Uni with Optional

[Meemaw]

How to map to Optionals using Unis?

  @Override
  public Uni<Optional<PageDTO>> getPage(UUID pageID, UUID sessionID, UUID deviceID) {
    Tuple values = Tuple.of(pageID, sessionID, deviceID);
    return pgPool
        .preparedQuery(SELECT_PAGE_RAW_SWL, values)
        .map(
            rowSet -> {
              if (!rowSet.iterator().hasNext()) {
                return Optional.empty();
              }
              Row row = rowSet.iterator().next();
              return Optional.of(
                  new PageDTO(
                      row.getString("org_id"),
                      row.getUUID("uid"),
                      row.getString("url"),
                      row.getString("referrer"),
                      row.getString("doctype"),
                      row.getInteger("screen_width"),
                      row.getInteger("screen_height"),
                      row.getInteger("width"),
                      row.getInteger("height"),
                      row.getInteger("compiled_timestamp")));
            })
        .onFailure()
        .invoke(
            throwable -> {
              log.error("Failed to get page id={}", pageID);
              throw new DatabaseException();
            });
  }

This code will fails with the error:

Required type: Uni<Optional<PageDTO>>
Provided: Uni<Optional<?>>

[cescoffier]

At first glance, it looks like a bug. It should work.

[cescoffier]

Ok, actually no it's normal. You need to help JAva a little bit:

.<Optional< PageDTO >>map(rowSet -> {

[Meemaw]

Interestingly this works with the CompletionStage API (different piece of code, but "same typing"):

  @Override
  public CompletionStage<Optional<UserDTO>> findUser(String email) {
    Tuple values = Tuple.of(email);
    return pgPool
        .preparedQuery(FIND_USER_BY_EMAIL_RAW_SQL, values)
        .thenApply(
            pgRowSet -> {
              if (!pgRowSet.iterator().hasNext()) {
                return Optional.empty();
              }
              Row row = pgRowSet.iterator().next();
              return Optional.of(
                  new UserDTO(
                      row.getUUID("id"),
                      row.getString("email"),
                      UserRole.valueOf(row.getString("role")),
                      row.getString("org")));
            });
  }

[cescoffier]

Yes because you don't have the last "stage" (onFailure().invoke), so it is able to deduce the generic type.

This would also work:

Uni<Optional<Integer>> uni = Uni.createFrom().item(Collections.emptyList())
                .map(list -> {
                    if (list.isEmpty()) {
                        return Optional.empty();
                    } else {
                        return Optional.of(12345);
                    }
                })