ignore me, you do actually need both ? extends on the type constructor and the inner type – dunno what I was thinking.
On 15 July 2013 13:02, Jed Wesley-Smith <j...@wesleysmith.io> wrote: > 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 >> >>>> >> >>>> >> >>>> >> >>> >> >>> >> >> >> >> >> > >> > >
