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Comment by bee_rider | Hacker News Reader

The author of the post says:

> For loops are not idiomatic in Rust - writing Fortran-ey code instead of iterators will lead to pain and slower loops. Spending time reading the examples in the iterator docs and the community solutions in the exercisms will help a lot.

I think they are talking about C style for loops or Fortran style do loops, they explicitly contrast against iterators.

Edit: Although, I wonder, what does an iterator like you’ve shown compile down to? I’d imagine Rust is smart/modern enough and that iterator is common enough that the assembly emitted looks a lot like a c for loop. Or better.

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I understood them to be saying that they would prefer

  (0..10).iter().for_each(|x| println!("{x}"))

over the thing that I wrote.

Many folks do not think of Rust's for loops as using iterators under the hood, and so will often talk about them as though they don't use iterators, even though they technically do.

Author here, long time no see Steve!

When I wrote this, my main takeaways were numerical computations with arrays, which don't tend to use for loops in Rust, and which require grokking iterators for maximum performance and rayon-parallelism boons.

To a Julia audience, seeing this style of iterator chaining is quite a new flavor to think about composing computation, so I wanted it was with that audience in mind that I wrote this.

Hey hey!

Yeah I think that makes sense! I don't do that kind of work and so it's more usual in the kinds of code I write.

IIRC for loops compile down to a while loop with the same body, that drives the given iterator with .next(), until it returns None.

Since the .next() implementation of std::ops::Range is essentially an i++, you'll quickly arrive at a similar representation as classic C for-loop.

Rust only "really" has one loop, the one named loop. Loop is an infinite loop that you can break out of (using the break keyword of course).

All the other syntax, including both "for" and "while" loops and the related but technically different "while let" loop, while useful to humans to communicate intent, aren't semantically different - and so they're transformed early in compilation, they are just "syntax sugar" and are de-sugared into rather elaborate loops with their exit condition now a loop break condition etc.

But yes, as in C++ and similar languages, the iterators generate the same machine code as you'd have gotten for writing the C-style for loop from the 1970s, it's just that in Rust you literally can't write that loop.

Now I’ve become confused as to what it means for a language to have a thing.

Hypothetically if you do

for i in 1..10 {

      println!("{i}");

  }

(From earlier) the compiler should have enough information to fully unroll the loop if it feels like it. So it is a different syntax from the programmer’s point of view. Maybe it goes through some “this is a loop” stage inside the compiler. But then hypothetically the assembly output could… end up not even having a loop at all (fully unrolled, no jump back, haha).

What your parent is saying is that the compiler treats that loop as

  {
      let result = match IntoIterator::into_iter(1..10) {
          mut iter => loop {
              let next;
              match iter.next() {
                  Some(val) => next = val,
                  None => break,
              };
              let i = next;
              let () = { println!("{i}"); };
          },
      };
      result
  }

(95% sure I got that correct)

And so in some sense, "loop" is more primitive than "for".

> But then hypothetically the assembly output could… end up not even having a loop at all

Absolutely, with any of these loops, the compiler may not emit a loop in assembly. And in this case, that seems to be true: https://godbolt.org/z/7Gq69Gvre

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