How to conditionally deserialize JSON to two different variants of an enum?

Question

Let's say I have JSON data like the following:

{
  "type": "A",
  "value": [ 1, 2, 3, 4, 5 ]
}

{
  "type": "B",
  "value": [ [ 1, 2, 3, 4, 5 ], [ 6, 7, 8 ] ]
}

type determines the type of value, which in the first example is Vec<u32> and in the second is Vec<Vec<u32>>.

If I represent the above data as follows:

enum DataValue {
  TypeA(Vec<u32>),
  TypeB(Vec<Vec<u32>>)
}

struct Data {
  data_type: String,
  value: DataValue
}

How do I implement serde deserialization to properly decode these values?

well... like that xd

Stargateur
– Stargateur

2019-08-25 13:48:04 +00:00
Commented Aug 25, 2019 at 13:48 — Stargateur
– Stargateur, Commented Aug 25, 2019 at 13:48

Sven Marnach · Accepted Answer · 2019-08-24 19:16:12Z

You can deserialize your JSON data directly to an instance of DataValue if you give Serde enough information to know how to do this:

#[derive(Debug, Deserialize)]
#[serde(tag = "type", content = "value")]
enum DataValue {
    #[serde(rename = "A")]
    TypeA(Vec<u32>),
    #[serde(rename = "B")]
    TypeB(Vec<Vec<u32>>),
}

let data_a = r#"
    {
        "type": "A",
        "value": [1, 2, 3, 4, 5]
    }"#;
let a: DataValue = serde_json::from_str(data_a)?;

Playground

If you name your enum variants A and B, you can omit the #[serde(rename = "…")] attributes.

This way of serializing enums is called "adjacent tagging". You can learn about the various options of tagging enums in the Serde documentation on enum serialization.

Your Data struct contains a redundant additional tag data_type. This information is already encoded in the enum, so I don't think you need this. If you need this information as a string, you can add a method to the enum:

impl DataValue {
    fn variant_name(&self) -> &'static str {
        match self {
            DataValue::TypeA(_) => "A",
            DataValue::TypeB(_) => "B",
        }
    }
}

edwardw · Accepted Answer · 2019-08-24 19:08:26Z

5

Fortunately serde has build-in support for enum type:

//# serde = { version = "1.0.99", features = ["derive"] }
//# serde_json = "1.0.40"

use serde::Deserialize;

#[derive(Deserialize, Debug)]
#[serde(tag = "type")]
enum Data {
    A { value: Vec<u32> },
    B { value: Vec<Vec<u32>> },
}

fn main() {
    let a: Data = serde_json::from_str(r#"{"type": "A", "value": [ 1, 2, 3, 4, 5 ]}"#).unwrap();
    let b: Data =
        serde_json::from_str(r#"{"type": "B", "value": [[1, 2, 3, 4, 5], [6, 7, 8 ]]}"#).unwrap();

    println!("{:?}", a);
    println!("{:?}", b);
}

answered Aug 24, 2019 at 19:08

edwardw

14.2k4 gold badges44 silver badges54 bronze badges

Comments

tricknology · Accepted Answer · 2019-08-24 15:12:53Z

-7

This may be a person opinion but I would generally try to avoid serialization / deserialization on enums.

Would this not be close to the same thing in C++?

struct DataValue final{
  TypeA(Vec<u32>) final,
  TypeB(Vec<Vec<u32>>) final
}

answered Aug 24, 2019 at 15:12

tricknology

1,1481 gold badge15 silver badges27 bronze badges

2 Comments

trent Over a year ago

That's... not even legal C++? It's not clear to me what argument you're making besides "don't do that", which is not an answer to the question.

Listerone Over a year ago

This neither fits the expression of the problem nor semantically make sense. Even after pretending it is valid C++, it would say that you have both TypeA and TypeB data valid. This represents a huge class of bugs in software. Please unlearn this and do not perpetuate this.

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