Fallible defaults for non-present fields

[junbl]

I want to be able to get configuration from a file, where individual fields can also be presented as environment variables.

The closest things to what I want are these attribute macros:

1. deserialize_with: as far as I can tell, this isn't run at all if the field isn't present
2. default: has to be infallible

So either a version of deserialize_with that is run even if the field isn't present at all (has some logic that can product a value in spite of this), or a version of default that returns a Result. If the field isn't present, and the default fails, that error will be propagated up to the deserialization caller.

Currently I'm using a default that panics if the values aren't found. This is fine for my current use case, but I'd like to be able to more gracefully handle that error.

[jofas]

Hi there,

I've a somewhat similar problem when a field is missing and I want to use deserialize_with. When using serde_json, I'm accustomed to the fact that fields of type Option<T> can be omitted from the json string if they are null and deserialization will still work fine. If I provide a deserialize_with function, this behavior is disabled and parsing fails for fields that are omitted.

Here an example from the playground:

use serde::Deserialize;

#[derive(Deserialize)]
struct Foo {
    #[serde(deserialize_with = "deserialize_wrapper")]
    bar: Option<bool>,
}

fn deserialize_wrapper<'de, D>(
    d: D,
) -> Result<Option<bool>, D::Error>
where
    D: serde::de::Deserializer<'de>,
{
    Option::deserialize(d)
}

fn main() {
    let json = "{}";

    let f: Foo = serde_json::from_str(json).unwrap();

    assert_eq!(f.bar, None);
}

The wrapper function does nothing really, but parsing the empty json object panics in line 21 with the following error:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error("missing field `bar`", line: 1, column: 2)', src/main.rs:21:45

If you comment out the deserialize_with field attribute in line 5, parsing works as expected.

[jsscheller]

I believe adding #[serde(default)] to the optional field will fix the issue. See this related issue: https://github.com/serde-rs/serde/issues/1728#issuecomment-579480080

[jofas]

Yes, adding the #[serde(default)] tag solves my problem, thanks

[kangalio]

I want to be able to get configuration from a file, where individual fields can also be presented as environment variables.

The closest things to what I want are these attribute macros:

1. `deserialize_with`: as far as I can tell, this isn't run at all if the field isn't present

2. `default`: has to be infallible

So either a version of deserialize_with that is run even if the field isn't present at all (has some logic that can product a value in spite of this), or a version of default that returns a Result. If the field isn't present, and the default fails, that error will be propagated up to the deserialization caller.

Currently I'm using a default that panics if the values aren't found. This is fine for my current use case, but I'd like to be able to more gracefully handle that error.

I propose #[serde(try_default = function)], where function is a function with no arguments returning Result. All error types that serde::de::Error::custom accepts should be supported, i.e. everything that implements Display.

[dtolnay]

I think I would prefer not to build something for try_default into serde_derive. But if someone wanted to put together a more fully featured derive macro as a separate library, plenty of people would be excited about that.