Return Generic Type in Rust

wulf, 8 months ago

SOLUTION:

Here is the solution I came up with:

<span style="font-weight:bold;color:#a71d5d;">pub struct </span><span style="color:#323232;">Configuration {}
</span><span style="color:#323232;">
</span><span style="font-weight:bold;color:#a71d5d;">impl </span><span style="color:#323232;">Configuration {
</span><span style="color:#323232;">    </span><span style="font-weight:bold;color:#a71d5d;">fn </span><span style="font-weight:bold;color:#795da3;">get </span><span style="color:#323232;">std::str::FromStr</span><span style="font-weight:bold;color:#a71d5d;">></span><span style="color:#323232;">() -> Result {
</span><span style="color:#323232;">        </span><span style="color:#0086b3;">Ok</span><span style="color:#323232;">(T::from_str(</span><span style="color:#183691;">"1234"</span><span style="color:#323232;">);
</span><span style="color:#323232;">    }
</span><span style="color:#323232;">}
</span><span style="color:#323232;">
</span><span style="font-weight:bold;color:#a71d5d;">fn </span><span style="font-weight:bold;color:#795da3;">main</span><span style="color:#323232;">() {
</span><span style="color:#323232;">    </span><span style="font-weight:bold;color:#a71d5d;">let</span><span style="color:#323232;"> my_conf_val </span><span style="font-weight:bold;color:#a71d5d;">= </span><span style="color:#323232;">Configuration::get();
</span><span style="color:#323232;">}
</span>

chrismit3s, 8 months ago

Well how do you want to use said get function?

<span style="font-weight:bold;color:#a71d5d;">let</span><span style="color:#323232;"> x </span><span style="font-weight:bold;color:#a71d5d;">=</span><span style="color:#323232;"> config.</span><span style="color:#62a35c;">get</span><span style="color:#323232;">(</span><span style="color:#183691;">"key"</span><span style="color:#323232;">).await;
</span>

So what type should x have?

Turun, 8 months 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.

Traister101, 8 months ago

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

Ogeon, 8 months ago (edited 8 months ago)

It may be possible to use the https://doc.rust-lang.org/std/any/trait.Any.html trait to “launder” the value by first casting it to &Any and then downcasting it to the generic type.

<span style="color:#323232;">let any_value = match tmp_value {
</span><span style="color:#323232;">    serde_json::Value::Number(x) => x as &amp;Any,
</span><span style="color:#323232;">    // ...
</span><span style="color:#323232;">};
</span><span style="color:#323232;">
</span><span style="color:#323232;">let maybe_value = any_value.downcast_ref::&lt; T >();
</span>

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.

wulf, 8 months ago

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

I_like_cats, 8 months 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

wulf, 8 months 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!