Possible, yes. Advisable, no. There is no guarantee that the finaliser is ever run, they are expensive (since they need to be kept separately and checked after each GC) and there are easier methods. Lava [1] uses a very lightweight sharing detection mechanism. Depending on your problem you might also want to look at Oleg's approach to explicit sharing in a DSL [2]. [1]: http://www.cs.chalmers.se/~koen/Lava/papers.html [2]: http://www.mail-archive.com/haskell-cafe@haskell.org/msg37765.html 2008/11/26 Andrew Coppin :

Here's an interesting question... Is it possible to attach finalisers to a value? (That is, have some Haskell code executed when the item in question is reclaimed by the GC.) I'm interested in knowing whether a particular data structure is shared (i.e., whether it's safe to mutate it or whether it must be copied first), and a simple reference-counting scheme looks like the easiest option.

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