Custom deserialization of an array or array of values using Serde

I would like to build a custom Serde deserializer to deserialize an array of array of values into a Vec<Child> where I have already written a custom deserializer to parse an array of values into a Child.

What would be the best way to do that?

Thanks in advance for the help :slight_smile:


As an illustration Iā€™m trying to make something like this work

struct Parent {
    #[serde(deserialize_with = "parse_child")]
    field: Vec<Child>,
}

where the below already exists

fn parse_child<'de, D>(deserializer: D) -> Result<Child, D::Error>
where
    D: Deserializer<'de>,
{
    struct ChildParser;
    impl<'de> Visitor<'de> for ChildParser
    {
        type Value = Child;

        fn visit_seq<A: SeqAccess<'de>>(self, seq: A) -> Result<Self::Value, A::Error> {
            // create a Child object
        }
    }

    deserializer.deserialize_any(ChildParser{})
}

Sample input I deserialize
[[49, 11.75, 0], [42, 9, 1]]

With a workable example and link to a playground

One idea would be to add a customer deserializer for a Vec<Child> directly but I was wondering whether a more elegant solution would exist?

extern crate serde_json; // 1.0.32
extern crate serde; // 1.0.80
#[macro_use] extern crate serde_derive;

use serde::de::{Deserializer, SeqAccess, Visitor};
use std::fmt;

#[derive(Debug, Deserialize)]
struct Parent {
    #[serde(deserialize_with = "parse_child")]
    single: Child,
    //#[serde(deserialize_with = "parse_child")]
    //array: Vec<Child>,
}

#[derive(Default, Debug, Deserialize)]
struct Child {
    a: u64,
    b: f32,
    c: usize,
}

fn parse_child<'de, D>(deserializer: D) -> Result<Child, D::Error>
where
    D: Deserializer<'de>,
{
    struct ChildParser;
    impl<'de> Visitor<'de> for ChildParser
    {
        type Value = Child;

        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            formatter.write_str("[u64, f32, usize]")
        }
        
        fn visit_seq<A: SeqAccess<'de>>(self, mut seq: A) -> Result<Self::Value, A::Error> {
            println!("In custom deserializer");
            let mut child = Child { ..Default::default() };
            
            let tmp = seq.next_element::<u64>()?;
            if let Some(a) = tmp {
                child.a = a;
            };
            
            let tmp = seq.next_element::<f32>()?;
            if let Some(b) = tmp {
                child.b = b;
            };
            
            let tmp = seq.next_element::<usize>()?;
            if let Some(c) = tmp {
                child.c = c;
            };
            
            Ok(child)
        }
    }

    deserializer.deserialize_any(ChildParser{})
}

fn main() {
    let child_data = r#"[49, 11.75, 0]"#;
    let child : Child = serde_json::from_str(child_data).unwrap();
    println!("Child = {:?}", &child);

    let parent_data = r#"{"single": [49, 11.75, 0]}"#;
    let parent : Parent = serde_json::from_str(parent_data).expect("to be able to deserialize it");
    println!("Parent = {:?}", &parent);

}