This is exactly the kind of breakage I had in mind: With the proposed change, the storage layout would change, and the compiler wouldn't warn you about that at all. Note that I'm not arguing about memory efficiency, it's all about a subtle semantic change for the sake of a single library, wanting to change something which was in place for 10-20 years. Seems like an extremely bad move from the POV of the Haskell ecosystem: It's exactly this kind of ad hoc changes which annoys people.