Haskell中的管道和回調

Question

我正在使用portaudio處理一些音頻。 只要有要處理的音頻數據，haskell FFI綁定就會調用用戶定義的回調。 應該非常快速地處理此回調，理想情況下不需要I / O. 我想保存音頻輸入並快速返回，因為我的應用程序不需要實時響應音頻（現在我只是將音頻數據保存到文件中;稍后我將構建一個簡單的語音識別系統） 。

我喜歡pipes的想法，並認為我可以使用該庫。 問題是我不知道如何創建一個返回通過回調傳入的數據的Producer 。

我該如何處理我的用例？

---

以下是我現在正在使用的內容，如果有幫助的話（基准mvar現在不能正常工作，但我不喜歡將所有數據存儲在seq中...我寧願處理它而不是它來代替剛剛結束）：

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}

module Main where

import Codec.Wav

import Sound.PortAudio
import Sound.PortAudio.Base
import Sound.PortAudio.Buffer

import Foreign.Ptr
import Foreign.ForeignPtr
import Foreign.C.Types
import Foreign.Storable

import qualified Data.StorableVector as SV
import qualified Data.StorableVector.Base as SVB

import Control.Exception.Base (evaluate)

import Data.Int
import Data.Sequence as Seq

import Control.Concurrent

instance Buffer SV.Vector a where
  fromForeignPtr fp = return . SVB.fromForeignPtr fp
  toForeignPtr = return . (\(a, b, c) -> (a, c)) . SVB.toForeignPtr

-- | Wrap a buffer callback into the generic stream callback type.
buffCBtoRawCB' :: (StreamFormat input, StreamFormat output, Buffer a input, Buffer b output) =>
    BuffStreamCallback input output a b -> StreamCallback input output    
buffCBtoRawCB' func = \a b c d e -> do
    fpA <- newForeignPtr_ d -- We will not free, as callback system will do that for us   
    fpB <- newForeignPtr_ e -- We will not free, as callback system will do that for us
    storeInp <- fromForeignPtr fpA (fromIntegral $ 1 * c)
    storeOut <- fromForeignPtr fpB (fromIntegral $ 0 * c)
    func a b c storeInp storeOut

callback :: MVar (Seq.Seq [Int32]) -> PaStreamCallbackTimeInfo -> [StreamCallbackFlag] -> CULong 
            -> SV.Vector Int32 -> SV.Vector Int32 -> IO StreamResult
callback seqmvar = \timeinfo flags numsamples input output -> do
  putStrLn $ "timeinfo: " ++ show timeinfo ++ "; flags are " ++ show flags ++ " in callback with " ++ show numsamples ++ " samples."  
  print input
  -- write data to output
  --mapM_ (uncurry $ pokeElemOff output) $ zip (map fromIntegral [0..(numsamples-1)]) datum
  --print "wrote data"

  input' <- evaluate $ SV.unpack input  
  modifyMVar_ seqmvar (\s -> return $ s Seq.|> input')

  case flags of
    [] -> return $ if unPaTime (outputBufferDacTime timeinfo) > 0.2 then Complete else Continue
    _ -> return Complete

done doneMVar = do
  putStrLn "total done dood!"
  putMVar doneMVar True
  return ()

main = do

  let samplerate = 16000

  Nothing <- initialize

  print "initialized"

  m <- newEmptyMVar
  datum <- newMVar Seq.empty

  Right s <- openDefaultStream 1 0 samplerate Nothing (Just $ buffCBtoRawCB' (callback datum)) (Just $ done m)
  startStream s

  _ <- takeMVar m -- wait until our callbacks decide they are done!
  Nothing <- terminate

  print "let's see what we've recorded..."

  stuff <- takeMVar datum
  print stuff

  -- write out wav file

  -- let datum = 
  --       audio = Audio { sampleRate = samplerate
  --                   , channelNumber = 1
  --                   , sampleData = datum
  --                   }
  -- exportFile "foo.wav" audio

  print "main done"

Answer 1

最簡單的解決方案是使用MVar在回調和Producer之間進行通信。 這是如何做：

import Control.Proxy
import Control.Concurrent.MVar

fromMVar :: (Proxy p) => MVar (Maybe a) -> () -> Producer p a IO ()
fromMVar mvar () = runIdentityP loop where
    loop = do
        ma <- lift $ takeMVar mvar
        case ma of
            Nothing -> return ()
            Just a  -> do
                respond a
                loop

您的流回調將寫入Just input到MVar ，您的終結回調將寫入Nothing以終止Producer 。

這是一個ghci示例，演示它是如何工作的：

>>> mvar <- newEmptyMVar :: IO (MVar (Maybe Int))
>>> forkIO $ runProxy $ fromMVar mvar >-> printD
>>> putMVar mvar (Just 1)
1
>>> putMVar mvar (Just 2)
2
>>> putMVar mvar Nothing
>>> putMVar mvar (Just 3)
>>>

編輯： pipes-concurrency庫現在提供此功能，它甚至在教程中有一節專門解釋如何使用它來從回調中獲取數據。