A potential solution to the current recursive schema limitation.

[iwoplaza]

Hey everyone! I implemented a way of defining type-safe recursive schemas for my binary library, and I was wondering if the same could be implemented in Zod to eliminate the current limitation stated in docs 👀.

Essentially, the technique I am proposing would involve a level of indirection when defining such schemas. Here is what that could look like (based on the example by crasite).

const Category = z.indirect('category', (Category) => // shadowing for convenience
//                                       ^? z.ZodIndirectRef<'category'>
  z.object({
    name: z.string(),
    subcategories: array(Category),
  })
);

type CategorySchema = typeof Category;
//   ^? z.ZodIndirect<'category', z.ZodObject<{ name: z.ZodString, subcategories: z.ZodArray<z.ZodIndirectRef<'category'>> }>>
// (some of the generic type params omitted for brevity)

// The z.infer type function keeps track of which key corresponds to which indirect schema. This can be
// done by adding a new generic type parameter that defaults to an empty record, but allows schemas
// to register ZodTypes under specific keys when traversing the tree, calling z.infer recursively with the extended record.
// - if a z.ZodIndirect<'key', T> is found, register T under 'key' for recursive calls.
// - if a z.ZodIndirectRef<'key'> is found, retrieve the schema type from the record.
type Category = z.infer<typeof Category>;
//.  ^? { name: string, subcategories: ...[] }

categorySchema.parse({
  name: "People",
  subcategories: [
    {
      name: "Politicians",
      subcategories: [
        {
          name: "Presidents",
          subcategories: [],
        },
      ],
    },
  ],
}); // passes

This can be seen in action in an equivalent typed-binary example: https://stackblitz.com/edit/vitejs-vite-ngngr1?file=src%2Flinked-list.ts (or as seen on the screenshot)

Potential drawbacks

This does increase the complexity of the TypeOf, input, and output generic functions, requiring them to traverse the '_output' and '_input' types recursively.

[iwoplaza]

As recursive schemas are already planned for Zod 4, I will be closing this discussion 🙏