Default values for missing fields during deserialization

[zesterer]

Hello. Is there a sensible way to have EnumMap use default values for each key if their key is not present when deserializing from a human-readable format?

[KamilaBorowska]

It's possible to manually implement deserialization function that provides a default using serde(deserialize_with) functionality. Hopefully it will help, I may later consider adding functionality to use default values for enum maps to serde_with crate.

use enum_map::{EnumArray, EnumMap};
use serde::de::{Deserializer, MapAccess, Visitor};
use serde::Deserialize;
use std::fmt;
use std::marker::PhantomData;

#[derive(Deserialize, Debug)]
struct DefaultEnumMap(#[serde(deserialize_with = "default_values")] EnumMap<u8, String>);

fn default_values<'de, D, K, V>(deserializer: D) -> Result<EnumMap<K, V>, D::Error>
where
    D: Deserializer<'de>,
    K: Deserialize<'de> + EnumArray<V>,
    V: Deserialize<'de> + Default,
{
    struct EnumMapVisitor<K, V>(PhantomData<(K, V)>);
    impl<'de, K, V> Visitor<'de> for EnumMapVisitor<K, V>
    where
        K: Deserialize<'de> + EnumArray<V>,
        V: Deserialize<'de> + Default,
    {
        type Value = EnumMap<K, V>;
        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            write!(formatter, "a map")
        }
        fn visit_map<M: MapAccess<'de>>(self, mut access: M) -> Result<EnumMap<K, V>, M::Error> {
            let mut entries = EnumMap::default();
            while let Some((key, value)) = access.next_entry()? {
                entries[key] = value;
            }
            Ok(entries)
        }
    }
    deserializer.deserialize_map(EnumMapVisitor(PhantomData))
}

fn main() {
    println!(
        "{:?}",
        serde_json::from_str::<DefaultEnumMap>(r#"{"2": "abc", "5": "def"}"#),
    );
}

[zesterer]

Thanks. I've ended up implementing something similar myself, but I guess there's not really a good way to have this included upstream without breaking the current semantics. Thanks!