unsafePerformIO and CSE

[Geoffrey Irving]

I'd like to do something like this:

lastWrite = unsafePerformIO $ newRef 0
lastRead = unsafePerformIO $ newRef 0

to declare two global mutable variables for use in fixpoint iteration.
Am I correct that Haskell is within its rights to merge these into a
single reference? Is there some clean way of telling Haskell not to
do this?

In this case I can avoid the potential problem in a variety of ways
(changing one of the 0's to 1's, using newRef (0,0), etc.), but it'd
be nice to have a safe general solution.

Thanks,
Geoffrey

[Dylan Simon]

From Geoffrey Irving <irv...@naml.us>, Sun, Jun 12, 2011 at 09:46:11PM -0700:

You are correct. The standard idiom is:

lastWrite :: IORef Int
{-# NOINLINE lastWrite #-}

lastWrite = unsafePerformIO $ newRef 0

It has been suggested (though I haven't seen concrete evidence) that as of 7.0
this is insufficient and you also need:

{-# OPTIONS_GHC -fno-cse #-}

But of course impacts the whole module, and so people sometimes put all their
"globals" into a single module without other code.

There are also people who claim this whole thing is bad and you should find
other ways to pass globals through functions.