Jaki będzie wynik i jak TypeScript zapewnia bezpieczeństwo typów w pipe?

type Fn<A, B> = (a: A) => B;

const pipe2 = <A, B, C>(
  f: Fn<A, B>,
  g: Fn<B, C>
): Fn<A, C> => (a) => g(f(a));

const parse = (s: string): number => parseInt(s, 10);
const double = (n: number): number => n * 2;
const format = (n: number): string => `Result: ${n}`;

const transform = pipe2(pipe2(parse, double), format);
console.log(transform("21"));