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Return Generic Type in Rust (lemmy.world)

submitted 1 year ago* (last edited 1 year ago) by [email protected] to c/[email protected]

8 comments fedilink hide all child comments

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();
}

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[–] [email protected] 2 points 1 year ago* (last edited 1 year ago) (1 children)

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.

[–] [email protected] 2 points 1 year ago

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