multiple futures in one function

futures.rs

use futures::future::*;

fn either(x: Option<i32>) -> impl Future<Item = i32, Error = i32> {
    match x {
        Some(x) => Either::A(ok(x).map(|x| x)),
        None => Either::B(ok(42).and_then(|x| err(x)))
    }
}

fn boxed(x: Option<i32>) -> Box<Future<Item = i32, Error = i32>> {
    match x {
        Some(x) => Box::new(ok(x).map(|x| x)),
        None => Box::new(ok(42).and_then(|x| err(x)))
    }
}

fn chained(x: Option<i32>) -> impl Future<Item = i32, Error = i32> {
    let fut = match x {
        Some(x) => ok(x),
        None => err(42),
    };
    let fut = fut.map_err(|error| error);
    let fut = fut.and_then(|x| ok(x + 1));
    fut.map(|x| x)
}

fn main() {
    
    let mut future = either(None);
    println!("Either: {:#?}", future.poll());
    let mut future = either(Some(32));
    println!("Either: {:#?}", future.poll());
    
    let mut future = boxed(None);
    println!("Either: {:#?}", future.poll());
    let mut future = boxed(Some(32));
    println!("Either: {:#?}", future.poll());
    
    let mut future = chained(None);
    println!("Chained: {:#?}", future.poll());
    let mut future = chained(Some(32));
    println!("Chained: {:#?}", future.poll());
}