Why won't Write<R> initialize R if it's missing?

[Jakub Okoński]

I'm kind of missing the point of Write. It requires a Default instance to initialize the resource, but still panics if used without initializing it manually. So what is the difference from WriteExpect?

[Thomas Schaller]

It should not panic, and I never heard of the initialization not working. Do you have a small example?

[Jakub Okoński]

Not on hand, I'll try to reproduce a small one today.

[Jakub Okoński]

OK, I think this is it, on specs 0.12.1:

extern crate specs;

use specs::prelude::*;

pub struct TestData {
    test: Option<u64>,
}

impl Default for TestData {
    fn default() -> TestData {
        TestData {
            test: None,
        }
    }
}

pub struct TestSystem;

impl<'a> System<'a> for TestSystem {
    type SystemData = Write<'a, TestData>;

    fn run(&mut self, mut test_data: Self::SystemData) {
        test_data.test = Some(16);
    }
}

fn main() {
    let mut world = specs::World::new();
    let mut dispatcher = specs::DispatcherBuilder::new()
        .with(TestSystem, "test_system", &[])
        .build();
    dispatcher.dispatch(&world.res);
}

I get this when running:

thread '<unnamed>' panicked at 'Tried to fetch a resource, but the resource does not exist.
Try adding the resource by inserting it manually or using the `setup` method.
You can get the type name of the missing resource by enabling `shred`'s `nightly` feature', /home/kuba/.cargo/registry/src/github.com-1ecc6299db9ec823/shred-0.7.0/src/res/mod.rs:215:48
note: Run with `RUST_BACKTRACE=1` for a backtrace.

If I skip the implementation of Default, it won't compile, but it doesn't seem to use it?

[Thomas Schaller]

Very short answer: `Dispatcher::setup`

…

On Tue, Aug 14, 2018, 16:19 Jakub Okoński ***@***.***> wrote: OK, I think this is it, on specs 0.12.1: extern crate specs; use specs::prelude::*; pub struct TestData { test: Option<u64>, } impl Default for TestData { fn default() -> TestData { TestData { test: None, } } } pub struct TestSystem; impl<'a> System<'a> for TestSystem { type SystemData = Write<'a, TestData>; fn run(&mut self, mut test_data: Self::SystemData) { test_data.test = Some(16); } } fn main() { let mut world = specs::World::new(); let mut dispatcher = specs::DispatcherBuilder::new() .with(TestSystem, "test_system", &[]) .build(); dispatcher.dispatch(&world.res); } I get this when running: thread '<unnamed>' panicked at 'Tried to fetch a resource, but the resource does not exist. Try adding the resource by inserting it manually or using the `setup` method. You can get the type name of the missing resource by enabling `shred`'s `nightly` feature', /home/kuba/.cargo/registry/src/github.com-1ecc6299db9ec823/shred-0.7.0/src/res/mod.rs:215:48 note: Run with `RUST_BACKTRACE=1` for a backtrace. If I skip the implementation of Default, it won't compile, but it doesn't seem to use it? — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#96 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AWFFuX-B0NmhzZtFgqIGTeheolrwFn_gks5uQtyIgaJpZM4V7oEs> .

[Jakub Okoński]

My bad, there's a whole chapter on this in the book 🤦‍♂️