Closures for Java and other thoughts on language evolution The document discusses goals for language changes including simplifying programs, reducing bugs, and adapting to changing requirements like multicore processors and concurrency. It proposes adding closures to Java to help meet these goals by allowing for more concise code, avoiding repetition, and making programming with concurrency easier. Specific examples are given of how closures could help implement common patterns like try-with-resources and iteration in a more readable way while also enabling flexibility for programmers to extend the language.

1. Closures for Java
…and other thoughts on language evolution
Neal Gafter
Google
1680

2. 2
Goals for Language Change
> Simplify programs
– More concise
– Don’t Repeat Yourself (DRY)
– Make reading, writing easier
> Fewer bugs
> Adapt to changing requirements
– Multicore and concurrency
– Enable programmer flexibility (expressiveness)

3. 3
What is a Closure
> A closure is an anonymous function expression
> Like
– Block in Smalltalk, Ruby, Scala
– Lambda in Lisp
– Anonymous function in Javascript

4. 4
DRY (Don’t Repeat Yourself)
InputStream s = openStream();
try {
doSomething(s);
} finally {
where
try { s.close(); }
catch(IOException ex) {
<T extends Closeable>
LOGGER.log(…);
void with(T s, {T => void} block) {
} try {
} block.invoke(s);
} finally {
becomes try ( s.close(); }
catch(IOException ex) {
LOGGER.log(…);
with (InputStream s : openStream()) {
}
doSomething(s);
}
} }

5. 5
DRY (Don’t Repeat Yourself)
long t0 = System.nanoTime();
boolean success = false;
try {
doSomething();
where
success = true;
} finally { void time(String key, Block block) {
recordTime(“doSomething”, success, long t0 = System.nanoTime();
System.nanoTime() – t0); boolean success = false;
try {
}
block.invoke();
success = true;
becomes } finally {
recordTime(
time (“doSomething”) { key, success,
System.nanoTime() – t0);
doSomething();
}
}
}

6. 6
DRY (Don’t Repeat Yourself)
for (Map.Entry<String,Thing> e : myMap.entrySet()) {
String name = e.getKey();
Thing thing = e.getValue();
doSomething(key, value);
}
becomes
for each(String name, Thing thing : myMap) {
doSomething(name, thing);
}
<K,V> void for each(Map<K,V> map, {K,V => void} block) {
for (Map.Entry<K,V> e : myMap.entrySet()) {
where
block.invoke(e.getKey(), e.getValue());
}
}

7. 7
Make reading, writing easier
void addListener(final ItemSelectable is) {
is.addItemListener(
new ItemListener() {
public void itemStateChanged(ItemEvent e)
{ doSomething(e, is); }
}
);
}
becomes
void addListener(ItemSelectable is) {
is.addItemListener(
{ ItemEvent e => doSomething(e, is); }
);
}

8. 8
Make reading, writing easier
void launch(Executor ex) {
ex.execute(new Runnable() {
public void run() {
doSomething();
}
});
}
becomes
void launch(Executor ex) {
ex.execute({ =>
doSomething();
});
}

9. 9
Benefits of being DRY
> Less boilerplate
– Programs easier to read
– Fewer bugs
> Hard things become easy
– Fork- join concurrency

10. 10
Multicore and Concurrency
> Need concurrency to exploit Moore’s Law

11. 11
Multicore and Concurrency
> Need concurrency to exploit Moore’s Law
> Thread- based concurrency is hard

12. 12
Multicore and Concurrency
> Need concurrency to exploit Moore’s Law
> Thread- based concurrency is hard
> Fork- Join concurrency is easier
– Concurrent Loops

13. 13
Multicore and Concurrency
> Need concurrency to exploit Moore’s Law
> Thread- based concurrency is hard
> Fork- Join concurrency is easier
– Concurrent Loops
> But more will be needed in the future
– Actors
– Functional Programming
– Software Transactional Memory

14. 14
Enable Programmer Flexibility
(expressiveness)
> Programmers “grow” a language by adding APIs
> See Guy Steele’s “Growing a Language”
If you give a person a fish, he can eat for a day.
If you teach a person to fish, he can eat his whole life long.

15. 15
Enable Programmer Flexibility
(expressiveness)
> Programmers “grow” a language by adding APIs
> See Guy Steele’s “Growing a Language”
> Support more flexible “language extension” by API
> Reduce the need for future language change

16. 16
There is more to Java (SE) than Java (PL)
> The VM
> Deployment
> Other languages
>…

17. Neal Gafter http:/ / www.javac.info/
Google neal@gafter.com