> In the Javascript world's defense, I don't see this as a practical solution in very many languages at all. I don't think I can name any language that doesn't

> In the Javascript world's defense, I don't see this as a practical solution in very many languages at all. I don't think I can name any language that doesn't have a compiler-level enforced definition of "pure function" somehow where its programmers will across-the-board reliably know what a "pure function" is. IME Scala programmers generally don't have much trouble with this, and the language doesn't do anything to prevent side-effects wherever you feel like putting them. It's just culturally not very common. Similarly, you can use `null` for almost anything in Scala, but it's extremely rare to run into one in code that isn't interacting with Java. But nothing really stops you from using basically any feature as Java++. For your Python programmer, like I mentioned they already have to deal with some form of no-side-effects with for loops: you should not modify the length of a data structure that you are iterating over (`for x in foo`), or you'll get weird/unexpected results (e.g. [0]). You don't need to even go full UB, and could just specify that the order that the arguments to chained folds get called in is unspecified, and that code that needs to care should use `forEach` or a for loop. IME if you're writing in a pipelined map/filter/fold style in the first place, the ordering question just doesn't come up much. With that style it's rare to need to mutate anything yourself because the mutation you need is baked into those loop primitives. [0] https://stackoverflow.com/questions/38003119/what-are-the-rules-for-iterator-invalidation https://stackoverflow.com/questions/38003119/what-are-the-ru...