CODEWITHSUNDEEP
rustrust-tokio
Leonti
Leonti

Reputation: 10960

Can't infer type F when returning a Future type

I'm trying to write a function which would return a Future, so following a tutorial from Tokio, I came up with this:

extern crate tokio;

use std::time::{Duration, Instant};
use tokio::prelude::*;
use tokio::timer::Interval;

fn run<F>() -> impl Future<Item = (), Error = F::Error>
where
    F: Future<Item = ()>,
{
    Interval::new(Instant::now(), Duration::from_millis(1000))
        .for_each(move |instant| {
            println!("fire; instant={:?}", instant);
            Ok(())
        })
        .map_err(|e| panic!("interval errored; err={:?}", e))
}

fn main() {
    tokio::run(run());
}

playground

I get this error:

error[E0282]: type annotations needed
  --> src/main.rs:20:16
   |
20 |     tokio::run(run());
   |                ^^^ cannot infer type for `F`

I assume the error will go away once I specify the full return type, which I can't even figure out (my IDE gives me <futures::MapErr<futures::stream::ForEach<tokio::timer::Interval, [closure@src/ir.rs:24:23: 38:14 self:_], std::result::Result<(), tokio::timer::Error>>, [closure@src/ir.rs:39:22: 39:65]>)

  1. How can I figure out the type? Any IDE tips or tricks? (I'm using Atom with ide-rust)

  2. Can I somehow get away with just defining impl Future<Item = (), Error = F::Error> where F: Future<Item = ()>?

    I'm fine with defining the full type somewhere inside of run function, but to the outside of the function I'd like to expose <Future<Item = (), Error = F::Error>> or <Future<Item = (), Error = io::Error>>

Upvotes: 0

Views: 498

Answers (1)

attdona
attdona

Reputation: 18943

Look at tokio::run's signature:

pub fn run<F>(future: F) 
where
    F: Future<Item = (), Error = ()> + Send + 'static,

The consumed future must have the associated Error type equal to (). This implies that you cannot be generic over the error.

This works:

fn run() -> impl Future<Item = (), Error = ()> {
    Interval::new(Instant::now(), Duration::from_millis(1000))
        .for_each(move |instant| {
            println!("fire; instant={:?}", instant);
            Ok(())
        })
        .map_err(|e| panic!("interval errored; err={:?}", e))
}

fn main() {
    tokio::run(run());
}

Upvotes: 2

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