Getting a Future from a Vector and a function

Question

I'm pretty new to Rust and to it's async/await model, and I'm trying to do something that looks like a specialized Haskell's traverse function. Given a Vec<T> and a function T -> Future<Output = R> I want to get a Future<Output = Vec<R>>.

At the moment, I have the following:

use futures::{FutureExt, StreamExt};

pub async fn traverse<I, T, R, F, FN>(xs: I, f: FN) -> Vec<R>
where
    I: IntoIterator<Item = T>,
    F: FutureExt<Output = R>,
    FN: Fn(T) -> F,
{   
    futures::stream::iter(xs)
        .fold(vec![], |acc, item| {
            f(item).map(move |app| {
                let mut a = acc;
                a.push(app);

                a
            })
        })
        .await
}

It works as expected, but doesn't feel very idiomatic. Does anybody has suggestion how to improve this function?

Answer 1

I think the best is to simply use then() and collect():

use futures::{Future, StreamExt};

pub async fn traverse<I, T, F, Fut, O>(xs: I, f: F) -> Vec<O>
where
    I: IntoIterator<Item = T>,
    F: Fn(T) -> Fut,
    Fut: Future<Output = O>,
{
    futures::stream::iter(xs).then(f).collect().await
}

Note:

• futures generic should be name Fut
• function generic should be name F
• I rename R as O but I don't know if it's better but it's my way.
• Doesn't need FutureExt