Allow extraction from fields that may be collections.

[tmpfs]

The current API only supports fields that extract &str directly from the document however there are times when we need to borrow from a collection in the document.

My use case is that I have a HashSet<String> to represent tags associated with the document.

With this PR my extract functions look like this:

// Label
fn label_extract(d: &Document) -> Vec<&str> {
    vec![d.2.label()]
}

// Tags
fn tags_extract(d: &Document) -> Vec<&str> {
    d.2.tags().iter().map(|s| &s[..]).collect()
}

I expected the extra heap allocation for the Vec to have a minor impact on performance but running the benchmarks shows a small performance improvement 🤷

[marcus-pousette]

Thanks for the PR.

Thinking about this feature fundamentally, this is basically multi-valued field support. If there is a performance penalty (even though it is not shown in the benchmarks because the formatter did some other optimisation) I would say it is worth it.

Could you update the README also?

[tmpfs]

So should we update to have a single extractor function, this would simplify the code instead of passing around slices of FieldAccessor - what do you think?

Happy New Year too! 😁

[marcus-pousette]

I was working on this issue before #19, which basically does that, but I had some troubles keeping the performance on the same level. But that would basically eliminate the need for passing around these functions and you would just impl the trait instead

I would say, keep this PR as it is (except fixing the README), and then we can move on later and simplify things even more!

Happy new year to you too! 🎉

[tmpfs]

Ok thanks @marcus-pousette that makes sense, I have updated the README, lets get this merged as is and migrate to the trait based extractor later. Cheers 👍

[marcus-pousette]

Super! Great!