A record combinator for fp-ts TaskEither

TaskEither_record.ts

import * as te from 'fp-ts/lib/TaskEither';
import * as arr from 'fp-ts/lib/Array';
import { pipe } from 'fp-ts/lib/pipeable';

type TaskEither<A, B> = te.TaskEither<A, B>;

type TERight<T> = T extends TaskEither<any, infer R> ? R : never;

/**
 * Transforms an object of `TaskEither`s into a TaskEither of the resolved objects.
 * Keeping keys and values type safe
 */
export function record<L, T extends { [key: string]: TaskEither<L, any> }>(
  rec: T,
): TaskEither<L, { [key in keyof T]: TERight<(typeof rec)[key]> }> {
  return pipe(
    Object.entries(rec),
    arr.map(([key, value]) =>
      pipe(
        value,
        te.map<any, [keyof T, any]>(x => [key, x]),
      ),
    ),
    arr.array.sequence(te.taskEither),
    te.map(xs => {
      const x: { [key in keyof T]: TERight<(typeof rec)[key]> } = {} as any;
      for (const [key, val] of xs) {
        x[key] = val;
      }
      return x;
    }),
  );
}

usage.ts

// fetchJSON<T>(url): TaskEither<Error, T>

function fetchUserWithPosts(
  userId: string
): te.TaskEither<Error, { user: User, posts: Post[] }> {
  return record({
    user: fetchJSON<User>("/users/1"),
    posts: fetchJSON<Post[]>("/users/1/posts")
  });
}