Contextual stores

[Rich-Harris]

Currently, a store referenced with $ must be declared at the top level. But you might want to have an array of stores:

{#each todos as todo}
  <div>
    <input type=checkbox bind:checked={$todo.done}>
    {$todo.description}
  </div>
{/each}

[pngwn]

Closing in favour of #4079 as there is more discussion in that issue, even though it came later.