value-bag

A ValueBag is an anonymous structured bag that supports casting, downcasting, formatting, and serializing. The goal of a ValueBag is to decouple the producers of structured data from its consumers. A ValueBag can always be interrogated using the consumers serialization API of choice, even if that wasn't the one the producer used to capture the data in the first place.

Say we capture an i32 using its Display implementation as a ValueBag:

let bag = ValueBag::capture_display(42);

That value can then be cast to a u64:

let num = bag.as_u64().unwrap();

assert_eq!(42, num);

It could also be serialized as a number using serde:

let num = serde_json::to_value(bag).unwrap();

assert!(num.is_number());

Say we derive sval::Value on a type and capture it as a ValueBag:

#[derive(Value)]
struct Work {
    id: u64,
    description: String,
}

let work = Work {
    id: 123,
    description: String::from("do the work"),
}

let bag = ValueBag::capture_sval(&work);

It could then be formatted using Display, even though Work never implemented that trait:

assert_eq!("Work { id: 123, description: \"do the work\" }", bag.to_string());

Or serialized using serde and retain its nested structure.

The tradeoff in all this is that ValueBag needs to depend on the serialization frameworks (sval, serde, and std::fmt) that it supports, instead of just providing an API of its own for others to plug into. Doing this lets ValueBag guarantee everything will always line up, and keep its own public API narrow. Each of these frameworks are stable though (except sval which is 1.0.0-alpha).