feat: implement sequence helper function in array

[baetheus]

TypeScript still has issues inferring the correct type for:

  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)
  )

Since there is a call for an easy sequence (not over a tuple) of arrays
of various ADTs this commit implements a createSequence function in
array.ts.

  import * as A from "../array.ts";
  import * as O from "../option.ts";
  import { identity, pipe } from "../fns.ts";

  const sequence = A.createSequence(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" }

While it is not used to create an adt specific sequence function for
every ADT it's possible that it might be used thusly in the future.

Fixes #7

[noverby]

I'm not sure how to use the helper function, if you have an array of readonly Eithers returned from Array.map:

const seq = A.createSequence(E.Applicative);
const loadConfigFile = (confPath: string): E.Either<Error, ReadonlyArray<Config>>

F.pipe(
  A.map(loadConfigFile), // Type: readonly E.Either<Error, ReadonlyArray<Config>>[]
  seq,
)

It results in the error:

The type 'readonly Either<Error, readonly Config[]>[]' is 'readonly' and cannot be assigned to the mutable type 'Either<Error, readonly Config[]>[]'

The readonly keyword works fine with the following sequence helper function:

const traverseOption = A.IndexedTraversable.traverse(E.Applicative);
const seq = traverseOption((a: E.Either<Error, ReadonlyArray<Config>>) => a);

[baetheus]

I've modified the signature of the createSequence function, can you check to see that it works with your code now?

[noverby]

@baetheus Now it works, thanks! LGMT