A reference is a special layout that can be used to avoid directly embedding a value by instead embedding only an identifier to the value.
A reference layout can be built using the & character, followed by the type of reference.
For instance, we define here the Person type where each parent will be referenced in its default layout using the &Person layout.
type Person {
name: required xs:string,
parent: multiple &Person


A reference layout is exported into any type that can store both an IRI or blank node identifier. In most languages, it will be a string.
For instance, the above layout will be exported into the following JSON Schema:
"type": "object",
"properties": {
"name": {
"type": "string"
"parent": {
"type": "array",
"item": {
"type": "string"
"required": [
When the language permits it, a more precise type will be used. For instance in Rust, the Id type of the treeldr-rust-prelude will be used by default, which is defined as follows:
use iref::IriBuf;
use rdf_types::BlankIdBuf;
pub enum Id {
As a consequence, the following Rust type definition is generated for Person:
pub enum Person {
name: String,
parent: BTreeSet<Id>