Add createLens primitive

[nathanbabcock]

Describe The Problem To Be Solved

Utilities for working with nested reactivity in a modular way.

See previous discussion:

• Feature request: Lens like utility for stores solidjs/solid#506
• Feature request: Lens like utility for stores solidjs/solid#944

Originally described by @clinuxrulz:

Say our application state holds a collection of states of things in the tabs of the application. It can be handy to have a Lens to focus on a subset of the state in order to provide a store for use by the UI in the tabs state. So that rhe end-user does not need to alway push state changes to top level (under the hood the state still reaches the top, it is just hidden boiler plate).

Suggest A Solution

Proposed primitives:

• createLens - Given a path within a Store object, return a derived or "focused"
  getter and setter pair.

• createFocusedGetter - The first half of the lens tuple; a derived Signal
  using path syntax on an object.

• createFocusedSetter - The second half of the lens tuple; a Setter
  for a specific path within a Store.

I've started a PR to explore this concept (WIP): #452

It's essentially just syntactic sugar on top of regular Stores, with some tricks to maintain Typescript support.

In previous discussion people theorized about a performance benefit from this approach as well, by caching a partially applied path within the Store object -- for example: solidjs/solid#506 (comment).

I'm not as familiar under the hood of how the Store works, so my goal will be the replicate the intended DX and then unit test thoroughly enough that someone else could come in and work some performance magic. For now it is Lens-like DX with the same behavior/performance as a regular Store.

I'm also curious to get people's thoughts on:

• Does this seem useful?
• Are the types correct?
• What tests are missing?

Cheers 🍺

[nathanbabcock]

Source code: https://github.com/nathanbabcock/solid-primitives/tree/lens/packages/lenses
PR: #452