Deserialize json into enum

I am trying to deserialize a json but i am unable to get the expected result. Below is my code:

use serde::{Serialize, Deserialize};

#[derive(Debug, Clone, Serialize, Deserialize)]
struct StudentData{
    stu1: TestResult,
    stu2: TestResult,
    stu3: TestResult,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(untagged)]
enum TestResult{
    #[serde(rename = "fail")]
    Fail,
    #[serde(rename = "absent")]
    Absent,
    Pass(String)
}


fn main() {
    let json_string = "{\"stu1\":\"50\",\"stu2\":\"fail\",\"stu3\":\"absent\"}";
    let s: StudentData = serde_json::from_str(json_string).expect("should provide student data");
    println!("{:#?}", s);
}

The output

StudentData {
    stu1: Pass(
        "50",
    ),
    stu2: Pass(
        "fail",
    ),
    stu3: Pass(
        "absent",
    ),
}

What i am trying to get:

StudentData {
    stu1: Pass(
        "50",
    ),
    stu2: Fail,
    stu3: Absent,
}

Is this doable?

>Solution :

This behaviour surprised me a bit. Apparently, if you have unit variants in an untagged enum, they get serialized as a null JSON value.

println!("{}", serde_json::to_string(&TestResult::Fail).unwrap());

prints

null

I presume serde expects a null on deserialize likewise. You can get around this by not making the entire enum untagged, but only the Pass variant.

#[derive(Debug, Clone, Serialize, Deserialize)]
enum TestResult {
    #[serde(rename = "fail")]
    Fail,
    #[serde(rename = "absent")]
    Absent,
    #[serde(untagged)]
    Pass(String),
}

(I’m not entirely sure I like your representation, btw. Why is the number of points a string, and not a number? And it should really have its own tag, like "pass": {"score": 50}. (Rounding might be a concern if you have half points. (rel: arbitrary_precision)))