extension RostersState {
    static func reducer(state: inout RostersState, action: RostersAction) {
        switch action {
        case .addRosterDone(let jid):
            state.newAddedRosterJid = jid
            state.newAddedRosterError = nil

        case .addRosterError(let reason):
            state.newAddedRosterJid = nil
            state.newAddedRosterError = reason

        case .rostersListUpdated(let rosters):
            state.rosters = rosters

        case .markRosterAsLocallyDeleted, .deleteRoster:
            state.deleteRosterError = nil

        case .rosterDeletingFailed(let reson):
            state.deleteRosterError = reson

        default:
            break
        }
    }
}