> But if you do this people will argue this is inefficient, because you are copying a lot of objects. Syntactically it is annoying to some extent. But, just li

> But if you do this people will argue this is inefficient, because you are copying a lot of objects. Syntactically it is annoying to some extent. But, just like the most important data gets saved to a database with lots of "ceremony" involved -- separate protocols, transactions, SQL statements, etc, because it's pretty important to track updates carefully. Here ,it's a bit like that, but on a smaller scale as in memory program state is also important, and has to be tracked explicitly, and for that some "ceremony" is acceptable. Implementation-wise, because of immutability, there is copying but it is often not 100% duplication. The updated version and the previous one behind the scenes (in heap) might share a lot of common structures. For example, if we have a 1000 element list L, and we updated it with a new element at the front L1 = [H | L], then L1 and is not a complete copy of L, but instead is just element H and a pointed to the shared tail L. For dictionary data structures (maps) something similar happens but there it's a O(log n) order of updates with everything else being shared. Definitely not as efficient as in-place updates in say C++ or Java but that's a price I'd happily pay.