Compose/Transform Covariance

[benjchristensen]

Failing test while exploring generic variance for #1416

Code like this makes these generics work:

    @Test
    public void testCovarianceOfCompose() {
        Observable<HorrorMovie> movie = Observable.<HorrorMovie> from(new HorrorMovie());
        movie.compose(new Transformer<Movie, Movie>() {

            @Override
            public Observable<? extends Movie> call(Observable<? super Movie> t1) {
                return Observable.from(new Movie());
            }

        });
    }

    @Test
    public void testCovarianceOfCompose2() {
        Observable<Movie> movie = Observable.<Movie> from(new HorrorMovie());
        movie.compose(new Transformer<Movie, Movie>() {

            @Override
            public Observable<? extends Movie> call(Observable<? super Movie> t1) {
                return Observable.from(new HorrorMovie());
            }

        });
    }

however, I couldn't get the compose/Transformer types to correctly work.

Anyone else want to help figure out generics for this?

See https://github.com/Netflix/RxJava/blob/master/rxjava-core/src/main/java/rx/Observable.java#L195

[cloudbees-pull-request-builder]

RxJava-pull-requests #1481 FAILURE
Looks like there's a problem with this pull request

[benjchristensen]

@abersnaze Thank you for fixing this ... I think I tried every permutation of super/extends except the right one that you came up with!

[cloudbees-pull-request-builder]

RxJava-pull-requests #1486 SUCCESS
This pull request looks good

[zsxwing]

Returning Observable<? extends R> is not a good idea. The following codes won't be compiled:

    @Test
    public void testCovarianceOfCompose() {
        Observable<HorrorMovie> movie = Observable.<HorrorMovie> from(new HorrorMovie());
        Observable<Movie> movie2 =
                movie.compose(new Transformer<Movie, Movie>() {

            @Override
            public Observable<? extends Movie> call(Observable<? extends Movie> t1) {
                return Observable.from(new Movie());
            }

        });
    }

    @Test
    public void testCovarianceOfCompose2() {
        Observable<Movie> movie = Observable.<Movie> from(new HorrorMovie());
        Observable<HorrorMovie> movie2 =
                movie.compose(new Transformer<Movie, HorrorMovie>() {
            @Override
            public Observable<? extends HorrorMovie> call(Observable<? extends Movie> t1) {
                return Observable.from(new HorrorMovie());
            }
        });
    }

I propose zsxwing@855252b#diff-d41d8cd98f00b204e9800998ecf8427e just like other methods that avoids to return Observable<? extends R>.

[benjchristensen]

Can you submit that as a PR?

Why in this use case must R not extend ? whereas the lift case does?

[zsxwing]

Please take a look at #1577