[imoverclocked]

Functional languages that require calling out side-effects (eg: "\ IO" in Flix) allow you to write functions that are "pure" (aka: they can do no mutation of data or perform IO) and will always give you the same result for a given input. Writing the majority of your code as pure functions makes the code much easier to reason about, especially as the system scales. IMHO, it also makes testing simpler.

It's definitely a different way of thinking and it has a lot of benefits. However, some of the downsides (which Flix seems to fix with region-based local mutation) might be implementing a sort that keeps two copies of a list in memory just to return the second list and discard the first. Depending on your list, this may not be an issue.