This question was not snark for FP pure functions or otherwise; no ill-will! It needs a legit answer maybe with a small lecture on why radix trees can be FP not pure FP. Hey, I can read and learn.
Other sorts of streaming operations --- message in, transform to new object which is logged or put into a data store most certainly do not need mutation so long as they are cache friendly, performant. FP and friends might even argue: yah ok it's some 10 pct slower but no race conditions, no weird sync. Formal methods are easier there to deploy. Those secondary arguments can be effective too; I'd bite
Other sorts of streaming operations --- message in, transform to new object which is logged or put into a data store most certainly do not need mutation so long as they are cache friendly, performant. FP and friends might even argue: yah ok it's some 10 pct slower but no race conditions, no weird sync. Formal methods are easier there to deploy. Those secondary arguments can be effective too; I'd bite