The trait bound `(): futures::Future` is not satisfied when using TcpConnectionNew

Question

I am trying to write a simple TCP client in Rust using Tokio crate. My code is pretty close to this example minus the TLS:

extern crate futures;
extern crate         


        

Answer 1

TL;DR: remove the semicolon after io::write_all.

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Review the definition of and_then:

fn and_then(self, f: F) -> AndThen 
where
    F: FnOnce(Self::Item) -> B,
    B: IntoFuture,
    Self: Sized, 

The closure (F) has to return some type (B) that can be converted into a future (B: IntoFuture) with an error type that matches the starting closure (Error = Self::Error).

What does your closure return? (). Why is that? Because you've placed a semicolon (;) at the end of your line. () does not implement the trait IntoFuture, which is indicated by the error message part "on the impl of futures::IntoFuture for ()":

impl IntoFuture for F {
    type Future = F;
    type Item = F::Item;
    type Error = F::Error;
}

Removing the semicolon will cause the Future returned by io::write_all to be returned back to and_then and the program will compile.

In general, futures work by combining together smaller components which are themselves futures. All of this works together to build a single large future which is essentially a state machine. It's good to keep this in mind, as you will almost always need to return a future when using such combinators.