Options for accessing local variables

[Neal Gafter]

You failed to list the choices made in CfJ, the draft specification, and the
strawman:

CfJ) Full access to local variables, but a warning if a variable is accessed
when not annotated and not effectively final.
Draft) Access to local variables only if effectively final.
Strawman) Full access to local variables if effectively final or a keyword
appears on the variable declaration.

Cheers,
Neal

On Sat, Mar 20, 2010 at 5:05 AM, Stephen Colebourne <scolebourne at joda.org>wrote:

> This is a list of possible different ways to link local variables to
> lambdas. (I spoke to Alex at QCon London about the list).
>
> In each case, an example is given, but the syntax isn't that important
> - its the semantics we are picking. (I've used annotations instead of
> keywords as it gets the point across more easily:
>
> 1) Full access to local variables by reference:
>  int total = 0;
>  Utils.each(list, #(String str) { total += str.length() });
>  // total is sum of string lengths if each() is not threaded
>
> 2) Access to local variables if they are marked by a key word or
> annotation:
>  @Shared int total = 0;
>  Utils.each(list, #(String str) { total += str.length() });
>
> 3) No access to local variables
>  (example not possible in style it is written - use an AIC?)
>
> 4) Only final local variables can be accessed (as with AIC)
>  final int[] total = new int[1];
>  Utils.each(list, #(String str) { total[0] += str.length() });
>
> 5) Local variables are accessible but copied on construction:
>  int total = 0;
>  Utils.each(list, #(String str) { total += str.length() });
>  // total still zero here - need different approach for the example
>
> 6) Import local variables by reference:
>  int total = 0;
>  Utils.each(list, #(String str : @Share total) { total += str.length() });
> 6a) non-imported local variables cannot be seen
> 6a) non-imported local variables are copied on construction as per #5
>
> 7) Import local variables by copy on construction:
>  int total = 0;
>  Utils.each(list, #(String str : @Copy total) { total += str.length() });
>  // total still zero here - need different approach for the example
> 7a) non-imported local variables cannot be seen
> 7a) non-imported local variables are accessed by reference as per #1
>
> 8) Mark the lambda as local, allowing full access to local variables:
>  int total = 0;
>  Utils.each(list, @AccessLocal #(String str) { total += str.length() });
> 8a) if annotation not present local variables cannot be seen
> 8a) if annotation not present local variables are copied on
> construction as per #5
>
> 9) Mark the lambda as multi-thread, copying locals on construction:
>  int total = 0;
>  Utils.each(list, @CopyLocals #(String str) { total += str.length() });
> 9a) if annotation not present local variables cannot be seen
> 9a) if annotation not present local variables are accessed by
> reference as per #1
>
> 10) Mark the destination method as local with keyword or annotation:
>  int total = 0;
>  Utils.each(list, #(String str) { total += str.length() });
> method declared as:
>  public <T> void each(Collection<T> coll, @Local Block1Arg<T> block)
> which accesses locals by reference.
>
> 11) Mark the destination method as multi-thread with keyword or annotation:
>  int total = 0;
>  Utils.each(list, #(String str) { total += str.length() });
>  // total still zero here - need different approach for the example
> (not a closure)
> method declared as:
>  public <T> void each(Collection<T> coll, @MultiThread Block1Arg<T> block)
> which accesses locals by copy on construction.
>
> NOTE: with #8 and #9, you can't tell from the call-site which type it is.
>
>
> I'm sure that there are other possibilities. And I've not directly
> addressed the question of "this" (which can be inferred to some degree
> by picking one of the choices above).
>
> I'd also note that "copying on construction" might be a simple copy of
> the reference, or a full clone.
>
> So, in summary - variables can be accessed by reference, or copied on
> construction. They can be annotated in a variety of different ways to
> provide choice or to restrict between these two basic choices.
>
> Hope this helps someone. I'm offlist for about 3 weeks now.
> Stephen
>
>