RFR: 8015317: Optional.filter, map, and flatMap

[Henry Jen]

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 <jed at 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.jen at oracle.com
> <mailto:henry.jen at 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 <jed at wesleysmith.io
>     <mailto:jed at 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.duigou at oracle.com
>>     <mailto:mike.duigou at 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/
>>         <http://cr.openjdk.java.net/%7Ehenryjen/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
>>
>>
>>
>
>