test.ts

const ref = <T>(f: (x: T) => T) => {
  const r = { val: f };
  return () => r;
};
const id = <T>(x: T) => x;

// r: <T>() => { val: (x: T) => T }
const r = ref(id);
// unsafe instantiations of r
const r1 = r<number>();
const r2 = r<string>();
// update r1.val with a function specific to numbers
r1.val = (x: number) => x * 2;
// using r2.val will cause runtime failure
r2.val("foo");