Dynamic BSN expressions generate a macro error

[kitsunecafe]

Bevy version and features

bevy 0.19.0 with default features

What you did

tried to reproduce the dynamic expression behavior from the scene notation docs

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .run();
}

#[derive(Component, FromTemplate)]
pub struct Health {
    pub current: u32,
    pub max: u32,
}

#[derive(Component, FromTemplate)]
pub struct Sprite {
    pub image: Handle<Image>,
}

fn enemy(hp: u32, name: &str) -> impl Scene {
    let name_string = name.to_string();
    bsn! {
        #{name}
        Health { current: {hp / 2}, max: hp }
        Sprite { image: {name_string + ".png"} }
    }
}

fn setup(mut commands: Commands) {
    commands.spawn_scene(bsn! { enemy(200, "goblin") });
}

What went wrong

error: expected identifier
  --> src/main.rs:24:10
   |
24 |         #{name}
   |          ^
error: could not compile `dynamic_expr_repro` (bin "dynamic_expr_repro") due to 1 previous error

i'm not sure if this is meant to be a currently supported feature or if it's not yet fully implemented and scheduled for a full release, but it may be helpful if the docs reflected that if so

[mansiverma897993]

@kitsunecafe @kfc35 I have raised #24787 addressing this issue look at once and lemme know if any changes required !!

[laundmo]

I've written a reply/review of the PR #24787 which explains why we decided to remove this originally intended feature during development.

The short version is: Thers no way to make it work well within the constraints we have set (compile-time hashing possible), and theres a relatively simple workaround, patching the Name component after creating it using a #Name entity reference:

bsn! {
  #Root
  Name({name})
}

I want to note: The linked docs talk about using {} for scene or component value expressions, not #Name expressions.

[laundmo]

I suggest changing the title to: "Implement dynamic name references" or something similar, to clarify this is not about normal expressions in bsn. I've also adjusted the tags to this needing design, as we're not fundamentally opposed to this, just missing a satisfactory implementation.