How can I construct and pass an iterator of iterators?

Question

I am trying to grok Rust by implementing simple algorithms in it. I managed to make a generic merge_sorted, which ended up having the following signature:

fn merge_sorted<IL, ILL, I: Ord>(mut arrays: ILL) -> Vec<I>
where
    IL: Iterator<Item = I>,
    ILL: Iterator<Item = IL>,
{
    // ...
}

This seems to be compiling on its own. The signature makes sense to me, as the function consumes the top-level iterator, and all the iterators it returns too. However, I am unable to construct a valid value to pass to this function:

fn main() {
    let v1 = vec![1, 2];
    let vectors = vec![v1.iter()];
    merge_sorted(vectors.iter());
}

As expected, vectors in this sample has the type:

std::vec::Vec<std::slice::Iter<'_, i32>>

This is the error message I get:

error[E0277]: the trait bound `&std::slice::Iter<'_, {integer}>: std::iter::Iterator` is not satisfied
  --> src\main.rs:58:5
   |
58 |     merge_sorted(vectors.iter());
   |     ^^^^^^^^^^^^ `&std::slice::Iter<'_, {integer}>` is not an iterator; maybe try calling `.iter()` or a similar method
   |
   = help: the trait `std::iter::Iterator` is not implemented for `&std::slice::Iter<'_, {integer}>`
note: required by `merge_sorted`

Where does the & come from?

Boiethios · Accepted Answer · 2018-11-07 08:32:50Z

2

Vec::iter borrows the items it contains, so you are iterating over borrowed iterators (&std::slice::Iter) that do not implement Iterator. To consume a vector in order to have the ownership of the items, you must call Vec::into_iter:

fn main() {
    let v1 = vec![1, 2];
    let vectors = vec![v1.iter()]; // You can use `into_iter` there to iterate over ints.
    merge_sorted(vectors.into_iter());
}

You can also require IntoIterators that can make easier the usage of your API:

fn merge_sorted<IterT, IterIterT, T: Ord>(mut arrays: IterIterT) -> Vec<T>
where
    IterT: IntoIterator<Item = T>,
    IterIterT: IntoIterator<Item = IterT>,
{
    panic!();
}

fn main() {
    let v1 = vec![1, 2];
    let vectors = vec![v1];
    merge_sorted(vectors);
}

edited Nov 7, 2018 at 8:32

answered Nov 6, 2018 at 16:51

Boiethios

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5 Comments

LOST Over a year ago

BTW, any suggestions on how to best make an iterator of iterators for a number of constants? Going through Vec seems excessive.

Boiethios Over a year ago

@LOST You can write a macro, but there are no easier ways to do this AFAIK

trent Over a year ago

If you change merge_sorted to accept IntoIterator rather than Iterator, this becomes: let vectors = vec![&v1]; merge_sorted(vectors). (Or v1 in place of &v1 to iterate over values.) It's almost always better to use IntoIterator as a bound for arguments because all things that implement Iterator also implement IntoIterator.

trent Over a year ago

@LOST There is a merge function in the itertools crate which you may find interesting, although it only merges two iterators, not an arbitrary number.

LOST Over a year ago

@trentcl I am really just practicing writing short snippets.

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