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

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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
}

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