How to return two different type in rust?

Hello all,

I am trying to implement an Actix API where I will receive the stream response from OpenAI and stream it back to the user.

If the key is not provided, I will send an HTTP response. However, I am a bit confused about how to return two different types in Rust.

Thanks in advance :slight_smile:

Code:

use actix_web::{get, web, App, HttpResponse, HttpServer, Responder};
use actix_web_lab::sse;
use futures_util::stream::StreamExt;
use reqwest;
use reqwest_eventsource::{Event, EventSource};
use serde::{Deserialize, Serialize};
use serde_json;
use std::{convert::Infallible, time::Duration};

#[get("/")]
async fn hello() -> impl Responder {
    let open_ai_completion_url = "";
    let open_ai_key = "";

    if open_ai_key == "" {
        return HttpResponse::Ok().body("Please set the OPENAI_KEY environment variable");
    }
    let response = reqwest::Client::new()
        .post(open_ai_completion_url)
        .header("api-key", open_ai_key)
        .json(&serde_json::json!({
            "messages": [
                {"role": "system", "content": "Capital of USA?"},
            ],
            "max_tokens": 10,
            "temperature": 0,
            "stream": true,
        }));

    let mut es = EventSource::new(response).expect("Failed to create EventSource");
    let mut event_stream =
        futures_util::stream::iter([Ok::<_, Infallible>(sse::Event::Data(sse::Data::new("foo")))]);

    let event_stream = async_stream::stream! {
        while let Some(event) = es.next().await {
            match event {
                Ok(Event::Open) => {
                    println!("Connection Open!");
                }
                Ok(Event::Message(message)) => {
                    let chunk = message.data;
                    if chunk == "[DONE]" {
                        println!("Done!");
                        es.close();
                        break;
                    }
                    let openai_chunk = chunk.split(" ").collect::<Vec<&str>>()[0];

                    let parsed: Result<OpenAIData, serde_json::Error> = serde_json::from_str(&chunk);
                    match parsed {
                        Ok(data) => {
                            let event = sse::Event::Data(sse::Data::new_json(data).unwrap());
                            yield Ok::<_, Infallible>(event);
                        }
                        Err(err) => {
                            println!("Error: {:?} - {:?}", err, chunk);
                        }
                    }
                }
                Err(err) => {
                    println!("Error: {}", err);
                    es.close();
                }
            }
        }
    };
    sse::Sse::from_stream(event_stream).with_keep_alive(Duration::from_secs(5))
}

Error:

error[E0308]: mismatched types
  --> src/main.rs:67:5
   |
34 |       let event_stream = async_stream::stream! {
   |  ________________________-
35 | |         while let Some(event) = es.next().await {
36 | |             match event {
37 | |                 Ok(Event::Open) => {
...  |
65 | |         }
66 | |     };
   | |_____- the found `async` block
67 |       sse::Sse::from_stream(event_stream).with_keep_alive(Duration::from_secs(5))
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `HttpResponse`, found struct `Sse`

If you want to return two distinct types from a function, you can create a common type from both types by wrapping them in an enum with two variants, one variant containing a field with your first type and the other variant a field with the second type you want to return. I'd recommend using either::Either for that.

2 Likes

Or the one from the actix-web.

3 Likes

Way better given that it implements Responder.

1 Like

Also, please link cross-posts on other forums/qa-sites to avoid duplicated effort by the community:

1 Like

Thanks, this worked. It seems elegant!

That question is from 2018. Or is time traveling a thing now? :smile:

1 Like

Haha, I accidentally linked the question OP's question was marked as duplication of :smile:. OP deleted their SO question, so I can't rectify the link

1 Like

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