fp-ts FoldMap example

fp-ts.foldMap.ts

import * as A from "fp-ts/lib/Array";
import * as M from "fp-ts/lib/Map";
import { eqNumber } from "fp-ts/lib/Eq";

//My domain type
type User = {
  name: string;
  age: number;
};

//Get the monoid for Map<number, User[]>
//Notice how I don't have to define anything to do with combining Maps
//or how to combine User[]s
const monoidMapAgeUser = M.getMonoid(eqNumber, A.getMonoid<User>());

const foldMap = A.array.foldMap(monoidMapAgeUser);

//Now all we have to do is turn a single User into a 1 element Map<number, User>
//and the monoid will take care of combining the Map and Array parts
const groupByAge: (users: User[]) => Map<number, User[]> = users =>
  foldMap(users, user => M.singleton(user.age, [user]));

console.log(groupByAge([]));