Malcolm Wallace wrote (about PVar's, my proposed interface for useful mutable state in such a way that they can be implemented in NHC, GHC and Hugs): [snip]
I'll need to think some more about the details of PVars, but my first reaction is that they would very easy to implement. However, I fear it may be no good persuading me of their value if you cannot also persuade the other implementations to adopt them, so I'd like to hear what Hugs and ghc people have to say, especially whether they would also solve the problems with mutable shared state raised by Alastair. [snip] I don't see that it's necessary for us to come to a decision right now about PVars unless we want to put them in the FFI standard. Well we could do that, in the same way as we have unsafePerformIO, if it's what it takes us to see the back of this finalizers discussion, but I don't think it's particularly necessary; although I think it might be a good idea to have a standard definition of something like PVars so that people can do useful manipulations of mutable state in finalizers, such things don't really seem to me to belong in an FFI standard.
What's important now is that I think I've proved the point that there is an interface to mutable state which can be manipulated usefully and safely from Haskell finalizers in all of Hugs, GHC and NHC. Also, in view of Simon Marlow's patch and what Malcolm Wallace has said, it looks as if Haskell finalizers are at least implementable by all three. Therefore I see very little left to stop them going back into the FFI standard. I'm about to go on holiday for several days, so I look forward to coming back to find the whole issue safely buried in the FFI archives. 8-)