6 ms·

> It stops when one of the two iterators ends Right; my question is, suppose you're iterating over two slice iterators - won't each call to `a.next()` and `b.n

by defen 4y ago

> It stops when one of the two iterators ends Right; my question is, suppose you're iterating over two slice iterators - won't each call to `a.next()` and `b.next()` have to check whether that sub-iterator is done? One of the benefits of the Zig approach is that you can iterate over an arbitrary number of slices and do one check before entering the loop, followed by the compiler emitting unchecked index access in the loop. So it basically compiles down to the equivalent of a C `for` loop.

masklinn 4y ago
Rust's zip has a specialisation for iterators with a trusted length. Such as slice iterators. `zip` yields exactly the same assembly as a loop over the index range with an unsafe item access: https://godbolt.org/z/7ebfxbhxc https://godbolt.org/z/7ebfxbhxc
- brundolf 4y ago
  Interesting, are "trusted-length" iterators something that might ever make it into userspace? Maybe as const generics?
  - masklinn 4y ago
    It’s already in userspace, though nightly (and unsafe, obviously), so whether it’ll be stabilised, and in what form, is an open question: https://doc.rust-lang.org/std/iter/trait.TrustedLen.html https://doc.rust-lang.org/std/iter/trait.TrustedLen.html
- kristoff_it 4y ago
  You have to pass -O though, the point of Zig's for loop syntax is to get fast compile times and good performance also in debug mode :^)
- defen 4y ago
  That's cool. At the same time though, it almost feels like a distinction without a difference in some ways - Zig has a special built-in syntax; Rust doesn't use special syntax, but it does use complex special-cased unsafe code in the stdlib in order to implement a safe + performant API.
  - masklinn 4y ago
    On the other hand, the "special cased unsafe code" is applicable to more than just zip, more than just the one array type, and is available in userland (though currently unstable so nightly only, both to implement it on a bespoke type and to rely on it).
  - brundolf 4y ago
    Rust's is built on top of (and exposed to) Iterators, which are a very general concept that can be rooted in all kinds of data structures, composed in all kinds of ways, and collected/processed in all kinds of ways (i.e. the user's code might not even contain an actual loop). The code continues to work in many situations, even where the optimization doesn't apply You trade some special-case syntax and ergonomics for that generality, but it is very general even if not all of it is optimized in the same way
- the8472 4y ago
  For Zip it's TrustedRandomAccess[0] instead of TrustedLen. Imo the most radioactively unsafe trait in the standard library and will likely never be stabilized in its current form. [0] https://github.com/rust-lang/rust/blob/f540a25745e03cfe9eac7e3a5e5489d36e4ad58a/library/core/src/iter/adapters/zip.rs#L474-L530 https://github.com/rust-lang/rust/blob/f540a25745e03cfe9eac7...

by defen 4y ago

masklinn 4y ago

Rust's zip has a specialisation for iterators with a trusted length. Such as slice iterators. `zip` yields exactly the same assembly as a loop over the index range with an unsafe item access: https://godbolt.org/z/7ebfxbhxc https://godbolt.org/z/7ebfxbhxc

brundolf 4y ago
Interesting, are "trusted-length" iterators something that might ever make it into userspace? Maybe as const generics?
- masklinn 4y ago
  It’s already in userspace, though nightly (and unsafe, obviously), so whether it’ll be stabilised, and in what form, is an open question: https://doc.rust-lang.org/std/iter/trait.TrustedLen.html https://doc.rust-lang.org/std/iter/trait.TrustedLen.html
kristoff_it 4y ago
You have to pass -O though, the point of Zig's for loop syntax is to get fast compile times and good performance also in debug mode :^)
defen 4y ago
That's cool. At the same time though, it almost feels like a distinction without a difference in some ways - Zig has a special built-in syntax; Rust doesn't use special syntax, but it does use complex special-cased unsafe code in the stdlib in order to implement a safe + performant API.
- masklinn 4y ago
  On the other hand, the "special cased unsafe code" is applicable to more than just zip, more than just the one array type, and is available in userland (though currently unstable so nightly only, both to implement it on a bespoke type and to rely on it).
- brundolf 4y ago
  Rust's is built on top of (and exposed to) Iterators, which are a very general concept that can be rooted in all kinds of data structures, composed in all kinds of ways, and collected/processed in all kinds of ways (i.e. the user's code might not even contain an actual loop). The code continues to work in many situations, even where the optimization doesn't apply You trade some special-case syntax and ergonomics for that generality, but it is very general even if not all of it is optimized in the same way
the8472 4y ago
For Zip it's TrustedRandomAccess[0] instead of TrustedLen. Imo the most radioactively unsafe trait in the standard library and will likely never be stabilized in its current form. [0] https://github.com/rust-lang/rust/blob/f540a25745e03cfe9eac7e3a5e5489d36e4ad58a/library/core/src/iter/adapters/zip.rs#L474-L530 https://github.com/rust-lang/rust/blob/f540a25745e03cfe9eac7...