We want to see how we can improve our state machine so that it allows us to hold variables across wait points. To do that, we need to store them somewhere and restore the variables that are needed when we enter each state in our state machine.

Tip

Pretend that these rewrites are done by corofy (or the compiler). Even though corofy can’t do these rewrites, it’s possible to automate this process as well.

Or coroutine/wait program looks like this:
coroutine fn async_main() {
    println!(“Program starting”);
    let txt = Http::get(“/600/HelloAsyncAwait”).wait;
    println!(“{txt}”);
    let txt = Http::get(“/400/HelloAsyncAwait”).wait;
    println!(“{txt}”);
}

We want to change it so that it looks like this:
coroutine fn async_main() {
let mut counter = 0;
    println!(“Program starting”);
    let txt = http::Http::get(“/600/HelloAsyncAwait”).wait;
    println!(“{txt}”);
counter += 1;
    let txt = http::Http::get(“/400/HelloAsyncAwait”).wait;
    println!(“{txt}”);
counter += 1;
println!(“Received {} responses.”, counter);
}

In this version, we simply create a counter variable at the top of our async_main function and increase the counter for each response we receive from the server. At the end, we print out how many responses we received.

Note

For brevity, I won’t present the entire code base going forward; instead, I will only present the relevant additions and changes. Remember that you can always refer to the same example in this book’s GitHub repository.

The way we implement this is to add a new field called stack to our Coroutine0 struct:

ch09/a-coroutines-variables/src/main.rs
struct Coroutine0 {
stack: Stack0,
    state: State0,
}

The stack fields hold a Stack0 struct that we also need to define:

ch09/a-coroutines-variables/src/main.rs
#[derive(Default)]
struct Stack0 {
    counter: Option<usize>,
}

This struct will only hold one field since we only have one variable. The field will be of the Option<usize> type. We also derive the Default trait for this struct so that we can initialize it easily.