I know what optimisations identity-less aggregates allow. Objects can be passed around in O(1) time in any language that uses references. It's not the issue. In

I know what optimisations identity-less aggregates allow. Objects can be passed around in O(1) time in any language that uses references. It's not the issue. Indeed passing value types is O(n) in the size of the value type, so that's a loss. The goal is to avoid data dependent loads (i.e. pointer dereferences) by flattening/inlining data structures. As you point out, Haskell could in theory merge things together and lay out memory more effectively, but in theory so could Java with e.g. interprocedural escape analysis. But in practice there's a limit to what automatic compiler optimisations can do - learning this the hard way is a big part of the history of functional languages (like, how many functional language runtimes automatically parallelise apps with real speedups?). Java sometimes does convert object allocations into value types via the scalarisation optimisations, but not always, hence the desire to add it to the language.