Import structs from a library

[Leastrio]

Is there a way to import structs from another library in rust to elixir without having to rewrite the entire struct again? Been looking through the docs and cant find much on this.

[filmor]

This is a general (intentional) limitation of Rust's trait system: To implement a trait for a type in a crate, you have to either implement the trait or the type in the same crate. In this case, you have a type from a foreign crate SomeType and the traits come from Rustler (rustler::Encoder and rustler::Decoder).

The "common" approach to this for "non-generated" trait implementations is to wrap the type in a new-type (a tuple struct with a single element, pub struct NewSomeType(SomeType);), accompany it with a impl From<SomeType> for NewSomeType and implement the required trait on it.

That won't help you here, as the derive macros that provide the implementations for Encoder and Decoder need to "see" the shape of the struct to convert on the syntax-level.

The best I can think of right now is to implement Encoder and Decoder manually on a wrapped newtype.

[Leastrio]

Hmm okay im not sure if thats viable as im looking to convert a LOT of types over from this one library to elixir but maybe I need to look at this in another way

[sachanganesh]

@filmor Thank you for the thorough explanation!

I know the discussion may be stale, but I was wondering if you could provide a simple example of how this manual implementation of Encoder and Decoder would be done.

// external struct 
pub struct TestStruct {
    pub year: usize,
    pub month: usize,
}

struct Wrapper(TestStruct);

impl Encoder for DateTime {
    fn encode<'b>(&self, env: Env<'b>) -> Term<'b> {
        // I doubt this is correct

        match elixir_struct::make_ex_struct(env, "ICalendar.DateTime") {
            Ok(term) => term,
            Err(err) => panic!("{:#?}", err),
        }
    }
}

impl<'a> Decoder<'a> for DateTime {
    fn decode(term: Term<'a>) -> NifResult<Self> {
        // unsure
    }
}