Functor example

functor-union.ts

const selectTopic = functor({
  prompt: ({ query, topics }: { query: string; topics: string }) => `
    User query: ${query}

    Existing topics:
    ${topics}

    Match an existing topic if one fits, otherwise create a new one.
  `,
  returns: [
    {
      type: 'choose_existing',
      description: 'Pick an existing topic that matches',
      schema: z.object({ topicId: z.string() }),
    },
    {
      type: 'create_new',
      description: 'Create a new topic when none match',
      schema: z.object({
        name: z.string(),
        summary: z.string(),
      }),
    },
  ],
});

functor.ts

const extractPatient = functor({
  prompt: ({ text }: { text: string }) => `
    Extract patient info from the text below.
    Return whatever fields are present.

    ${text}
  `,
  returns: {
    description: 'Extract basic patient demographics if present',
    schema: z.object({
      name: z.string().optional(),
      birthDate: z.string().optional(),
      gender: z.string().optional(),
    }),
  },
});

const patient = await extractPatient({ text: documentText });

Need more than one return shape? The prompt stays a typed function, returns becomes an array, and you get a typed discriminated union. Dispatched in prose.

The trick that makes it work: tool calling, used to select and enforce return types.

Each return is a schema plus a short description.

The model thinks it’s choosing an action. We use it to select the shape of the return value.

https://x.com/ninjudd/status/2044098508920369535