Effectively final

Tim Fox timvolpe at gmail.com
Thu Aug 18 09:00:36 PDT 2011

On 18/08/11 16:30, Howard Lovatt wrote:
> On 16/08/2011, at 10:45 PM, Tim Fox <timvolpe at gmail.com 
> <mailto:timvolpe at gmail.com>> wrote:
>> The point I was making was in response to your assertion that 
>> inherently serial code will become less common as time goes on. 
>> Actors frameworks (and other hybrid frameworks like node.x) are all 
>> based around inherently serial code, and I don't see them dying out yet.
> I don't get what you are driving at, you have been posting that 
> Lambdas should capture mutable local variables and have given Actors 
> as an example of why that is necessary.
No, I didn't give actors as an example of why lambdas should mutate 
local vars, I gave actors as a counterpoint to your assertion that 
serial code will become less common as time came on (Actor code is all 
serial and these frameworks are gaining in popularity)

The point I was trying to make, in a nutshell, was this:

Conventional wisdom Java concurrency involves letting many threads 
execute your objects and using synchronization and what-not to make 
those objects threadsafe. This is difficult. Concurrent programming is hard.

There are other ways of making scalable concurrent systems that don't 
involve having your code executed by multiple threads concurrently. 
(Actor model is one example of this, there are others) This is very 
desirable, since if you don't have to worry about making your code 
threadsafe, you can open up to a legion of web developers who otherwise 
would find concurrency too hard.

My bet is the latter type of concurrent systems will eventually become 
more popular. So... if in this future, developers will be writing 
islands of single threaded coded, then it's safe to use lambdas that 
mutate local vars in those islands since zero race conditions are 

So far from being a throwback to the 50s, lambdas which mutate local 
vars could be a very useful tool for the next generation of frameworks. 
And if Java does not support such lambdas it will be sidelined as a good 
programming language for these kinds of frameworks.

Allowing such mutation would allow you to implement "node.js-like" 
frameworks on the JVM. This is already possible in JS/Ruby/Groovy/etc 
but is currently very clunky in Java because of lack of lambdas - but 
even with Java 8 proposed lambdas it would still be clunky because of 
the effectively-final rule.

> What I demonstrated, was that for Actors you wouldn't use Lambdas but 
> classes and therefore your argument that Lambdas should capture local 
> mutables because of Actors is non sequitur.
> Now you seem to be saying that you were talking about particular 
> styles of code. Are you arguing about capturing mutables or not?
> PS Capturing a mutable variable in an Actor and passing it via message 
> to another actor is problematic. Again indicating that Lambdas should 
> not capture mutable state.
> PPS Also note that Scala has introduced map/reduce style collections 
> (presumably because people prefer these to Actors - which it already 
> has). Again map/reduce is best suited to not capturing mutable variables.
>> Quite the opposite, I see them getting more popular :)
>> On 16/08/11 16:25, Howard Lovatt wrote:
>>> I don't see that the actor or FJ type frameworks are effected much, 
>>> because the messages are immutable, e.g. a standard actor example:
>>> http://java.dzone.com/articles/scala-threadless-concurrent
>>> Recoded in Java would be something like this:
>>> public class Actor extends FJTask {
>>>   private Queue messages ...
>>>   public void accept( final Object message ) ...
>>>   public Object acceptAndReply( final Object message ) ...
>>>   public Future<Object> acceptAndReplyLater( final Object message ) ...
>>>   protected void reply( final Object response ) ...
>>> protected Object getMessage() ...
>>> }
>>> |10.||public class Accumulator ||extends| |Actor {|
>>> |11.||public void run() {|
>>> |12.||int sum = ||0;|
>>> |13.||for(;;) {|
>>> |14.|final Object message = getMessage();
>>> |15.|if ( message instanceof |Accumulate ) { ||  sum += 
>>> ((Accumulate)message).n; ||}|
>>> |16.|else if ( message instanceof |Reset ) { sum = ||0; }|
>>> |17.|else if ( message instanceof Total ) {  reply(sum);  break;
>>> |18.||}|
>>> |19.||}|
>>> |20.||}|
>>> |21.||}|
>>> The lambdas might prove useful for the messages, that are immutable 
>>> anyway, but the actors themselves would always be instances of a 
>>> class because you need to be able to create more actors as the 
>>> program runs and you can't do this with a lambda (or a closure many 
>>> other language). As I said at the beginning I don't see that an 
>>> Actor framework would change much one way or the other if Lambdas 
>>> could mutate a variable or not.
>>> Cheers,
>>>  -- Howard.
>>> Sent from my iPad
>>> On 16/08/2011, at 6:57 PM, Tim Fox <timvolpe at gmail.com 
>>> <mailto:timvolpe at gmail.com>> wrote:
>>>> On 16/08/11 12:36, Howard Lovatt wrote:
>>>>> I think the correct decision has been made, inherently serial code 
>>>>> will become less common as time goes on.
>>>> The growing popularity of actor model implementations (actors of 
>>>> course
>>>> only allow serial execution of code in the actor), e.g. Erlang and 
>>>> Akka,
>>>> and the growing popularity of languages which are inherently serial
>>>> (e.g. JavaScript) are a clear counterpoint to your argument. Also look
>>>> at webworkers, node.js. I could go on.
>>>> I'd argue the current trend is quite opposite to what you see. Non
>>>> serial code will become unusual (because concurrency is hard), and
>>>> serial code will become the norm (because it's easier to code,
>>>> especially for the web developer masses). Systems will scale by having
>>>> multiple "islands" of serial code (e.g. multiple actors in the 
>>>> actor model).
>>>> Please note that just because all code is executed serially does not
>>>> mean you can't exploit parallelism. You can do fork/join type stuff by
>>>> sending messages to other "islands" and reassembling the results as 
>>>> they
>>>> come back.
>>>>>  Therefore optimizing new features for parallel execution is the 
>>>>> correct path to take and also constant with Java's past of taking 
>>>>> existing concepts into the mainstream. When Java was introduced 
>>>>> the concentration on objects and garbage collection was not 
>>>>> mainstream and was opposed by many. I think the same will happen 
>>>>> with mutable data and serial execution and it is nice that Java is 
>>>>> once again daring to be different and adhering to the mantra that 
>>>>> less is more.
>>>>> Cheers,
>>>>>  -- Howard.
>>>>> Sent from my iPad
>>>>> On 16/08/2011, at 4:56 PM, Steven Simpson<ss at comp.lancs.ac.uk 
>>>>> <mailto:ss at comp.lancs.ac.uk>>  wrote:
>>>>>> On 16/08/11 01:51, Stephen Colebourne wrote:
>>>>>> [snip: lots of syntax options for permitting mutable locals]
>>>>>>> int #total = 0;
>>>>>>> list.apply(#{item ->   total += item});
>>>>>>> ie. a way to introduce a local variable that can be managed safely.
>>>>>> For the simple example given, you could translate 'total' into an
>>>>>> AtomicInteger, but if there are other variables to be accessed, you'd
>>>>>> have to box them together, and make the whole lambda synchronize 
>>>>>> on it,
>>>>>> or at least from the first use of the box to the last.  Trying to 
>>>>>> patch
>>>>>> the call site like this doesn't seem to be particularly 
>>>>>> promising.  The
>>>>>> alternative is to require List.apply to make additional 
>>>>>> guarantees about
>>>>>> how it executes the lambda.
>>>>>> Would it not be better to let List.apply get on with its potential
>>>>>> parallelism, and define other methods that do make extra guarantees,
>>>>>> e.g. that the lambda will be executed serially, or even on the 
>>>>>> caller's
>>>>>> thread?
>>>>>> Tim's cases include, for example, setTimeout(int, Runnable), 
>>>>>> which must
>>>>>> make such a guarantee, if only informally in its documentation. 
>>>>>>  To be
>>>>>> more formal:
>>>>>>    * Declare setTimeout(int, @Callback Runnable).
>>>>>>    * When a lambda is assigned to a @Callback Runnable, allow the
>>>>>>      lambda body to mutate locals (without error or warning).
>>>>>>    * Don't permit a @Callback Runnable (which is tainted) to be
>>>>>>      assigned to a plain Runnable (without error or warning).
>>>>>> This way, the likes of List.apply don't have to make any guarantees,
>>>>>> requiring the caller to make corresponding ones (automatically 
>>>>>> achieved
>>>>>> by not being generally allowed to mutate locals).  Meanwhile, 
>>>>>> setTimout
>>>>>> makes additional guarantees, to the convenience of the caller, who is
>>>>>> specially permitted to mutate locals.
>>>>>> Cheers,
>>>>>> Steven

More information about the lambda-dev mailing list