## Writing an Accept Loop
Let's implement the scaffold of the server: a loop that binds a TCP socket to an address and starts accepting connections.
First of all, let's add required import boilerplate:
```rust,edition2018
# extern crate async_std;
use async_std::{
prelude::*, // 1
task, // 2
net::{TcpListener, ToSocketAddrs}, // 3
};
type Result<T> = std::result::Result<T, Box<dyn std::error::Error + Send + Sync>>; // 4
```
1. `prelude` re-exports some traits required to work with futures and streams.
2. The `task` module roughly corresponds to the `std::thread` module, but tasks are much lighter weight.
A single thread can run many tasks.
3. For the socket type, we use `TcpListener` from `async_std`, which is just like `std::net::TcpListener`, but is non-blocking and uses `async` API.
4. We will skip implementing comprehensive error handling in this example.
To propagate the errors, we will use a boxed error trait object.
Do you know that there's `From<&'_ str> for Box<dyn Error>` implementation in stdlib, which allows you to use strings with `?` operator?
Now we can write the server's accept loop:
```rust,edition2018
# extern crate async_std;
# use async_std::{
# net::{TcpListener, ToSocketAddrs},
# prelude::*,
# };
#
# type Result<T> = std::result::Result<T, Box<dyn std::error::Error + Send + Sync>>;
#
async fn accept_loop(addr: impl ToSocketAddrs) -> Result<()> { // 1
let listener = TcpListener::bind(addr).await?; // 2
let mut incoming = listener.incoming();
while let Some(stream) = incoming.next().await { // 3
// TODO
}
Ok(())
}
```
1. We mark the `accept_loop` function as `async`, which allows us to use `.await` syntax inside.
2. `TcpListener::bind` call returns a future, which we `.await` to extract the `Result`, and then `?` to get a `TcpListener`.
Note how `.await` and `?` work nicely together.
This is exactly how `std::net::TcpListener` works, but with `.await` added.
Mirroring API of `std` is an explicit design goal of `async_std`.
3. Here, we would like to iterate incoming sockets, just how one would do in `std`:
```rust,edition2018,should_panic
let listener: std::net::TcpListener = unimplemented!();
for stream in listener.incoming() {
}
```
Unfortunately this doesn't quite work with `async` yet, because there's no support for `async` for-loops in the language yet.
For this reason we have to implement the loop manually, by using `while let Some(item) = iter.next().await` pattern.
Finally, let's add main:
```rust,edition2018
# extern crate async_std;
# use async_std::{
# net::{TcpListener, ToSocketAddrs},
# prelude::*,
# task,
# };
#
# type Result<T> = std::result::Result<T, Box<dyn std::error::Error + Send + Sync>>;
#
# async fn accept_loop(addr: impl ToSocketAddrs) -> Result<()> { // 1
# let listener = TcpListener::bind(addr).await?; // 2
# let mut incoming = listener.incoming();
# while let Some(stream) = incoming.next().await { // 3
# // TODO
# }
# Ok(())
# }
#
// main
fn run() -> Result<()> {
let fut = accept_loop("127.0.0.1:8080");
task::block_on(fut)
}
```
The crucial thing to realise that is in Rust, unlike other languages, calling an async function does **not** run any code.
Async functions only construct futures, which are inert state machines.
To start stepping through the future state-machine in an async function, you should use `.await`.
In a non-async function, a way to execute a future is to hand it to the executor.
In this case, we use `task::block_on` to execute a future on the current thread and block until it's done.