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

Answer 1

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
}