$effect that separates tracking from execution ($reaction?)

[xaviergonz]

Describe the problem

In mobx there are several ways to run "effects" - https://mobx.js.org/reactions.html

• autorun - which is the equivalent to $effect
• reaction - allows you to pass two functions
  • a tracked function that returns a value
  • an untracked function that will run if that tracked value changes
  • an options object with things like if the reaction should run immediately or only after a first change, a comparator to compare values returned by the first function, etc.
• when - an auto disposable reaction that runs only once when tracked method returns true (plus a cool overload that turns that into a promise)

solid.js also has something similar to reactions:

• https://docs.solidjs.com/reference/secondary-primitives/create-reaction

that also separates what's tracked from what should run untracked when the tracked value changes

Describe the proposed solution

Although I think this could be implemented in userland with $effect + untracked, probably it would be worth to add a $reaction rune that's already implemented in the framework?

Importance

would make my life easier

[Conduitry]

Duplicate of #9248.