Hi, I have a question about sharing at the Haskell run-time level. Suppose we have a record update r { x = f (r x)} and suppose that most of the time f returns it's argument unchanged. I have the following questions: 1. Does the generated code for the record update build an identical record when f returns it's argument unchanged instead of sharing the old one? (I guess yes.) 2. Can we prevent building identical records? Recently I've heard about Q-combinators. Central idea: Change (f :: a -> a) to (f' :: a -> Maybe a) returning Nothing when the value didn't change. Then we can replace the record update with smarter code which preserves more sharing. My question is: Can (or could) we enable more sharing without changing the source code? Ideally the compiler would generate record update code which checks poiter-equality of the updated field value to decide whether a copy of the original record is needed or it can be shared (given an optimisation flag enabled). Thanks, Peter Background info: There was a discussion on the Agda mailing list about decreasing the Agda compiler memory-usage with more sharing[^1]. (The Agda compiler is written in Haskell.) I had the above idea and I was advised to ask about it on some Haskell mailing list. [^1]: https://lists.chalmers.se/pipermail/agda/2012/004485.html