How can I deserialize JSON with a top-level array using Serde?

Question

I have a some JSON data that is returned from a web service. The JSON is a top-level array:

[
    {
        "data": "value1"
    },
    {
        "data": "value2"
    },
    {
        "data": "value3"
    }
]

Using serde_derive to make structs I can can deserialize the data contained within the array, however, I am unable to get Serde to deserialize the top-level array.

Am I missing something, or can Serde not deserialize top level-arrays?

score 53 · Accepted Answer · edited Jun 25 '21 at 18:19

53

You can simply use a Vec:

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, Debug)]
struct Foo {
    data: String,
}

fn main() -> Result<(), serde_json::Error> {
    let data = r#"[
        {
            "data": "value1"
        },
        {
            "data": "value2"
        },
        {
            "data": "value3"
        }
    ]"#;

    let datas: Vec<Foo> = serde_json::from_str(data)?;

    for data in datas.iter() {
        println!("{:#?}", data);
    }

    Ok(())
}

If you wish, you could also use transparent:

#[derive(Serialize, Deserialize, Debug)]
#[serde(transparent)]
struct Foos {
    foos: Vec<Foo>,
}

let foos: Foos = serde_json::from_str(data)?;

This allows to encapsulate your data with your type.

edited Jun 25 '21 at 18:19

jonasbb

2,131
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answered Jun 18 '17 at 00:33

Stargateur

24,473
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What if the input JSON body is not an array? Is there any general way to handle it? Thansk! – prehistoricpenguin Jun 25 '21 at 09:55
@prehistoricpenguin there is no more simple https://github.com/serde-rs/json#parsing-json-as-strongly-typed-data-structures – Stargateur Jun 25 '21 at 11:55
Thank you, I just find the the answer in issue discussion, We can use `let foos: serde_json::Value = serde_json::from_str(data)?;` – prehistoricpenguin Jun 25 '21 at 14:46
But how do you parse this file efficiently if it's 10 GB size ? – R Sun Aug 03 '21 at 18:33

How can I deserialize JSON with a top-level array using Serde?

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