Type checking specific entities types

[DGriffin91]

I'm trying to figure out if there is a better way to require a specific entity type. With the specific type being in an attribute of the entity I haven't been able to figure out a good way to do this yet.

This is currently a pattern I am using:

fn do_something_with_polylines(polyline: Entity) {
    let specific = if let EntityType::LwPolyline(ref specific) = polyline.specific {
        specific
    } else {
        panic!("Must use LwPolyline in box_kind()")
    };
}

Is there a better way to do this? I was hoping for something along these lines (Though I can see why it wouldn't work this way):

fn do_something_with_polylines(polyline: Entity<EntityType::LwPolyline>)

Sorry if this is something obvious as I'm fairly new to Rust.

In many cases I need have a function be able to access things in the .common attribute (Like what layer the Entity) so it is not practical to only pass the .specific attribute to the function.

[brettfo]

As far as I'm aware there's not a way to pattern match in the function signature like you suggested. Even if there was a bare enum field on the root object, you'd still have to check it in the function body like you're doing in your example.

[DGriffin91]

In case it's on any interest to you, I ended up doing this: https://gist.github.com/DGriffin91/77390c206a841fc9b87ab0a11df152d8

I'm just changing the layout of the types.

I'm not really sure if this is the best way to do this, and it feels a bit excessive. But it's made the rest of my code base much cleaner. And thanks to multiple cursors in VSCode it was easy to implement.

I can then have functions like this:

fn do_something_with_polylines_and_inserts(polylines: &ddxf::DLwPolyline, inserts: ddxf::DInsert) -> ddxf::DInsert

I just convert types on when I import:

let mut dwg = Drawing::load_file("some_file.dxf").expect("");
let mut entities = ddxf::convert_entities(dwg);

Then extract specific types:

pub fn polylines(entities: &Vec<ddxf::DEntity>) -> Vec<ddxf::DLwPolyline>

[brettfo]

Thanks for looking at this. I'll experiment a bit to see if inverting the type composition makes other scenarios easier/harder.

[DGriffin91]

Thanks for looking into this!

I haven't played around with it, but I think it might be even better if it was structured like this with the specific fields directly in the main struct:

struct Line {
    thickness: f64,
    p1: Point,
    p2: Point,
    extrusion_direction: Vector,
    common: EntityCommon,
}

I'm not entirely sure since I am fairly new to Rust and this crate in particular. And I'm obviously much more aware of the implications in my own use of the crate.