Finally, I managed to implement a proof of concept of serde-select. But lets start at the beginning.

The Problem

The problem I tried to solve with this crate is rather simple: You need to be able to get values from a serde-compatible document (e.g. toml, json, yaml, ...) but you don't know the full schema of the document at compiletime of your crate.

The origin of the idea of serde-select was when I first started working on my imag project, where a lot of seperated crates coexist in one ecosystem, but all of them should be configured in one big configuration file. Of course I did not want to have one central crate just for defining the schema, especially since a user might not want to use all functionality from the ecosystem, thus not having a “full” configuration file, but only the parts they needed.

So I started writing “toml-query”, a crate which lets the programmer query a toml::Value with a “path”. For example:

list_format = "{{lpad 5 i}} | {{abbrev 5 uid}} | {{summary}} | {{location}}"
show_format = """
{{i}} - {{uid}}

music = "/home/user/music"
contacts = "/home/user/contacts"
calendars = "/home/user/calendars"

The document looks like this, but in the program code we only need calendar and its sub-values. So we can do

let r ="calendar.list_format");

in the code and get a Result<Option<&'document Value>> value back.

toml-query evolved over the time, now featuring more flexibility by implementing “Partials”, how I call them. These are structs that are Serialize + Deserialize and have a path attached to them, so deserializing the partial document is possible right away:

let r: Result<Option<CalendarConfig>, _> = document.read_partial::<CalendarConfig>();

where CalendarConfig: Serialize + Deserialize + Debug + toml_query::Partial (see here).

The evolution

toml-query works perfectly fine and I use it in my other projects a lot. It is fast and easy to use. Its error reporting is nice.

But an idea formed in the back of my head and I did not stop to think about it.

Can toml-query by generalized to work with all formats serde can handle?

So I started to experiement with a more general implementation: serde-select was born.

And today I managed to get the first bits working.

Meet serde-select

serde-select implements a “read” functionality for both JSON and TOML, depending on what features you enable. The inner implementation of the resolve-algorithm is agnostic of the actual format.

For a quick overview how to use the crate right now, have a look at the tests for toml for example.

I strongly advice against using this crate, though. It is only an experiement for now and shouldn't be used in production code. Nevertheless, I published the first preview on

tags: #rust #programming