How to update a mutable attribute?

[rirze]

Hi, I'm struggling to find an implementation of a complex_value type that allows for mutation of its attributes. I'm confused on a number of aspects and I was hoping to clear them up with some help. Suppose I want a Value like this:

#[derive(Debug, ProvidesStaticType, NoSerialize, Allocative, Clone, Default, Display)]
#[display("myvalue {:?}", providers)]
struct MyValueGen {
    mapping: HashMap<String, AnotherStruct>,
}

impl MyValueGen {
    fn new() -> Self {
        MyValueGen {
            mapping: HashMap::new(),
        }
    }
}

starlark_complex_value!(MyValue);

How do I implement a method that can be called from starlark that can update the HashMap with a new pair of items. Any attempt I try tells me that I can't get a mutable reference to that attribute.

I've found #126, but the complicated example confuses me here. Do I create another struct that is Mutable w/ a RefCell and use that for all operations? That seems like a lot of boilerplate....

If I could get a hint, then I think it would really help me get started. Thanks!

[ndmitchell]

You will need to use a RefCell for all operations.

Assuming you also want to use this for frozen values (e.g. non-mutable), you'll have to have both a frozen version and a non-frozen version.

And yeah, it is a fair bit of boilerplate. Unfortunately we couldn't figure out a way to abstract over mutability in Rust, so this was the best we could figure out.

[rirze]

I see... It would really help to have an illustrative example.

I'll try to get a working example and create a PR, but if you have a code sample in-mind/already that could be used, I'd love to learn from it.