Proposal idea - generators

[Neal Gafter]

For Java 7, I agree.  Until the thread leakage problem I mentioned can
be solved, I don't think it should be done in a standard library
either.

-Neal

On Wed, Mar 25, 2009 at 8:55 AM, Tim Peierls <tim at peierls.net> wrote:
> I don't think Java needs language support for generators.
> --tim
>
> On Wed, Mar 25, 2009 at 10:02 AM, Neal Gafter <neal at gafter.com> wrote:
>>
>> The problem with this approach is that, without cooperation from the
>> client, the other thread can stick around for arbitrarily long, which
>> is a severe resource leak.  I wrote about this at
>>
>> <http://gafter.blogspot.com/2007/07/internal-versus-external-iterators.html>.
>>  That's because the generating thread has no way of knowing when the
>> client did a "break" from the loop.
>>
>> You can solve this if your language has support for an AutoCloseable
>> interface, and the Iterator is closed at the end by the expansion of
>> the for-each loop when the Iterator is a subtype of AutoCloseable.
>> Unfortunately, the current ARM proposal does not do that.
>>
>> On Wed, Mar 25, 2009 at 6:33 AM, Tim Peierls <tim at peierls.net> wrote:
>> > My sense (bolstered by a quick Google code search) is that people have
>> > been
>> > coming up with this kind of functionality on their own as they encounter
>> > a
>> > need for it. Is it really something that needs special language support?
>> > Using another thread to do the generation is quite reasonable. You can
>> > build
>> > a nice coroutine-style facility with a pair of SynchronousQueues (or,
>> > more
>> > generally and flexibly, with TransferQueues, expected for Java 7; or
>> > maybe
>> > with Phasers). Using that as a building block you could then rewrite
>> > your
>> > example below as something like this:
>> >
>> > final List<User> allUsersList = ...;
>> > Iterator<User> userIterator = generatorToIterator(new Generator<User>()
>> > {
>> >    public void generate(Generation<User> generation) {
>> >        for (User user : allUsersList) {
>> >            if (user.isActive()) generation.yield(user);
>> >        }
>> >    }
>> > });
>> > // Now userIterator lazily produces active users.
>> >
>> >
>> > Getting this kind of thing right is hard enough that someone should
>> > propose
>> > putting a production version in the standard library, but I don't see
>> > the
>> > need for a language change.
>> >
>> > For your particular example, you could use Google Collections'
>> >
>> > Iterators.filter<http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Iterators.html#filter(java.util.Iterator,
>> > com.google.common.base.Predicate)>:
>> >
>> > Iterator<User> userIterator = filter(allUsersList.iterator(), new
>> > Predicate<User>() {
>> >    public boolean apply(User user) {
>> > return user.isActive();
>> >    }
>> > });
>> >
>> >
>> > That's not true generation, but I bet lazy filtering handles a lot of
>> > the
>> > common cases that people think they want generation for.
>> >
>> > --tim
>> >
>> >
>> >
>> > On Tue, Mar 24, 2009 at 3:07 PM, Mark Derricutt <mark at talios.com> wrote:
>> >
>> >> Hi all,
>> >>
>> >> This isn't an actual proposal, but more a proposal idea that hopefully
>> >> someone could run with.  I was thinking about all the closures issues
>> >> and looking at a lot of my code where I'm using closure like patterns
>> >> and started thinking what would really make things easier for me would
>> >> be some form of yield/generators api to give me a cleaner way of
>> >> making Iterable's that could be used in the JDK5 for loop.
>> >>
>> >> The usage I'd like would be something like:
>> >>
>> >> for ( User user : allUsersList) {
>> >>  if (user.isActive()) yield user;
>> >> }
>> >>
>> >> Effectively I could see this as being like comparing an XML pull
>> >> parser to SAX.  The problem I have is I can't think of a suitable way
>> >> of expressing, or implementing this in java, unless I split the code
>> >> into another thread that sleeps between the yield calling back to the
>> >> iterable next() call which is nasty as I could see a lot of thread
>> >> leakage there.
>> >>
>> >> Has anyone looked at, or already written some form of generator
>> >> proposal?
>> >>
>> >>
>> >> ...and then Buffy staked Edward.  The End.
>> >>
>> >>
>> >
>> >
>
>