> Source: it takes more lines of code to return deep copies of objects than to not do that.
Defensive copying and deep copying is not a thing you have to do in immutable languages. Even under the covers, it's not happening the way you seem to think it is. If I had a large immutable map in use by some other process, and needed a version of it with an element changed or added, why would I deep copy it when I can just point to that same map instance, and add a pointer to the key-value pair I want to substitute [1]? I think this is a common reservation people have about immutable programming because they come into it with a OO mindset. At least, I know I did.
In a really simplified example, a = (1, 2, 3, ..., 100) and b = (2, 3, ..., 100) are not allocated as two full lists in memory space. a contains 1 followed by a pointer to b. Because you have guarantees that b will never change, the single instance of b can be recycled in other data structures (or passed to many other functions and threads) and you avoid the complexity of managing race conditions, mutexes, semaphores, which are a significant source of bugs in other languages.
See [2] for a more realistic implementation.
