Writing a trait that is generic over lifetimes | Rust Programming Language Community | Page 1

heavy flax Jun 14, 2022, 9:01 PM

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Ah, empty threads

wispy wadi Jun 14, 2022, 9:04 PM

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i am sorry. i realized my question is not fleshed out last min

slightly related question: if I have an associated type, is there a way to constrain that type by its lifetime?

I want this:

trait RequestHandler {
type Response: 'self;
fn get_response(&self) -> Self::Response;
}

but of course self doesn't exist

I can't make get_response return &Self::Response, it may be a static struct or a smart pointer with a lifetime

heavy flax Jun 14, 2022, 9:04 PM

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I think this takes GATs.

wispy wadi Jun 14, 2022, 9:05 PM

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yeah. i can't find an equiv stdlib api which makes me wonder

heavy flax Jun 14, 2022, 9:05 PM

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trait RequestHandler {
  type Response<'this>;
  fn get_response<'this>(&'this self) -> Self::Response<'this>
}

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I think you can elide the lifetime params on the function, but I'd at least keep Self::Response<'_>

wispy wadi Jun 14, 2022, 9:06 PM

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that's GAT, right?

heavy flax Jun 14, 2022, 9:06 PM

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Yup

wispy wadi Jun 14, 2022, 9:06 PM

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wait GAT are stable?

heavy flax Jun 14, 2022, 9:07 PM

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They're close, but I don't think they're stable yet?

wispy wadi Jun 14, 2022, 9:08 PM

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if I changed the trait to work like trait RequestHandler<Response> I think that doesn't buy me anything, right?

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then it's just HKT or something else

heavy flax Jun 14, 2022, 9:09 PM

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Then you'd need

trait RequestHandler<Response<'_>> {
  fn get_response<'this>(&'this self) -> Response<'this>
}
```which is indeed a HKT

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(HKTs are basically type theory speak for generic generics)

wispy wadi Jun 14, 2022, 9:10 PM

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yeah

light briar Jun 14, 2022, 9:18 PM

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heavy flax Then you'd need ```rs trait RequestHandler<Response<'_>> { fn get_response<'th...

it's possible to have rs trait RequestHandler<Response<'resp>> { fn get_response(&'resp self) -> Response<'resp> } and you can use for<'a> RequestHandler<Response<'a>> and stuff

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yeah I just realized...

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the trait needs a lifetime then trait RequestHandler<'resp, Response<'resp>>

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you can do it with an associated type too

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I'm slightly confused

heavy flax Jun 14, 2022, 9:20 PM

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light briar it's possible to have ```rs trait RequestHandler<Response<'resp>> { fn get_res...

Wait, since when is that allowed

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Oh yeah, if you give the trait a lifetime

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No wait, not even then

heavy flax Jun 14, 2022, 9:20 PM

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light briar the trait needs a lifetime then `trait RequestHandler<'resp, Response<'resp>>`

I don't think that's legal.

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You're giving a generic parameter its own generic parameter

light briar Jun 14, 2022, 9:21 PM

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ah no

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right

heavy flax Jun 14, 2022, 9:21 PM

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Maybe Response: 'resp?

light briar Jun 14, 2022, 9:21 PM

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I confused impl with declaration

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Response was a struct in my head

heavy flax Jun 14, 2022, 9:21 PM

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Ah, I see

light briar Jun 14, 2022, 9:22 PM

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anyway, rs trait RequestHandler<'resp> { type Response: 'resp; fn get_response(&'resp self) -> Self::Response } should definitely (maybe) be possible

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if it's not, I'm going crazy

heavy flax Jun 14, 2022, 9:23 PM

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This is possible, yes. It's slightly weaker than GATs, but it might suffice

light briar Jun 14, 2022, 9:23 PM

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or just use nougat ferrisBigBrain

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once it's ready

wispy wadi Jun 14, 2022, 9:27 PM

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i already tried giving the trait a lifetime

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i didn't even grok why that didn't work

light briar Jun 14, 2022, 9:28 PM

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well, it usually works

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but is less ergonomic to use than GATs

wispy wadi Jun 14, 2022, 9:29 PM

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if you have a impl<'a> RequestHandler<'a> for Foo, how do you constrain 'a

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hmm wait

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yeah okay that worked actually, it just caused a lot of problems down the line

#Writing a trait that is generic over lifetimes