const $messages = createStore([])
const $users = createStore([])
const getUser = createEvent()
const $user = createStore(null)
sample({
source: $users,
clock: getUser,
fn: (users, id) => users.find(user =>
user.id === id),
target: $user
})
const $chat = combine([$messages, $user], ([messages, user]) => {...} )