io.validator.ts

const userSchema: t.type({
  id: t.string,
  name: t.string,
  age: t.number,
  roles: t.array(t.union([
    t.literal('ADMIN'),
    t.literal('GUEST'),
  ])),
});

// ...

const effect$ = r.pipe(
  r.matchPath('/'),
  r.matchType('POST'),
  r.useEffect(req$ => req$.pipe(
    use(requestValidator$({ body: userSchema })),
    // ..
  )));

👇

(typeof req.body) = {
  id: string;
  name: string;
  age: number;
  roles: ('ADMIN' | 'GUEST')[];
};