Return Generic Type in Rust

3 years ago by wulf to c/programming

Background: I have a large serde_json value that I want to be read-only (the authoritative source is an encrypted SQLite DB and should only be updated when that gets updated)

The issue, I would like a single get function that returns a generic type

use serde_json;

pub struct Configuration {
    config: serde_json::Value,
}

impl Configuration {
    async fn get(&amp;self, key: &amp;str) -> Result {
        let tmp_value: = &amp;self.config["test"];

        // This would be repeated for String, bool, etc
        if tmp_value.is_i64() {
            match tmp_value.as_i64 {
                Some(x) => Ok(x),
                Err(e) => Err(()),
            }
        } else {
            Err(())
        }
    }
}

However I get: "mismatched types expected type parameter T found type i64"

Is it even possible to return multiple types from a single function?

EDIT: SOLUTION

Here is the solution I came up with:

pub struct Configuration {}

impl Configuration {
    fn get std::str::FromStr>() -> Result {
        Ok(T::from_str("1234");
    }
}

fn main() {
    let my_conf_val = Configuration::get();
}

I_like_cats 12 points 3 years ago

No. You can only return a single type from the function. You could return the serde_json::Value though so that the code calling this function can get the value it needs itself

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wulf 1 point 3 years ago

Afraid this might have been the case, if Ogeon's suggestion doesn't work out, I'll probably end up with multiple getters, one per type. There aren't that many anyway

Thank you!

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Turun 3 points 3 years ago

I see two options immediately:

Make the function generic and return result(T, err), where T is the generic typed supplied by the caller (turbo fish syntax). Not sure if it will compile though.

Use whatever serve uses under the hood. They obviously have some way that allows them to return an arbitrary type. Alternatively implement it yourself by creating an enum that can be either string, int or bool. Will require matching by the caller after the function returns.

I know in e.g. java you regularly do if x.instanceof(y), but rust lends itself really badly to this type of programming.

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Traister101 1 point 3 years ago

Got yourself some interesting syntax there lol. It's x instanceof Y

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chrismit3s 3 points 3 years ago

Well how do you want to use said get function?

let x = config.get("key").await;

So what type should x have?

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Ogeon 2 points 3 years ago

It may be possible to use the Any trait to "launder" the value by first casting it to &Any and then downcasting it to the generic type.

let any_value = match tmp_value {
    serde_json::Value::Number(x) => x as &amp;Any,
    // ...
};

let maybe_value = any_value.downcast_ref::&lt; T >();

I haven't tested it, so I may have missed something.

Edit: to be clear, this will not actually let you return multiple types, but let the caller decide which type to expect. I assumed this was your goal.

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wulf 2 points 3 years ago

Correct, I would want the caller to know what they're getting, I'll see if this works, Thank you!

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wulf 2 points 3 years ago

SOLUTION:

Here is the solution I came up with:

pub struct Configuration {}

impl Configuration {
    fn get std::str::FromStr>() -> Result {
        Ok(T::from_str("1234");
    }
}

fn main() {
    let my_conf_val = Configuration::get();
}

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