public class TransactionExample {
    public static void main(String[] args) {
        Transactor<IO<?>, Unit, String> transactor = new Transactor<>() {
            @Override
            public <A> IO<Either<String, A>> runTransactional(Transactional<IO<?>, String, Unit, A> transactional) {
                Unit connection = UNIT;
                return IO.io(() -> System.out.println("Starting Transaction"))
                        .discardL(transactional.run(connection)
                                          .<IO<Either<String, A>>>coerce()
                                          .flatMap(e -> e.match(s -> IO.io(() -> System.err.printf("Rolling back transaction due to %s%n", s)),
                                                                __ -> IO.io(() -> System.out.println("Committing Transaction")))
                                                  .fmap(constantly(e))));
            }

        };
        Transactional<IO<?>, String, Unit, Integer> fetchOddNumber = new Transactional<>(pureIO(), readerT(context -> IO.io(() -> System.out.println("Fetching data"))
                .fmap(constantly(7))
                .fmap(Either::right)));
        Transactional<IO<?>, String, Unit, Integer> fetchEvenNumber = new Transactional<>(pureIO(), readerT(context -> IO.io(() -> System.out.println("Fetching data"))
                .fmap(constantly(8))
                .fmap(Either::right)));


        transactor.runTransactional(fetchOddNumber.flatMap(rollBackWhenOdd()))
                .<IO<Either<String, Unit>>>coerce().unsafePerformIO();
        transactor.runTransactional(fetchEvenNumber.flatMap(rollBackWhenOdd()))
                .<IO<Either<String, Unit>>>coerce().unsafePerformIO();
    }

    private static Fn1<Integer, Monad<Unit, Transactional<IO<?>, String, Unit, ?>>> rollBackWhenOdd() {
        return integer -> {
            if (integer % 2 == 0) {
                return new Transactional<>(pureIO(), readerT(__ -> io(right(UNIT)))); // Commit
            }
            return new Transactional<>(pureIO(), readerT(__ -> io(left("Trigger rollback because odd integer")))); // Cancel;
        };
    }
}