Type Narrowing in TypeScript Generator Functions: An Example

function demo(foo: string | undefined) {
  return Effect.gen(function* () {
    if (foo == null) {
      yield* Effect.fail(new RuntimeException());
    }
    return foo;
  });
}

function demo(foo: string | undefined) {
  return Effect.gen(function* () {
    if (foo == null) {
      yield* Effect.fail(new RuntimeException());
    }
    return foo;
  });
}