Rayon parallel iterator is not an iterator?

[tbreisacher]

I'm starting to experiment with the Rayon crate and I'm sure I must be missing something obvious here. If I call par_iter to get one of Rayon's parallel iterators, I can call the usual iterator methods on it (for_each, map, collect...) so it seems to be an iterator. But when I try to use it in a for loop, the compiler insists it is not:

error[E0277]: rayon::range::Iter<{integer}> is not an iterator

Rust Playground

What am I missing here?

[tbreisacher]

Aha!

Rayon par_iter not working on Vec<struct>

The for syntax in Rust only works with “ordinary” sequential iterators. The abstraction of parallel iteration provided by rayon does thus not work with for syntax. The way you can write something similar to a for loop is by using the for_each method. Of course, printing to stdout is not an operation for which parallelization makes much sense: The elements are potentially printed in a weird order and printing would have to coordinate through a global lock on stdout anyways. Nonetheless, using for…

So indeed, despite the name a ParallelIterator is in fact not an Iterator! If you want to turn it back into a normal Iterator, I guess you have to collect() it into a Vec or something like that?

[jofas]

@tbreisacher

If you want to turn it back into a normal Iterator, I guess you have to collect() it into a Vec or something like that?

That would be one option. See also this previous topic (I don't think much has changed since then): [rayon] How do I convert a `ParallelIterator` into a regular `Iterator` without collecting it in a `Vec` first?

[cuviper]

@tbreisacher

If I call par_iter to get one of Rayon's parallel iterators, I can call the usual iterator methods on it (for_each, map, collect...) so it seems to be an iterator.

It's actually a ParallelIterator, which is deliberately similar to Iterator, although some methods are a bit different to support parallel execution. For example, Iterator::map requires only FnMut while ParallelIterator::map requires Fn + Sync + Send.