StreamExt.scan() 方法上的“预期绑定生命周期参数，找到具体生命周期”

Question

I am using async-tungstenite to listen to a websocket, and async-std's StreamExt to operate on the resulting stream.

I want to use a HashMap to accumulate the latest Ticker values from the websocket. These Ticker values will be looked up later to be used in calculations. I'm using the symbol (String) value of the Ticker struct as the key for the HashMap. I'm using the.scan StreamExt method to perform the accumulation.

However, I get a compilation error related to lifetimes. Here's some stripped-down code:

        let tickers = HashMap::new();
        let mut stream = ws.
            .scan(tickers, accumulate_tickers);
        while let msg = stream.next().await {
            println!("{:?}", msg)
        }

...and the accumulate_tickers function:

fn accumulate_tickers(tps: &mut HashMap<String, Ticker>, bt: Ticker) -> Option<&HashMap<String, Ticker>> {
    tps.insert((*bt.symbol).to_string(), bt);
    Some(tps)
}

The compilation error I receive is as follows:

error[E0271]: type mismatch resolving `for<'r> <for<'s> fn(&'s mut std::collections::HashMap<std::string::String, ws_async::model::websocket::Ticker>, ws_async::model::websocket::Ticker) -> std::option::Option<&'s std::collections::HashMap<std::string::String, ws_async::model::websocket::Ticker>> {accumulate_tickers} as std::ops::FnOnce<(&'r mut std::collections::HashMap<std::string::String, ws_async::model::websocket::Ticker>, ws_async::model::websocket::Ticker)>>::Output == std::option::Option<_>`
--> examples/async_std-ws.rs:64:4
   |
64 |         .scan(tickers, accumulate_tickers);
   |          ^^^^ expected bound lifetime parameter, found concrete lifetime

I'm unaware of a way to provide a lifetime parameter to the scan method.

I wonder whether the issue may be related to the fact that I modify the HashMap and then try to return it (is it a move issue?). How could I resolve this, or at least narrow down the cause?

Answer 1

I was able to get this working. Working my way through the compiler errors, I ended up with this signature for the accumulate_tickers function:

fn accumulate_tickers<'a>(tps: &'a mut &'static HashMap<String, Ticker>, bt: Ticker) -> Option<&'static HashMap<String, Ticker>>

I do want the accumulator HashMap to have a static lifetime so that makes sense. tps: &'a mut &'static HashMap... does look a bit strange, but it works.

Then, this issue was was that tickers also had to have a static lifetime (it's the initial value for the accumulator. I tried declaring it as static outside the main but it wouldn't let me set it to the result of a function - HashMap::new() .

I then turned to lazy_static which allows me to create a static value which does just that:

lazy_static! {
    static ref tickers: HashMap<String, Ticker> = HashMap::new();
}

This gave me a HashMap accumulator that had a static lifetime. However, like normal static values declared in the root scope, it was immutable. To fix that, I read some hints from the lazy_static team and then found https://pastebin.com/YES8dsHH . This showed me how to make my static accumulator mutable by wrapping it in Arc<Mutex<_>> .

lazy_static! {
    // from https://pastebin.com/YES8dsHH
    static ref tickers: Arc<Mutex<HashMap<String, Ticker>>> = {
        let mut ts = HashMap::new();
        Arc::new(Mutex::new(ts))
    };
}

This does mean that I have to retrieve the accumulator from the Mutex (and lock it) before reading or modifying it but, again, it works.

Putting it all together, the stripped-down code now looks like this:

#[macro_use]
extern crate lazy_static;

lazy_static! {
    // from https://pastebin.com/YES8dsHH
    static ref tickers: Arc<Mutex<HashMap<String, Ticker>>> = {
        let mut ts = HashMap::new();
        Arc::new(Mutex::new(ts))
    };
}

// SNIP

    // Inside main()
    let mut ticks = ws
        .scan(&tickers, accumulate_tickers);
    while let Some(msg) = ticks.next().await {
        println!("{:?}", msg.lock().unwrap());
    }

// SNIP

fn accumulate_tickers<'a>(tps: &'a mut &'static tickers, bt: Ticker) -> Option<&'static tickers> {
    tps.lock().unwrap().insert((*bt.symbol).to_string(), bt);
    Some(tps)
}

I'd be happy to hear suggestions for ways in which this could be made simpler or more elegant.