Writeable Atom with derived default value

I just recently went through a refactoring from recoil. Everything was pretty straightforward but one use case I couldn’t build in jotai.

I have a list of countries I can fetch from the backend:

const availableCountriesApi = atom<Array<Country>>(async () => fetchEntities("/core/countries"))

I built a defaultCountry atom based on this fetch

const defaultCountry = atom<Country>(
  (get) => get(availableCountries).find(country => country.id === "US") || {id: "US", name: "United States"},
)

And now I have a selectedCountry atom which I want to initialize with the default country. But everything I tried resulted either in a type error or a readable only atom:

const selectedCountryState = atom<Country>((get) => get(defaultCountry))
 
const [selectedCountry, setSelectedCountry] = useAtom(selectedCountryState)

setSelectedCountry({id: "DE", name: "Germany"}) // This expression is not callable, Type 'never' has no call signatures.

I think the async fetch is not super relevant, because when I change my defaultCoutry atom to this, I’ll run in the same problem:

const defaultCountry = atom<Country>({id: "US", name: "United States"})

So is there a way to build a writeable atom that I can initialize with a derived atom value?

[leifg]

Oh sorry, didn’t see. Now it works. Thank you so much!

[dai-shi]

So, the three atom solution is a valid approach currently. It would be nice to have a uitil as it’s quite common.

Actually, it might be possible to implement it with just one atom, using some undocumented properties. We already did atomWithReducer similarly, so let me work on it.