Effectively final

[Tim Fox]

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>  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
>>