TypeScript Linked List (Haskell/OCaml)

list.ts

type Item<T> = { val: T, rest: List<T> } | null;
type List<T> = Item<T>;

const cons = <T>(val: T, rest: List<T>) => ({ val, rest });
const empty = () => null;
const fold_left = <T, A>(func: (acc: A, val: T) => A, list: List<T>, acc: A): A => {
  //poor man's pattern matching
  switch (list) {
    case empty(): return acc;
    default: {
      const { val, rest } = list;
      return func(fold_left(func, rest, acc), val);
    }
  }
};
const filter = <T>(func: (val: T) => boolean, list: List<T>): List<T> =>
  fold_left((acc: List<T>, val: T) => func(val) ? cons(val, acc) : acc, list, empty());
const map = <T, N>(func: (val: T) => N, list: List<T>): List<N> =>
  fold_left((acc: List<N>, val: T) => cons(func(val), acc), list, empty())


const list: List<number> = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  .reduce((lst, val) => cons(val, lst), empty());

const filtered_and_mapped = map(i => i.toString(), filter(i => i % 2 === 0, list)); //[]
const js_array = fold_left((a, i) => [...a, i], list, []); // [1,2,3,4,5]
const sum = fold_left((a, i) => a + i, list, 0);           // 15