Hands-on Rust: How do I debug a call chain in Rust?

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Hi,

Thanks for reading the book!

Unfortunately, Legion (the ECS) is responsible for the World access (which is what the read_component is handling) - and doesn’t flag that one at compile time. I really wish it did!

The issue can be caught by changing the if let. if let is like a single-case match, so it’s only running the enclosed code if get_component_mut succeeds - and continues on its way if it fails.

get_component_mut returns a Result type. So you have a few options for handling this:

The simplest way, not very specific:

if let Ok(mut health) = ecs.entry_mut(*victim).unwrap().get_component_mut::<Health>() {
} else {
     // Print an error here
}

Using match:

match ecs.entry_mut(*victim).unwrap().get_component_mut::<Health>() {
    Ok(mut health) => { // do the health deduction }
    Err(msg) => { // Handle the error, msg will tell you what went wrong }
}

Or you can choose to crash when things go wrong (surprisingly helpful for debugging):

let mut victim = ecs.entry_mut(*victim).unwrap().get_component_mut::<Health>().unwrap();
// Health deduction code

Hope that helps!

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