Haskell 中通过 unsafePerformIO 的全局变量
GHC API 要求在调用之前进行一些初始化。具体来说，parseStaticFlags 只能调用一次。
我有可以调用 runGhc :: MaybeFilePath :: Ghc a -> 的函数多次 IO a 来运行某些 GHC API 方法。然而，某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 源中可以创建一个全局变量，
ghcInitialised :: MVar (Bool,[String])
ghcInitialised = unsafePerformIO $ newMVar (False,[])
这样在调用 runGhc 的单子操作中我们就可以拥有
(init,flags) <- readMVar ghcInitialised
when (not init) $ do
...
(_,_,staticFlagWarnings) <- parseStaticFlags ...
...
putMVar ghcInitialised (True,staticFlagWarnings)
但是，我不记得了到底是如何完成的。此代码位于包装 GhcMonad 的 monad 的 runMonad 函数中。我很清楚使用 unsafePerformIO 不是纯粹的或功能性的，但（在当时）这是实现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,[])
以便在调用 runGhc 的单子操作中，我们可以
(init,flags) <- takeMVar ghcInitialised
when (not init) $ do
...
(_,_,staticFlagWarnings) <- parseStaticFlags ...
...
putMVar ghcInitialised (True,staticFlagWarnings)
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)
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您需要关闭内联。
另一件重要的事情：类型必须是单态的（=没有类型变量），否则你可能会为每个实际类型评估 unsafePerformIO 。
You need to turn inlining off.
The other important thing: The type has to be monomorph (= no type variables), otherwise you might evaluate unsafePerformIO for each actual type.
请参阅此答案。它展示了如何使用每次查看时都会“滴答作响”的全局计数器。您不需要计数器，但只需将
True放入其中即可，而不是+1。或者，更好的是，您将初始化代码放入
unsafePerformIO中（当然由if保护）。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 putTrueinto it.Or, even better, you put the initialisation code into the
unsafePerformIO, (guarded by anifof course).