CPUTime

import qualified Data.PSQueue as PSQ
--import Language.Haskell.Pretty

main = do
     time <- getCPUTime
     let q = PSQ.singleton "msg" time
     r <- PSQ.lookup "msg"
     print (r)

解决方案

You've written r <- PSQ.lookup "msg". <- is the syntax to extract a monadic value from within a block of do-notation. You should instead use let r = ..., the syntax to bind pure values.

You also forgot the queue parameter itself. This is what the error message is telling you: the right-hand side of <- has to be of type IO a for some a, but instead it's a function from a PSQ k p to the result of a lookup (Maybe p).

After those two fixes, the corrected line is let r = PSQ.lookup "msg" q.

Perhaps what you instead want is a State monad with a PSQ as the state; e.g. StateT PSQ IO. This would let you rewrite your snippet as follows:

main :: IO ()
main = flip runStateT PSQ.empty $ do
  time <- liftIO getCPUTime
  modify $ PSQ.insert "msg" time
  r <- gets $ PSQ.lookup "msg"
  liftIO . print $ r

If you're intending to write a concurrent program, the best solution is probably an MVar or TVar containing a PSQ instead.

You might also be interested in the fingertree-psqueue package, an implementation of priority search queues based on finger trees. I haven't used it or the PSQueue package you're considering, but it seems to be more actively maintained.

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