haskell - unsafePerformIO怎么能用来写unsafeCoerce呢？

Question

人们普遍认为这unsafePerformIO不是类型安全的。这通常通过使用它来实现来演示unsafeCoerce：

box :: IORef a
box = unsafePerformIO (newIORef undefined)
{-# NOINLINE box #-}

unsafeCoerce :: a -> b
unsafeCoerce a = unsafePerformIO $
  writeIORef box a >> readIORef box

正如我几年前所展示的，这种实现不是线程安全的。一个线程可以写入盒子，然后另一个线程可以在第一个线程可以读取之前再次写入盒子。哎呀！如何解决这个问题？

Answer 1

正如我曾经展示过的那样，正确的做法是通过 an 使用强制IORef来产生unsafeCoerce函数本身，而不是产生其应用的个别结果。

box :: IORef x
box = unsafePerformIO (newIORef undefined)
-- This NOINLINE is essential. If this binding is inlined,
-- then unsafeCoerce = undefined.
{-# NOINLINE box #-}

unsafeCoerce :: a -> b
unsafeCoerce = unsafePerformIO $
  writeIORef box id >> readIORef box

-- Inlining this wouldn't break anything,
-- but it'd waste time with unnecessary IORef operations.
{-# NOINLINE unsafeCoerce #-}