我如何使用REPL和CPS功能？ [英] How can I use REPL with CPS function?

问题描述

我刚刚遇到 withSession ::（Session - > IO a） - > IO a wreq 包。我想逐行评估延续，但我无法找到任何方法。

  import Network.Wreq。 Session作为S 
 withSession $ \sess  - > 
 res<  -  S.getWith opts sesshttp://stackoverflow.com/questions
  -  print res 
  -  ..其他东西
  

在上面的代码片段中，如何评估ghci中的 print res ？换句话说，我可以在ghci中获得 Session 类型吗？

解决方案

精彩的问题。

我知道没有方法可以重新输入GHCi REPL，因此我们可以在CPS功能中使用它。也许其他人可以提出一些建议。

然而，我可以提出一个黑客攻击。基本上，如果是基于IO monad，就可以利用并发性将CPS从内部转出来。

下面是黑客攻击：在GHCi会话中使用它

 > sess<  -  newEmptyMVar :: IO（MVar Session）
>停止<  -  newEmptyMVar :: IO（MVar（））
> forkIO $ withSession $ \s  - > putMVar sess s>> takeMVar stop 
> s<  -  takeMVar sess 
> - 在这里使用，就好像你在里面withSession 
>让s =（） - 建议
> putMVar stop（）
> - 我们现在在Session的外面，不要试图在这里访问！ 
  

一个自动化黑客攻击的小型库：

< pre $ data CPSControl b = CPSControl（MVar（））（MVar b）

startDebugCps ::（（a - > IO（）） - > IO b） - > IO（a，CPSControl b）
startDebugCps cps = do
cpsVal< - newEmptyMVar
retVal< - newEmptyMVar
stop< - newEmptyMVar
_< - forkIO $ do
x< - cps $ \c - > putMVar cpsVal c>> takeMVar stop
putMVar retVal x
s < - takeMVar cpsVal
return（s，CPSControl stop retVal）

stopDebugCps :: CPSControl b - > IO b
stopDebugCps（CPSControl stop retVal）= do
putMVar stop（）
takeMVar retVal

testCps ::（String - > IO（）） - > ; IO String
testCps act = do
putStrLntestCps：begin
act这里有一些字符串！
putStrLntestCps：end
返回一些返回值


A快速测试：

 > （x，ctrl）<  -  startDebugCps testCps 
 testCps：begin 
> x 
这是一些字符串！ 
> stopDebugCps ctrl 
 testCps：end 
一些返回值
  

I've just encountered withSession :: (Session -> IO a) -> IO a of wreq package. I want to evaluate the continuation line by line, but I can't find any way for this.

import Network.Wreq.Session as S
withSession $ \sess -> do
  res <- S.getWith opts sess "http://stackoverflow.com/questions"
  -- print res
  -- .. other things

In above snippet how can I evaluate print res in ghci? In other words, can I get Session type in ghci?

解决方案

Wonderful question.

I am aware of no methods that can re-enter the GHCi REPL, so that we can use that in CPS functions. Perhaps others can suggest some way.

However, I can suggest an hack. Basically, one can exploit concurrency to turn CPS inside out, if it is based on the IO monad as in this case.

Here's the hack: use this in a GHCi session

> sess <- newEmptyMVar :: IO (MVar Session)
> stop <- newEmptyMVar :: IO (MVar ())
> forkIO $ withSession $ \s -> putMVar sess s >> takeMVar stop
> s <- takeMVar sess
> -- use s here as if you were inside withSession
> let s = () -- recommended
> putMVar stop ()
> -- we are now "outside" withSession, don't try to access s here!

A small library to automatize the hack:

data CPSControl b = CPSControl (MVar ()) (MVar b)

startDebugCps :: ((a -> IO ()) -> IO b) -> IO (a, CPSControl b)
startDebugCps cps = do
   cpsVal <- newEmptyMVar
   retVal <- newEmptyMVar
   stop   <- newEmptyMVar
   _ <- forkIO $ do
      x <- cps $ \c -> putMVar cpsVal c >> takeMVar stop
      putMVar retVal x
   s <- takeMVar cpsVal
   return (s, CPSControl stop retVal)

stopDebugCps :: CPSControl b -> IO b
stopDebugCps (CPSControl stop retVal) = do
   putMVar stop ()
   takeMVar retVal

testCps :: (String -> IO ()) -> IO String
testCps act = do
   putStrLn "testCps: begin"
   act "here's some string!"
   putStrLn "testCps: end"
   return "some return value"

A quick test:

> (x, ctrl) <- startDebugCps testCps
testCps: begin
> x
"here's some string!"
> stopDebugCps ctrl
testCps: end
"some return value"

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