let's say I have a list of items

const list = $state([{ val: 1 }, { val: 2 }, { val: 3}]);

and I want adeeply reactive variable that's derived from it. it must:

• cause an update when I mutate it.
• reset if the original variable changes.

the only way I currently know is using$effect.

function double(item) {  return { val: item.val * 2 }}// workslet doubled = $state(list.map(double));$effect(() => {  doubled = list.map(double);});

$derived doesn't make the variabledeeply reactive, so it does not work:

https://svelte.dev/playground/5f8a2ab8ce1d4834a5ba16fce64fada6?version=latest

// not deeply reactiveconst doubled = $derived(list.map(double));