value-bag 1.0.0-alpha.9

Anonymous structured values


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Getting started

Add the value-bag crate to your Cargo.toml:

version = "1.0.0-alpha.9"

You'll probably also want to add a feature for either sval (if you're in a no-std environment) or serde (if you need to integrate with other code that uses serde):

version = "1.0.0-alpha.9"
features = ["sval1"]
version = "1.0.0-alpha.9"
features = ["serde1"]

Then you're ready to capture anonymous values!

struct MyValue {
    title: String,
    description: String,
    version: u32,

// Capture a value that implements `serde::Serialize`
let bag = ValueBag::capture_serde1(&my_value);

// Print the contents of the value bag
println!("{:?}", bag);

Cargo features

The value-bag crate is no-std by default, and offers the following Cargo features:

  • std: Enable support for the standard library. This allows more types to be captured in a ValueBag.
  • error: Enable support for capturing std::error::Errors. Implies std.
  • sval: Enable support for using the sval serialization framework for inspecting ValueBags by implementing sval::value::Value. Implies sval1.
    • sval1: Enable support for the stable 1.x.x version of sval.
  • serde: Enable support for using the serde serialization framework for inspecting ValueBags by implementing serde::Serialize. Implies std and serde1.
    • serde1: Enable support for the stable 1.x.x version of serde.
  • test: Add test helpers for inspecting the shape of the value inside a ValueBag.

What is a 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();


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

struct Work {
    id: u64,
    description: String,

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

let bag = ValueBag::capture_sval1(&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).