Datatype-generic lists

lists.js

const Arr = (() => {
  const Nil = [];
  const Cons = head => rest => [head, ...rest];
  const match = ({ Cons, Nil }) => a =>
    a.length === 0 ? Nil : Cons(a[0])(a.slice(1));

  return { Nil, Cons, match };
})();

const GenericList = ({ Cons, Nil, match }) => {
  // Misc
  const length = match({ Nil: 0, Cons: _ => rest => 1 + length(rest) });

  // Functor
  const map = f =>
    match({ Nil, Cons: head => rest => Cons(f(head))(map(f)(rest)) });

  return { length, map };
};

console.log(GenericList(Arr).length([0, 1, 2]));
// 3
console.log(GenericList(Arr).map(x => x * 2)([1, 2, 3]));
// [2, 4, 6]