Proposal idea - generators

[Tim Peierls]

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.
>
>