ACAccountStore *accountStore = [[ACAccountStore alloc] init];
ACAccountType *type = [accountStore accountTypeWithAccountTypeIdentifier:ACAccountTypeIdentifierTwitter];

[[[accountStore
    rac_requestAccessToAccountsWithType:type options:nil]
    concat:[RACSignal defer:^{
        ACAccount *account = [accountStore accountsWithAccountType:type][0];
        account.accountDescription = @"description";
        return [accountStore rac_saveAccount:account];
    }]] subscribeError:^(NSError *error) {
        // Failed to save or access was not granted.
    } completed:^{
        // Succeeded to save.
    }];