Aanish Sachdev

01 the origin: API drift

I learned this wiring a TypeScript frontend to a separate backend through an OpenAPI contract. (the pattern came from CoralEHR's frontend/backend contract work)

// the backend renamed a field:
type User = { id: string; displayName: string };

// the frontend still reads the old name:
user.name
//   ~~~~ Property 'name' does not exist on type 'User'.

TypeScript turns a rename into a compiler-guided checklist.

02 what TypeScript achieves: impossible states

type State = {
  loading: boolean;
  error?: string;
  user?: User;
}; // loading + error + user can all be true at once

type State =
  | { status: "loading" }
  | { status: "error"; message: string }
  | { status: "success"; user: User };

Good types make invalid states unrepresentable, a class of tests you never write.

03 how a compiler thinks: type-loupe

Edit the code. A hand-built checker runs live in your browser. No TypeScript shipped; type-loupe is its own tiny parser and checker.

type User = { name: string };
let user: User = { name: 42 };

04 the agent loop: model drafts, compiler checks

Watch it run: the agent writes FHIR, the compiler rejects it, the agent patches the code with fhir-craft, and the check passes. (fixture replay; real tool output; no live model)

import { createPatient, humanName } from "fhir-craft";

const patient = createPatient({
  given: "John", family: "Smith",
});

Animation: an agent drafts a FHIR Patient with name as a string; the compiler reports it is not assignable to HumanName[]; the agent reasons and rewrites it with fhir-craft's createPatient; the type check passes. Loops continuously.