How do I update stores from reactive declaration updates

Route.html

{#if match !== null}
  <slot></slot>
{/if}

<script>
import { getHistory, matchPath, ROUTING_CONTEXT } from './index.js';
import { writable } from 'svelte/store.js';
import { setContext, onDestroy } from 'svelte';
const history = getHistory();
const matchStore = writable(null);
setContext(ROUTE_MATCH, matchStore);

export let path = '';
export let exact = false;
export let strict = false;

let pathname = history.location.pathname;
let match;
// How do I update matchStore with this derived value?
$: match = matchPath(pathname, { path, exact, strict });

const unlisten = history.listen(location => {
  pathname = location.pathname;
});

onDestroy(unlisten);
</script>