Add array index as a final argument to createArraySelector combiner

[pbadenski]

In order to make this possible:

const mul = createArraySelector(
  numbers,
  (number, index) => [index, number]
);

This is similar to what's already possible with keys for maps and objects.

[heyimalex]

This was intentional; memoization is bound to a "key" and array index doesn't make sense as a key because re-orders trigger length - index total changes. Changing this would opt everyone into that behavior, since it makes the mapping function dependent on the index and the mapping functions need to be pure. Some more discussion about this choice here.

[pbadenski]

Makes sense. Although it does optimise towards reselect-map in computation use case (which what this library was created for, so that makes sense 😆). I'm plugging in reselect-map for inputArray -> outputArray transformation to ensure output array only changes at particular indices where inputArray also changed and other elements remain unchanged. The behaviour with ordering is actually what I would be expecting.

Hope you can help... What would be the easiest way for me to roll out the custom implementation? Would it be possible to expose a bit more of the API (eg. createMappedSelectorCreator and memoizeMap)?

[heyimalex]

Yeah, we can definitely expose those. I think just exporting and then adding to the typescript definitions is all that needs to be done if you want to submit a pr, or I can try and tackle it sometime this week.

[pbadenski]

cool, I'll have a go at it, thanks!