Instead of returning

Either<Failure, List<User>>

in the UseCase, why not just return

FriendsState

(I would rename it to FriendsResult or something similar) and return the error as a type of the sealed class? Like:

sealed class FriendsState {        
   object Loading : FriendsState()        
   object Empty : FriendsState()        
   data class Success(val users: List<UserUiModel>) :FriendsState()    
   object Error : FriendsState()
}

?