Add API for converting StoreWritable and EventCallable to readonly

[chshanovskiy]

For good and consistent API design it would be nice to have a simple helper, that converts StoreWritable/EventCallable to their readonly types.

Something like this:

function readonly(store: StoreWritable<T>): Store<T> {
  return store.map(store => store)
}
function readonly(event: EventCallable<T>): Event<T> {
  return event.map(event => event)
}

Example – dummy user factory:

export const createUser = createFactory(() => {
  const check = createEvent()
  const authorized = createEvent<User>()

  const $user = createStore<User | null>(null)
    
  sample({
    clock: check,
    target: checkFx,
  })

  sample({
    clock: checkFx.doneData,
    target: $user,
  })

  sample({
    clock: $user,
    filter: Boolean,
    target: authorized,
  })

  return {
    check,
    authorized: authorized.map((user) => user), // or readonly(authorized)
    $user: $user.map((user) => user), // or readonly($user)
  }
})

Here authorized is EventCallable, but for factory consumers it must be readonly event, only for subscribing. Also $user is readonly store for consumers.

Naming is under discussion, maybe readonly is too ambiguous.

[chshanovskiy]

Maybe readonly must support object form like this readonly({$someStore, someEvent})

[chshanovskiy]

Also, readonly helper can support already readonly stores and events, in favor of factory developers and producing strong api for consumers

[sergeysova]

Done by #320