A Kind Introduction

7 minute read Published: 2023-11-15

How to leverage the Rust compiler to prevent identifier misuse ?

How to go from ec2ba151-7acf-43a9-bb98-6f5331992f42 in your database to "Cust_ec2ba151-7acf-43a9-bb98-6f5331992f42" in your REST/JSON API ?

How to do it at zero runtime cost, with no boilerplate, with a derive macro ?


Wingback is a B2B SaaS app that helps B2B SaaS companies price, sell and bill their services. It uses a classical architecture : front-end and other API users communicate in REST with a Rust backend.

We have many tables in our PostgreSQL database. All are mapped to Rust structs which are then, more or less directly, exchanged in JSON through our REST API and described in OpenAPI schemas.

In the first few months of our startup, our Rust objects had Uuid fields everywhere: objects were having ids, and were referring to other objects by their ids. And functions were taking several Uuid as arguments.

This was obviously a dangerously loose typing, and made us miss a lot of the Rust safety.

And even if we could have managed this with conventions in our Rust codebase, our API would have still been unclear and examples unhelpful.

We wanted to

We made a library to solve this problem, and we're now making it public: kind.

What it looks like

In database, an id is just your standard UUID, and is initialized db-side (we use gen_random_uuid() but you can use any function or version of UUID).

It looks like ec2ba151-7acf-43a9-bb98-6f5331992f42.

In JSON, URLs, documentations, exports, the identifier is always prefixed.

For example "Cust_ec2ba151-7acf-43a9-bb98-6f5331992f42" for a customer.

This makes all examples in documentation and OpenAPI interfaces obvious. This makes exports self documented. And self-checked.

In Rust, the mandatory type prevents any confusion:

fn get_contracts(
    customer_id: Id<Customer>,
    plan_id: Id<Plan>,

You can't pass the wrong id: the compiler will tell you what type is expected.

What it really is, how it works

We declare a class for each kind of object.

This is done with a derive attribute. To say that a Customer has an id prefixed with Cust_, we write this:

#[derive(Debug, Clone, Serialize, Deserialize, sqlx::FromRow, Identifiable)]
#[kind(class = "Cust")]
pub struct Customer {
    pub name: String,

With just this declaration, you can use the Id<Customer> type:

let id: Id<Customer> = "Cust_371c35ec-34d9-4315-ab31-7ea8889a419a"
    .parse()?; // fail if the prefix isn't right

What the procedural macro really does is it makes Customer implement a special trait, Identifiable, which allows it to parameterize an id.

Here's the generic Id type:

pub struct Id<O: Identifiable> {
    uuid: Uuid,
    phantom: PhantomData<O>,

The PhantomData<O> ensures that Id<A> and Id<B> are different and incompatible types.

As soon as the compiler has checked your code and prevented any misuse, its strips the phantom. This id is exactly like a single UUID, once compiled. This is a zero-cost safety abstraction.

The Identifiable trait is

pub trait Identifiable {
    fn class() -> IdClass;

You never see this trait as Kind library user. It's used by Kind to ensure that two ids aren't mixed in Rust , and provides the "Cust" prefix which is appended on serialization and checked on deserialization.

You could easily implement it yourself, IdClass just wraps a &'static str (eg "Cust"), but it's much simpler to use the derive macro.

So the first part of the magic is combining the PhantomData of Rust which enables a zero-cost sub typing and a trait returning the kind of id.

And the rest of the Kind magic is made of the derive macro and a bunch of generic implementations for Id<O> and Ided<O>.

Identified object

You may have noticed the Customer struct has no id field.

This is very important: we split the id part so that we can both have objects with id (most of the time) and objects without id (e.g. before receiving one).

Of course, they’re of different types.

An object with an id is of type Ided<T>.

For example, loading a customer could be

pub async fn get_customer(id: Id<Customer>) -> Result<Ided<Customer>> {

and storing a just created one would be

pub async fn store_customer(customer: Customer) -> Result<Ided<Customer>> {

The Ided<Customer> has functions returning the &Id<Customer> and the &Customer, but it also dereferences into a Customer so you can directly address its fields and functions.

Just like the id, the identified type can be used in functions and structs, for example

pub struct SomeThing {
    pub id: Id<Customer>, // we refer to the customer here
    pub plan: Ided<Plan>, // the whole plan is wrapped here

Parse/Write, (de)serialize

Parsing an id is as simple as

let id: Id<Plan> = "Cust_ec2ba151-7acf-43a9-bb98-6f5331992f42".parse()?;

(yes, this one will throw an error, it's not a plan)

Deserialize an id or a struct containing ids is as simple with serde as the Id type.

An Ided type is automatically assumed to have an id field on (de)serialization.

So the JSON for an identified customer according to the struct above would be similar to

    "id": "Cust_ec2ba151-7acf-43a9-bb98-6f5331992f42",
    "name": "Alfred Einstein"

You can't deserialize an id whose type doesn't match, it would be a deserialization error.

Query the database

We use sqlx, so we've added some impl to make reading/writing our identifier and identifiable objects transparent and efficient, just as for serialization.

You may have missed the tiny sqlx::FromRow in the Customer definition above, so here it is again:

#[derive(Debug, Clone, Serialize, Deserialize, sqlx::FromRow, Identifiable)]
#[kind(class = "Cust")]
pub struct Customer {
    pub name: String,

This is enough to make Ided<Customer> implement FromRow too: the row just has to contain an id column.

If you use another DB layer, it’s probably easy to add the relevant extension.

And more

The crate comes with more features.

This introduction is too short to detail them all, the documentation is better suited, but here's for a taste of it.

When you use this kind of struct in the Rust ecosystem, you usually need to implement a few standard traits, like PartialEq and Eq, Hash, PartialOrd and Ord, Debug. Kind comes with those implementations.

We also support the cases when you want the user to pass an id, but it could be of any kind among a selection. For example your REST API lets you link two objects, but they may be customers, plans, payments, invoices, etc.

Kind made it possible at wingback, and for our API users, to have safe and clear identifiers.

We think it could help the Rust communities, as it’s quite a frequent concern. In case there are shortcomings for your use case (maybe a different database?), we probably can extend it together.

At the worst, the core idea and this specific combination of PhantomData, an exported class, and a derive macro, could be applied in a different library.