ordcontramap.ts

export const contramap = <A, B>(f: (b: B) => A, O: Ord<A>): Ord<B> => ({
  compare: (x: B, y: B) => O.compare(f(x), f(y))
});

const stringOrd: Ord<string> = ({
  compare: (x: string, y: string) => x < y ? Ordering.LT : x > y ? Ordering.GT : Ordering.EQ
})

//extending stringOrd to work on Person objects
const firstName: Ord<Person> = contramap(x => x.first, stringOrd);
const lastName: Ord<Person> = contramap(x => x.last, stringOrd);

//combining Ord<Person> instances
const lastThenFirst: Ord<Person> = append(lastName, firstName);

//composing Ord<Person> instances, inverting the lastName sort
const firstThenLastDescending: Ord<Person> = append(firstName, invert(lastName))