全局变量通过Haskell中的unsafePerformIO [英] Global variables via unsafePerformIO in Haskell

问题描述

GHC API要求在调用之前进行一些初始化。具体而言， parseStaticFlags 只能调用一次。

我有可以调用 runGhc :: MaybeFilePath :: Ghc a - > IO a 多次运行一些GHC API方法。但是，一些初始化应该只在第一次调用该函数时发生。

我似乎记得从 Yi 源代码，可以创建一个全局变量，如

  ghcInitialised :: MVar（Bool，[String]）
 ghcInitialised = unsafePerformIO $ newMVar（False，[]）
  

runGhc 我们可以有

 （init，flags）<  -  readMVar ghcInitialised 
 when（not init）$ do 
 ... 
（_，_，staticFlagWarnings）<  -  parseStaticFlags ... 
 ... 
 putMVar ghcInitialised （True，staticFlagWarnings）
  

然而，我不记得它是如何完成的。此代码位于包装 GhcMonad 的monad的 runMonad 函数中。我清楚地知道，使用 unsafePerformIO 不是纯粹的或功能性的，但是（当时）这是获得实际结果的最佳方式。

<编辑：工作解决方案：

  { - ＃NOINLINE ghcInitialised＃ - } 
 ghcInitialised :: MVar（Bool，[String]）
 ghcInitialised = unsafePerformIO $ newMVar（False，[]）
  

，因此在调用 runGhc 的单调动作中，我们可以有

 （init，flags）<  -  takeMVar ghcInitialised 
 when（not init）$ do 
 ... 
（_，_，staticFlagWarnings）<  -  parseStaticFlags .. 。
 ... 
 putMVar ghcInitialised（True，staticFlagWarnings）
  

解决方案

请参阅此答案。它展示了如何使用一个全局计数器，每次你看它时都会'勾号'。你不需要一个计数器，而是 +1 ，你只需要把 True 加入它。

或者，更好的办法是将初始化代码放入 unsafePerformIO 中，（由 if 当然）。

The GHC API requires that some initialisation occurs before invocation. Specifically, parseStaticFlags can only be called once.

I have functions that can call runGhc :: MaybeFilePath :: Ghc a -> IO a multiple times to run some GHC API methods. However, some of that initialisation should only occur the first time that function is called.

I seem to remember from Yi source that it is possible to create a global variable something like

ghcInitialised :: MVar (Bool,[String])
ghcInitialised = unsafePerformIO $ newMVar (False,[])

so that in the monadic action that calls runGhc we can have

(init,flags) <- readMVar ghcInitialised
when (not init) $ do
   ...
   (_,_,staticFlagWarnings) <- parseStaticFlags ...
   ...
   putMVar ghcInitialised (True,staticFlagWarnings)

However, I can not recall exactly how it is done. This code is in the runMonad function for the monad that wraps a GhcMonad. I am well aware that using unsafePerformIO is not pure or functional, but (at the time) this was the best way of achieving a practical result.

[Edit: the working solution:

{-# NOINLINE ghcInitialised #-}
ghcInitialised :: MVar (Bool,[String])
ghcInitialised = unsafePerformIO $ newMVar (False,[])

so that in the monadic action that calls runGhc we can have

(init,flags) <- takeMVar ghcInitialised
when (not init) $ do
   ...
   (_,_,staticFlagWarnings) <- parseStaticFlags ...
   ...
   putMVar ghcInitialised (True,staticFlagWarnings)

解决方案

See this answer. It shows how to use a global counter that 'ticks' everytime you look at it. You don't need a counter, but instead of +1, you just put True into it.

Or, even better, you put the initialisation code into the unsafePerformIO, (guarded by an if of course).

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