How can I put an async function into a map in Rust?

Question

I cannot handle async functions when writing an async router for hyper.

This code:

use std::collections::HashMap;
use std::future::Future;

type BoxedResult<T> = Result<T, Box<dyn std::error::Error + Send + Sync>>;
type CalcFn = Box<dyn Fn(i32, i32) -> dyn Future<Output = BoxedResult<i32>>>;

async fn add(a: i32, b: i32) -> BoxedResult<i32> {
    Ok(a + b)
}

async fn sub(a: i32, b: i32) -> BoxedResult<i32> {
    Ok(a - b)
}

fn main() {
    let mut map: HashMap<&str, CalcFn> = Default::default();
    map.insert("add", Box::new(add));
    map.insert("sub", Box::new(sub));

    println!("map size: {}", map.len());
}

Generates the following compiler error:

error[E0271]: type mismatch resolving `<fn(i32, i32) -> impl std::future::Future {add} as std::ops::FnOnce<(i32, i32)>>::Output == dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`
  --> src/main.rs:17:23
   |
17 |     map.insert("add", Box::new(add));
   |                       ^^^^^^^^^^^^^ expected opaque type, found trait std::future::Future
   |
   = note: expected type `impl std::future::Future`
              found type `dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`
   = note: required for the cast to the object type `dyn std::ops::Fn(i32, i32) -> dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`

error[E0271]: type mismatch resolving `<fn(i32, i32) -> impl std::future::Future {sub} as std::ops::FnOnce<(i32, i32)>>::Output == dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`
  --> src/main.rs:18:23
   |
18 |     map.insert("sub", Box::new(sub));
   |                       ^^^^^^^^^^^^^ expected opaque type, found trait std::future::Future
   |
   = note: expected type `impl std::future::Future`
              found type `dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`
   = note: required for the cast to the object type `dyn std::ops::Fn(i32, i32) -> dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`

It seems there is a conflict between impl Future and dyn Future, but I have no idea how to handle it.

Shepmaster · Accepted Answer · 2020-09-28 13:36:30Z

16

This happens because impl Future is a concrete unique type while dyn Future is an abstract type. HashMap expects the abstract type since it can only hold instances of a single type.

If we can box the return type of the async functions, we will able to add these futures into a HashMap.

First we need to change the type of CalcFn:

type CalcFn = Box<dyn Fn(i32, i32) -> Pin<Box<dyn Future<Output = i32>>>>;

Then this can do the trick:

let mut map: HashMap<&str, CalcFn> = Default::default();
map.insert("add", Box::new(|a, b| Box::pin(add(a, b))));
map.insert("sub", Box::new(|a, b| Box::pin(sub(a, b))));

println!("map size: {}", map.len());

//map.get("add").unwrap()(2, 3).await

This complete example simplified Future's Item type, using an i32 instead of a Result. Please also check the full code for your case.

You can also use types from the futures crate like LocalBoxFuture and BoxFuture created by the FutureExt::boxed and FutureExt::boxed_local methods respectively:

use futures::future::{FutureExt, LocalBoxFuture}; // 0.3.5
use std::collections::HashMap;

type BoxedResult<T> = Result<T, Box<dyn std::error::Error + Send + Sync>>;
type CalcFn = Box<dyn Fn(i32, i32) -> LocalBoxFuture<'static, BoxedResult<i32>>>;

async fn add(a: i32, b: i32) -> BoxedResult<i32> {
    Ok(a + b)
}

async fn sub(a: i32, b: i32) -> BoxedResult<i32> {
    Ok(a - b)
}

async fn example() {
    let mut map: HashMap<&str, CalcFn> = Default::default();
    map.insert("add", Box::new(|a, b| add(a, b).boxed()));
    map.insert("sub", Box::new(|a, b| sub(a, b).boxed()));

    println!("map size: {}", map.len());

    //map.get("add").unwrap()(2, 3).await
}

edited Sep 28, 2020 at 13:36

Shepmaster

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answered Nov 5, 2019 at 10:26

Ömer Erden

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2 Comments

Ömer Erden Over a year ago

Full example with executor

Shepmaster Over a year ago

See also Why can impl trait not be used to return multiple / conditional types?; How do I create a heterogeneous collection of objects?; How can I have a collection of objects that differ by their associated type?.

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