Status: implemented

A schema can use itself by being a thunk that names itself:

const tree = () => ['const', { value: number, children: array(tree) }]

A thunk is a function returning a descriptor, ['const', c] here, and the readers call it only when they reach it, so the definition can mention tree before tree is finished. A constant inside a recursive definition is wrapped the same way, () => ['const', c], which keeps every name the definition uses a function.

The data form turns each recursive definition into one named rule, which the JSON Schema writer emits once under $defs and refers to with $ref.

Ts<> of a recursive schema can exceed TypeScript's instantiation depth (TS2589); ts/README.md describes the WithOut phantom output type that avoids it.

A sub-schema used in two places is one value, as any JavaScript value named by a const is:

const address = { street: string, city: string }
const order = { billing: address, shipping: address }

The demo's graph draws address once, with an edge per use. A value can share a member the same way, and the two readers treat that differently: validate hands back the value it was given, sharing included, while parse builds a fresh copy for each use.

The demo on this page runs the Recursion lesson, an arrow back to tree's definition where it uses itself, and the Shared parts lesson, where address is used twice and drawn once; the drop-down switches between them. The lessons are defined in sample.f.mjs, and the module page's demo lists them among all the others.

• TypeScript: recursive type aliases
• JSON Schema: $defs and $ref (recursion)

Thunk and Info in types.ts; toData's named rules in data; WithOut in ts/types.ts.