“unsafePerformIO (newTVarIO 0)”有多安全？
uniqSource :: TVar Integer
uniqSource = unsafePerformIO (newTVarIO 0)
{-# NOINLINE uniqSource #-}
是否保证只运行一次？
I've noticed this idiom in Data.Unique:
uniqSource :: TVar Integer
uniqSource = unsafePerformIO (newTVarIO 0)
{-# NOINLINE uniqSource #-}
Is it guaranteed to only run once?
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在 GHC 中，是的。1 请参阅 文档了解更多信息；有一个变体
unsafeDupablePerformIO可以多次执行，以避免专门用于实现此保证的开销。请注意，创建可变变量的
unsafePerformIO通常并不安全；如文档中所述，您可以创建多态引用并使用它来实现unsafeCoerce。不过，这不是您可能会意外执行的操作，并且它不适用于相关代码（因为引用的类型是明确指定的）。safe-globals 包抽象了这个“习惯用法”（虽然在某些情况下很有用，但它是通常被认为是反模式，并且不应该用于正常代码）以确保安全的方式。
另请参阅我的之前的答案< /a>
unsafePerformIO以及应用它时必须注意的事项。1 我很确定它也适用于所有其他实现； GHC 为避免重复执行而采取的特殊措施仅在线程设置中才必要，而且我不知道 Haskell 的任何其他线程实现。不过，GHC 是人们现在真正使用的唯一实现......
In GHC, yes.1 See the documentation for more information; there is a variant
unsafeDupablePerformIOthat can be executed multiple times that avoids the overhead dedicated to achieving this guarantee.Note that
unsafePerformIOto create mutable variables isn't safe in general; as described in the documentation, you can create a polymorphic reference and use it to implementunsafeCoerce. That's not something you're likely to do accidentally, though, and it doesn't apply to the code in question (since the type of the reference is specified explicitly).The safe-globals package abstracts this "idiom" (while useful in some cases, it's generally considered an antipattern, and should not be used in normal code) in a way that ensures safety.
See also my previous answer on
unsafePerformIOand the caution that must be used when applying it.1 I'm pretty sure it applies to all other implementations, too; the special care GHC takes to avoid repeated execution is only necessary in a threaded setting, and I don't know of any other threaded implementations of Haskell. GHC is the only implementation people really use these days, though...