Matthew Naylor wrote: [snip]
Finally, when I say "observable sharing", I don't necessarily mean it as defined by Koen Claessen and David Sands. I simply mean the use of unsafePerformIO to detect sharing, whether or not this is done by an "eq" predicate on Refs. (I say this because I think there are simpler ways to detect sharing, though these will probably not have the nice semantic properties of observable sharing.)
ghc actually provides a primop for this: reallyUnsafePtrEquality# :: a -> a -> Int# Use at your own risk. Note that you can only check for equality uing that primop. To detect cycles in data structures efficiently, a total order would be better, but in the presence of copying garbage collection that's asking too much. Bertram