On: Copying data is wasteful, mutating data is dangero...

[perfunctory]

> can be implemented like a persistent data structure

you don't get much benefit from persistent structures if every element of an array is modified, like in the examples in the post.

[pipeep → perfunctory]

If the data structure has a reference count of one, you can safely mutate the data structure instead. Many persistent immutable data structure libraries use this as an additional optimization.

[capableweb → perfunctory]

I missed that, sorry. Thanks for clarifying!