> The canonical Haskell compiler, GHC, is excellent at transforming operations on immutable data, as Haskell programs are written, into efficient mutations, at the runtime level.
Yes, at the level of native machine code and memory cells, there's not that much of a difference between immutability + garbage collection, and higher level source code that mutates. Thanks to GC you are going to overwrite the same memory locations over and over again, too.
Programmers for some reason really don't understand that generational garbage collection provides locality. I am really surprised how often I see C/C++/Rust types not understand this.
I think that only applies to a moving GC. A conservative GC (like the Boehm GC for C) doesn't move any items around, and thus doesn't do anything for locality.
Of course, even a moving GC has limits, itwon't turn a hashtable into something that has local accesses.