this should not refer to the lambda

[Rémi Forax]

Le 21/02/2010 20:11, Alex Blewitt a écrit :
> On Sun, Feb 21, 2010 at 6:51 PM, Rémi Forax<forax at univ-mlv.fr>  wrote:
>    
>>> This argument is somewhat tautologous. "I'd have difficulty in explaining
>>> it because I don't like what it means".
>>>        
>> I have no difficulty to explain it. I don't want to explain it.
>>      
> I'm not sure that is a technical reason to not include something.
>
>    
>> Alex, try to compile:
>> int a =a;
>> and take a look to the error message.
>>      
> Test.java:2: illegal forward reference
> int a=a;
>        ^
>
> That's the point - it's an illegal reference. Yet, were 'this' used to
> describe only the enclosing object instance (and not the lambda
> instance), you wouldn't be able to do recursive lambdas.
>    

The message is not something like "can't find symbol".
The compiler knows what 'a' means.

> This prints out a countdown from n to 0:
>
> #(i) { System.out.println(i); if(i>0) this.(i-1) };
>
> So without 'this' referring to the enclosing lambda, you can't do
> this. An earlier proposal would be to have an illegal forward
> reference like:
>
> countdown = #(i) { System.out.println(i); if(i>0) countdown.(i-1) }
>
> So, you either allow 'this' as referring to the lambda (to support the
> recursive case), or disallow 'this' to refer to the lambda; at which
> point, you either disallow recursive cases or use hackery forward
> references to enable recursion.
>
> Given that the example I posted earlier lets your refer to any number
> of nested lambdas, and that in the most case, the compiler should be
> smart enough to resolve the correct enclosing scope where necessary
> (i.e. you won't need to put 'this' in that frequently, since it will
> work up the stack to find it, much like inner classes work today) -
> what exactly is the problem with letting 'this' refer to the lambda?
>
> The alternative seems to be to allow 'this' to refer to the lambda,
> and then use 'A.this' when you want to access the enclosing classes
> 'this'. I doubt many Java programmers would find that difficult,
> especially since that's how inner classes work today.
>
> I have yet to see a technical argument of what can be done if you
> disallow 'this' as a lambda reference, other than having to type in
> n+1 extra characters should you ever want to refer to the enclosing
> scope.
>
> Alex
>    

Rémi