如何将两个消费者合并成一个Haskell Pipes？ [英] How to merge two Consumer into one in Haskell Pipes?

问题描述

我使用Haskell流处理库管道编写命令行工具。每个命令行操作都可以将结果输出到 stdout ，并使用 pipes 记录到 stderr c $ c $> API。



我需要 Consumer ，其类型为 Consumer打印数据块（ Left ）至 stderr ，<$ c> $ c> Right 至 stdout ）与单个消费者。

我写的代码（应该改进）

这个函数 consumeEither doesn'

  consumeEither ::（MonadIO m）=>所以我想改进它。消费者（或者字符串字符串）m（）
 consumeEither = do 
或者S<  - 等待
 case $ s 
（Left l） - > for（yield l）（liftIO。（IO.hPutStrLn IO.stderr））
（Right r） - > for（yiled r）（liftIO。putStrLn）
  

此外，提供一个函数会很有用两个消费者 s并将它们合并为一个消费者。

问题

是否有人知道以下接口的良好示例或实现？

  merge ::（Monad m）=>消费者a m r  - >消费者b m r  - >消费者（或者ab）mr 
  

• 第一个参数为 stderr


• 第二个参数为 stdout

函数的用法

  import Pipes 
导入合格Pipes.Prelude作为P 
将合格System.IO导入为IO 
 $ b stdoutOrErr :: Consumer（或者String String）IO（）
 stdoutOrErr = merge（P.toHandle IO.stderr）P.stdoutLn 
  

谢谢

在 pipes-extras 。请记住消费者是管道（无处），所以 P.toHandle IO .stderr +++ P.stdoutLn :: MonadIO m => Pipe（String String）（或bd）m（）。

获取 Consumer ，你必须去掉 Lefts ，例如用> - > P.concat 或> - > P.drain 。用 Fold s来做到这一点的方法更加健壮和美观。

I use Haskell stream processing library pipes to write a command line tool. Each command line actions may output result to stdout and logs to stderr with pipes API.

I need Consumer which has type as Consumer (Either String String) m r to print chunk of data (Left to stderr, Right to stdout) with single Consumer.

Code I wrote (should be improved)

This function consumeEither doesn't have flexibility so I want to improve it.

consumeEither :: (MonadIO m) => Consumer (Either String String) m ()
consumeEither = do
  eitherS <- await
  case eitherS of
    (Left l)  -> for (yield l) (liftIO . (IO.hPutStrLn IO.stderr))
    (Right r) -> for (yiled r) (liftIO . putStrLn)

Furthermore it would be useful to provide a function which takes two Consumers and merge them into one Consumer.

Question

Does anybody know good example or implementation of the following interface?

merge :: (Monad m) => Consumer a m r -> Consumer b m r -> Consumer (Either a b) m r

• 1st argument as stderr
• 2nd argument as stdout

Usage of the function

import           Pipes
import qualified Pipes.Prelude as P
import qualified System.IO as IO

stdoutOrErr :: Consumer (Either String String) IO ()
stdoutOrErr = merge (P.toHandle IO.stderr) P.stdoutLn

Thanks

解决方案

(This is @Michael's answer, but I'd like to write it up here so we can move the question out of the unanswered queue for the Haskell tag.)

See (+++) in pipes-extras. Keep in mind a Consumer is a Pipe (to nowhere), so P.toHandle IO.stderr +++ P.stdoutLn :: MonadIO m => Pipe (Either String String) (Either b d) m ().

To get a Consumer, you would have to get rid of the Lefts e.g with >-> P.concat or >-> P.drain. There are more robust and handsome ways of doing this with Folds.

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