I'm unsure what the difference is between the first two…are they not equivalent? The lexical scoping could be interpreted differently, sure (allowing your third example the benefit of the doubt) but this is where the "Swift is consistent" comes into play. You already know Swift's scoping rules, it's one of the first things you learn, so using that knowledge you can figure out this code as well.

No, they're not equivalent. See, that requires making an assumption that I didn't about lexical nesting. They merely do the same thing in this example.

I do not know Swift's scoping rules and having read through the introductory guide and the flow control chapter I still can't tell you if the loop variable remains visible after the loop, and what it is bound to if so.

No doubt that if I sat down with a REPL or an IDE it'd hopefully be evident by example within a few seconds, but the claim was about dry-reading, not interactive experience.

I'll leave it there with an admonishment against making assumptions/declarations about the obvious-ness of something you're already familiar with.

In every language I’ve used with a for-in construct (or equivalent), the iterated variable is scoped to the loop. That seems to be implied by the syntax, too; it’s equivalent to “for all i in [1,9] where i mod 2 = 0”. If you saw that in a math context, you’d expect i to have no meaning beyond the iteration.

Is there a case to be made for other behavior? Or an example of a language that intentionally treats it any differently? (Unless you go out of your way to iterate over a variable you’ve declared in an outer scope in C, but then that’s not really a for-in construct.)

Python and JavaScript are languages that do not behave the way you mentioned.

JS works that way with plain for loops, but not for-in or for-of. The following fails on the last line, for example:

  for (const i in [1,2,3,4]) {
    alert(i);
  }
  alert("i is now " + i); // ReferenceError

(Not that JS is exactly known for consistent and predictable behavior on edge cases, of course...)

Python I don’t have experience with, and it appears I stand corrected on that front! I wonder how intentional it is, and what sort of use case it enables (and if it’s considered good practice to use).

Python does this mostly because it doesn't have block scope, only function scope like JS's `var`. After using Python for a decade, I think this is mostly a mistake. I would configure (or edit) my linter to prevent me from using it if I needed to write a lot more Python.

Lexical nesting? While I agree with the statement you’re making — that reading code requires you to understand more about what the code means, in that context/runtime, I also think the example you’re picking apart reminds me of the dangers of undefined behaviour in C++. If the language specification plus its test cases still has implementation-specific details, then while by all means measure or discover them, but I would both say this is not unique to Swift and is just part of the complexity that is not writing CPU instructions directly that target only one type of CPU design. Maybe I’m taking this too far, but my point I suppose is that I know what was intended by the swift code even if the exact details are optimized out from underneath me by a compiler or language library. Might as well argue that you should have full unit and integration tests for every target platform to ensure the code behaves the way you expect, and even then you’ll still hit platform-specific edge cases, particularly on the wide variety of platforms Linux supports... I do tend to trust Clang, but even it can vary greatly from (hopefully major) version to version.

Yes, I'm definitely trying to point out the distinction between reading code and understanding its intention vs understanding implementation mechanics and awareness of the possible alternative behaviours. It's easy to point out that C++ is amongst the worst offenders in that regard, but even a language with a reputation for "programmer happiness" like Ruby (which I absolutely adore) is full of magic, almost every nontrivial expression glossing over a huge pile of conceptual and mechanical object-functional devices that beginners trip over quite routinely (don't even get me started on the thick layer of sorcery that Rails slathers over the top of that).

So I'm not actually calling Swift a major offender, but noting generally that I don't there's ever been a language in which the intuition gap between intention and mechanics was small enough to be irrelevant.

(something at the back of my mind is now whispering "Scheme", but too quietly to be taken seriously)