How to wrap an existing readable store to add extra parameters?

I am new to Svelte. I'm trying to wrap a small wrapper around svelte-i18n to allow relative translation key.

This is how the library can be used:

<script>
import { _ } from 'svelte-i18n';
</script>

<p>{$_('absolute_key')}</p>

What I would like to achieve

<script>
import { tr } from "my_wrapper.js"

const _ = tr("base.path.relative");
</script>

<p>{$_('.relative_key')}</p>

Closest I was able to achieve is

// my_wrapper.js
import { _ } from 'svelte-i18n';

export const tr = (basePath) => (path, options = {}) => {
  let returnVal;
  if (path.startsWith(".")) {
    _.subscribe((t) => returnVal = t(`${basePath}${path}`, options));
  } else {
    _.subscribe((t) => returnVal = t(path, options));
  }
  return returnVal;
}

It kinda work but I have to omit the $ sign in {$_('.relative_key')}. Is there a way to keep its usage with a dollar sign as if I was using the default API?

Solution

I have implemented exactly that before, you just have to return a function from the store. Looks something like this:

// better name for _
import { format } from 'svelte-i18n';

export function section(path) {
    const { subscribe } = derived(
        format,
        $format => (idOrMessage, options) => {
            if (typeof idOrMessage == 'string')
                return $format(`${path}.${idOrMessage}`, options);

            return $format(
                {
                    ...idOrMessage,
                    id: `${path}.${idOrMessage.id}`
                },
                options,
            );
        },
    );

    return {
        path,
        subscribe,
        // allows chaining
        section: (subPath) => section(`${path}.${subPath}`),
    }
}

Would recommend not including the joining . anywhere except the internals, it's pretty ugly otherwise.

<script>
  // ...
  const s = section('section');
  const sub = s.section('subSection');
  // or
  const sub2 = section('section.subSection');
</script>

Should be equivalent:
{$_('section.subSection.value')}
{$s('subSection.value')}
{$sub('value')}
{$sub2('value')}

REPL