Add sequence functions to Array, Option, Either modules

[noverby]

There are no convenience sequence functions like in fp-ts for doing:

• Option<T>[] -> Option<T[]>
• Either<E,T>[] -> Either<E,T[]>

[baetheus]

This was a decision made early on when working out the type classes in hkts. You can see a discussion on it here. You can create a sequence function for any ADT with an Applicative class by doing this:

import * as A from "https://deno.land/x/fun/array.ts";
import * as O from "https://deno.land/x/fun/option.ts";
import { identity, pipe } from "https://deno.land/x/fun/fns.ts";

const sequence = pipe(
  O.map(identity),
  A.traverse(O.Applicative)
)

console.log(sequence([O.some(1), O.some(2)])) // { tag: "Some", value: [ 1, 2 ] }
console.log(sequence([O.none, O.some(2)])) // { tag: "None" }

I have two reasons for not creating a sequence function for each adt.

1. It's trivial to create that behavior from Array's traverse export.
2. I want to keep cross module imports low (ie. not pulling in all of the array.ts file for people that only want option or either).

I hope that previous issue link and the above code work for you!

[baetheus]

After playing around with the traverse function for awhile I found that the generic is not inferred properly by TypeScript still in the above example. I've implemented a createSequence function that coerces to the correct type in PR #8. Perhaps you wouldn't mind reviewing that?

[noverby]

@baetheus Nice, thanks! I will do a review.