I'm not entirely sure that is a problem, have a look at the following: https://gist.github.com/jedws/5993596#file-variancetest-java
it is only the one with a covariance annotation on the parameter that fails… On 15 July 2013 12:52, Zhong Yu <zhong.j...@gmail.com> wrote: > Another example of possible missing a wildcard > > > http://gee.cs.oswego.edu/dl/jsr166/dist/docs/java/util/concurrent/CompletionStage.html#thenCompose%28java.util.function.Function%29 > > thenCompose(Function<? super T,? extends CompletionStage<U>> fn) > > should be > > thenCompose(Function<? super T,? extends CompletionStage<? extends U>> > fn) > > The problem is probably wide spread, and we need a tool to find these > mistakes. > > Zhong Yu > > > On Sun, Jul 14, 2013 at 8:04 AM, Jed Wesley-Smith <j...@wesleysmith.io> > wrote: > > (accidentally didn't post to the list) > > > >> You probably know that the example provided is not completed ported to > > work with our Optional implementation, > > > > It should be, for the example I wrote an Optional that is final and > should > > be otherwise identical. It should certainly be fairly easy for any > > committer to try. If you can make it work without the ? extends Optional > > I'd love an explanation of how. > > > >> and fugue works around the type system with Option as abstract class. > > > > As I've tried to explain, this isn't about the implementation of the > > container class, but how covariance works with a parameterised class. > > > > We originally had the non-, but in a discussion with Brian he alerted us > to > > the fact that the signature was wrong. We hastily fixed it: > > > > > https://bitbucket.org/atlassian/fugue/commits/9eca663326a5baeb8f23974732ec585d5627a05c > > > > To further demonstrate, I give you a minimal example of a final Optional > > implementation that does not compile for this test: > > > > https://gist.github.com/jedws/5993596#file-gistfile1-java-L57 > > > > cheers, > > jed. > > > > > > > > On 14 July 2013 15:02, Henry Jen <henry....@oracle.com> wrote: > > > >> I think I understand what you are saying. However, unless we make > >> Optional not final, the extends part just doesn't matter. > >> > >> You probably know that the example provided is not completed ported to > >> work with our Optional implementation, and fugue works around the type > >> system with Option as abstract class. > >> > >> Cheers, > >> Henry > >> > >> On Jul 13, 2013, at 4:35 PM, Jed Wesley-Smith <j...@wesleysmith.io>< > j...@wesleysmith.io>wrote: > >> > >> I did supply a test that you can use to try it. > >> > >> What we are talking about is whether type Box<Parent> is substitutable > >> by Box<Child> in the contravariant position. Intuitively we think we > only > >> need Box<? extends Parent> because we only care about the type > parameter, > >> but the type – as you point out – is actually different. Box<Parent> is > not > >> "inherited by" Box<Child>. > >> > >> Specifically, if we have a consuming Box, and we replace it with a Box > >> of a more specific type parameter, we could attempt feed the more > general > >> type into it, ie. a Box<Child> isn't going to appreciate having Parent > fed > >> to it. This is why covariance and mutable containers don't mix well, and > >> why Java's covariant arrays are problematic. > >> > >> In this situation we have an immutable container, and we can substitute > >> the type of our container with one of a more specific type, as it will > only > >> ever supply a value – and a value of Child will suffice as a Parent. So, > >> for this case we need a Box that is substitutable, and therefore we > need to > >> add the covariance to our box. > >> > >> <? extends Box> is simply adding covariance to our Box type. > >> > >> For a much better explanation than I can give about this, see this > >> excellent post describing generics in Scala, which – apart from have > >> declaration-site variance and using [A] in place of <A> – generally > follow > >> the same pattern: > >> > >> > >> > http://termsandtruthconditions.herokuapp.com/blog/2012/12/29/covariance-and-contravariance-in-scala/ > >> > >> cheers, > >> jed. > >> > >> > >> On 14 July 2013 04:49, Henry Jen <henry....@oracle.com> wrote: > >> > >>> I think the type you talking about here is Optional<? extends U> > instead > >>> of ? extends Optional<U>. > >>> > >>> IIRC, Optional<? extends U> is not a subtype of Optional<U>, just like > >>> any other Collection class. List<Child> is not a List<Parent>. > >>> > >>> Cheers, > >>> Henry > >>> > >>> > >>> On Jul 13, 2013, at 3:15 AM, Jed Wesley-Smith <j...@wesleysmith.io> > >>> wrote: > >>> > >>> > The ? extends Optional is unnecessary in flatMap as Optional is > final. > >>> > >>> interestingly enough, it actually is. > >>> > >>> try the following test: > >>> > >>> class OptionalTest { > >>> class Parent {}; > >>> > >>> class Child extends Parent {}; > >>> > >>> @Test public void covariantReturn() { > >>> Optional<Parent> some = some(new Parent()); > >>> Function<Parent, Optional<Child>> f = new Function<Parent, > >>> Optional<Child>>() { > >>> @Override public Optional<Child> apply(Parent p) { > >>> return some(new Child()); > >>> } > >>> }; > >>> Optional<Parent> mapped = some.<Parent> flatMap(f); > >>> assertThat(mapped.get(), notNullValue()); > >>> } > >>> } > >>> > >>> adapted from the fugue test suite: > >>> > >>> > >>> > https://bitbucket.org/atlassian/fugue/src/96a65067fb7aaf1edae1bffa07167a5865cbebec/src/test/java/com/atlassian/fugue/OptionTest.java#cl-155 > >>> > >>> The point to remember is that Optional<Child> is a type and as such is > >>> actually a subtype of Optional<Parent> – and therefore requires a > >>> covariant return. > >>> > >>> cheers, > >>> jed. > >>> > >>> > >>> > >>> > >>> On 13 July 2013 04:15, Mike Duigou <mike.dui...@oracle.com> wrote: > >>> > >>>> The ? extends Optional is unnecessary in flatMap as Optional is final. > >>>> Otherwise this looks good. > >>>> > >>>> Mike > >>>> > >>>> On Jul 5 2013, at 14:37 , Henry Jen wrote: > >>>> > >>>> > Hi, > >>>> > > >>>> > Please review the webrev at > >>>> > > >>>> > http://cr.openjdk.java.net/~henryjen/ccc/8015317.0/webrev/ > >>>> > > >>>> > Which adds following method to Optional, > >>>> > > >>>> > public static <T> Optional<T> ofNullable(T value) {} > >>>> > public Optional<T> filter(Predicate<? super T> predicate) {} > >>>> > public<U> Optional<U> map(Function<? super T, ? extends U> mapper) > {} > >>>> > public<U> Optional<U> flatMap(Function<? super T, ? extends > >>>> Optional<U>> > >>>> > mapper) {} > >>>> > > >>>> > Also included is some cleanup on javadoc. > >>>> > > >>>> > Cheers, > >>>> > Henry > >>>> > >>>> > >>>> > >>> > >>> > >> > >> > > >
