traversePar.ts

import { pipe } from 'fp-ts/lib/function';
import type { Separated } from 'fp-ts/lib/Separated';
import * as S from 'fp-ts/lib/Separated';
import type { Task } from 'fp-ts/lib/Task';
import type { TaskEither } from 'fp-ts/lib/TaskEither';
import * as TE from 'fp-ts/lib/TaskEither';

export const traversePar =
  <T, E, A>(poolLimit: number, task: (t: T) => TaskEither<E, A>) =>
  (dataList: T[]): Task<Separated<E[], A[]>> =>
  async () => {
    const okList: A[] = [];
    const errList: E[] = [];
    const iterator = dataList.values();

    const workers = Array.from({ length: Math.max(1, poolLimit) }, async () => {
      for (const data of iterator) {
        await pipe(
          task(data),
          TE.match(
            (err) => {
              errList.push(err);
            },
            (ok) => {
              okList.push(ok);
            },
          ),
        )();
      }
    });

    await Promise.all(workers);
    return S.separated(errList, okList);
  };