Derived store function with no arguments is never set

[jacwright]

In the following example, the value t is never set, it remains undefined.

import i18next from 'i18next';

export const language = writable(i18next.language || '');
language.subscribe(value => i18next.changeLanguage(value));

// Update the function because internally the language has been updated
export const t = derive([ language ], () => i18next.t.bind(i18next));

The derived function is run, but the value is never set.

[Rich-Harris]

Thanks for the PR. Admit it took me a few beats to understand what was going on — I have to wonder (and there may be a good reason; I haven't used i18next — maybe it's needed outside) if it should be this instead?

import i18next from 'i18next';

export const language = writable(i18next.language || '');

export const t = derive([language], language => {
  i18next.changeLanguage(language);
  return (key, options) => i18next.t(key, options);
});

[Rich-Harris]

goddammit, i didn't mean to close that, i meant to delete my comment. second time i've done that this week. (i changed my mind, i don't think derive callbacks should have side-effects unless it's unavoidable)