How can unsafePerformIO be used to write unsafeCoerce?

Question

It's widely understood that unsafePerformIO is not type safe. This is typically demonstrated by using it to implement unsafeCoerce:

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

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

As I showed a few years ago, this implementation is not thread-safe. One thread could write to the box, and then another thread could write to the box again before the first thread can read. Oops! How can this be fixed?

dfeuer · Answer 1 · 2021-07-13T18:31:19.693

8

As I showed once upon a time, the right way to do this is to use coercion through an IORef to produce the unsafeCoerce function itself, rather than to produce individual results of its application.

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 #-}

edited Jul 13 '21 at 18:31

answered Jul 09 '21 at 20:57

dfeuer

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4

I confess I chuckled at "the right way": we are creating an abomination, but hey, we should be careful not to do that in the wrong way! ;-) – chi Jul 09 '21 at 21:28
1

I had trouble understanding "to coerce the coercion function" might it be better to write "to coerce the identity function" or "to coerce the identity function into a coercion function"? – Noughtmare Jul 09 '21 at 21:37
2

@chi, yes, it is a bit silly, but I think fun. – dfeuer Jul 09 '21 at 21:37
@Noughtmare, is it better now? – dfeuer Jul 09 '21 at 21:39
1

Yes, I think it is a bit better now. – Noughtmare Jul 09 '21 at 21:48

How can unsafePerformIO be used to write unsafeCoerce?

1 Answers1