How to Deserialize a Map into a Vector of Enums with Serde in Rust?

Question

I am working on a Rust project where I need to deserialize YAML configuration data into a more structured Rust type. The YAML data represents output specifications for different data sinks (like CSV and JSON files) and looks something like this:

output_data_specs:
  csv:
    file_path: "/output-data.csv"
    delimiter: ","
  json:
    file_path: "/output/"
    another: "foo"

I want to deserialize this data into a Rust Vec<Sink>, where Sink is an enum that can represent either a CSV sink or a JSON sink:

pub enum Sink {
    Csv(CsvSink),
    Json(JsonSink),
}

pub struct CsvSink {
    file_path: String,
    append: bool,
}

pub struct JsonSink {
    file_path: String,
    another: String
}

I'm looking for a way to implement this with Serde, it should deserialize the map of output data specs map into a list of Sinks in something like:

pub struct InferenceConfig {
    pub name: String,
    pub output_data_specs: Vec<Sink>,
}

I'd preferably like to implement this without a custom deserializer. Any examples or pointers to relevant documentation would be very helpful.

Answer 1

If you can change your Map to a List (simply add - in front of csv and json) — which it seems you want to do, since output_data_specs is a Vec<Sink> — you can use #[serde(with = "serde_yaml::with::singleton_map_recursive")] to use the keys csv and json as enum tags in serde’s default enum representation.

use serde::Deserialize;

#[derive(Debug, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum Sink {
    Csv(CsvSink),
    Json(JsonSink),
}

#[derive(Debug, Deserialize)]
pub struct CsvSink {
    file_path: String,
    append: bool,
    delimiter: String,
}

#[derive(Debug, Deserialize)]
pub struct JsonSink {
    file_path: String,
    another: String,
}

#[derive(Debug, Deserialize)]
pub struct InferenceConfig {
    pub name: String,
    #[serde(with = "serde_yaml::with::singleton_map_recursive")]
    pub output_data_specs: Vec<Sink>,
}

fn main() {
    let s = r#"
        name: "xyz"
        output_data_specs:
            - csv:
                file_path: "/output-data.csv"
                append: true
                delimiter: ","
            - json:
                file_path: "/output/"
                another: "foo"
        "#;
    let inference_config = serde_yaml::from_str::<InferenceConfig>(s).unwrap();
    println!("{inference_config:#?}");
}

InferenceConfig {
    name: "xyz",
    output_data_specs: [
        Csv(
            CsvSink {
                file_path: "/output-data.csv",
                append: true,
                delimiter: ",",
            },
        ),
        Json(
            JsonSink {
                file_path: "/output/",
                another: "foo",
            },
        ),
    ],
}