What's the most idiomatic way of working with an Iterator of Results?

rust iterator map-function filterfunction rust-result
Tim McLean picture Tim McLean · Apr 2, 2016 · Viewed 20.4k times · Source

I have code like this:

let things = vec![/* ...*/]; // e.g. Vec<String>
things
    .map(|thing| {
        let a = try!(do_stuff(thing));
        Ok(other_stuff(a))
    })
    .filter(|thing_result| match *thing_result {
        Err(e) => true,
        Ok(a) => check(a),
    })
    .map(|thing_result| {
        let a = try!(thing_result);
        // do stuff
        b
    })
    .collect::<Result<Vec<_>, _>>()

In terms of semantics, I want to stop processing after the first error.

The above code works, but it feels quite cumbersome. Is there a better way? I've looked through the docs for something like filter_if_ok, but I haven't found anything.

I am aware of collect::<Result<Vec<_>, _>>, and it works great. I'm specifically trying to eliminate the following boilerplate:

  • In the filter's closure, I have to use match on thing_result. I feel like this should just be a one-liner, e.g. .filter_if_ok(|thing| check(a)).
  • Every time I use map, I have to include an extra statement let a = try!(thing_result); in order to deal with the possibility of an Err. Again, I feel like this could be abstracted away into .map_if_ok(|thing| ...).

Is there another approach I can use to get this level of conciseness, or do I just need to tough it out?

Answer

Veedrac picture Veedrac · Apr 2, 2016

There are lots of ways you could mean this.

If you just want to panic, use .map(|x| x.unwrap()).

If you want all results or a single error, collect into a Result<X<T>>:

let results: Result<Vec<i32>, _> = result_i32_iter.collect();

If you want everything except the errors, use .filter_map(|x| x.ok()) or .flat_map(|x| x).

If you want everything up to the first error, use .scan((), |_, x| x.ok()).

let results: Vec<i32> = result_i32_iter.scan((), |_, x| x.ok());

Note that these operations can be combined with earlier operations in many cases.

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