RE: [aspectj-dev] Restricting type from user defined pointcut

["Wes Isberg" <wes@xxxxxxxxxxxxxx>]

To say what you're trying to say now, I prefer your suggestion:

  pointcut p(Foo foo) : ...;

  before(Foo foo) : p(foo) && if(foo instanceof Bar) { ... }

because it's explicit and doesn't need to know how foo is bound 
(and thus doesn't involve rewriting cflow, right?).

+ Another advantage of your proposal: to bind the subtype and avoid a downcast:

now:

  before(Foo foo) : p(foo) && if(foo instanceof Bar) {
    Bar bar = (Bar) foo;
    ...
  }

under your proposal?:

  before(Bar bar) : p(bar) { ... }

- There's no subtype requirement when binding on interfaces, so
the compiler could provide no warnings there in case of error.

- I'd feel more comfortable if the semantics of this(), 
target(), and args() were exactly "instanceof"; exposing more
cases where something can be proveably of a certain type 
but possibly null can make the semantics tricky.  That's why
I prefer the clarity of the runtime if().

- I don't like that different clients of the "same" pointcut have
different semantics.  I tried thinking of this in terms of parameterized types, but that hurt, too.  I tried thinking about how tool support could show the clients of the pointcuts or show the explicit form.

So all-in-all, your proposal is attractive but not convincing
(to me). I imagine there are other more compelling opportunities 
for syntactic sugar.  I do think it's worth tracking as a bug,
so that when variable binding or this/target/args is revisited,
it can be a consideration.

Wes

> ------------Original Message------------
> From: "Ron Bodkin" <rbodkin@xxxxxxxxxxxxxx>
> To: "'AspectJ developer discussions'" <aspectj-dev@xxxxxxxxxxx>
> Date: Wed, Aug-31-2005 2:11 PM
> Subject: RE: [aspectj-dev] Restricting type from user defined pointcut
>
> As a follow up, the "hold your nose" version gets even worse in the 
> case I'm
> dealing with, where the expression is bound in a cflow:
> 
> ...
>     public pointcut inRequest(RequestContext requestContext) :
>         cflow(requestExecution(requestContext))
> 
>     //worse redundancy:
>     public pointcut inDerivedRequest(DerivedRequestContext drc) : 
>         cflow(requestExecution(*) && this(drc));
> 
> -----Original Message-----
> From: aspectj-dev-bounces@xxxxxxxxxxx
> [mailto:aspectj-dev-bounces@xxxxxxxxxxx] On Behalf Of Ron Bodkin
> Sent: Wednesday, August 31, 2005 2:01 PM
> To: 'AspectJ developer discussions'
> Subject: [aspectj-dev] Restricting type from user defined pointcut
> 
> I would like to be able to test if a variable bound in a named pointcut 
> is
> an instance of a subtype. For example, with
> 
> public abstract aspect Base {
>     public pointcut requestExecution(RequestContext requestContext) :
>         execution(* RequestContext.execute(..)) && 
> this(requestContext);
> }
> 
> I'd like to write a pointcut to say "a requestExecution where the bound
> variable is an instance of the subclass DerivedRequestContext." The
> "natural" thing to do fails:
> 
>     public pointcut derivedRequestExecution() : 
>         requestExecution(DerivedRequestContext);
> 
> C:\devel\scratch\inner\Base.aj:9 [error] incompatible type, expected
> RequestContext found DerivedRequestContext
> requestExecution(DerivedRequestContext);
>                  ^^^^^^^^^^^^
> 
> 1 error
> 
> Now usually people faced with this situation will just hold their nose 
> and
> write redundant code (which needs to be coupled to the details of how 
> the
> used pointcut binds the variable):
> 
>     public pointcut derivedRequestExecution() : 
>         requestExecution(*) && this(DerivedRequestContext);
> 
> It is actually possible to write a modular version of this with AspectJ
> pointcuts, but it's verbose and ugly:
> 
>     private pointcut derivedRequestExecutionParam(RequestContext
> requestContext) : 
>         requestExecution(requestContext) && if(requestContext 
> instanceof
> DerivedRequestContext);
> 
>    public pointcut derivedRequestExecutionOk() :
> derivedRequestExecutionParam(*);
> 
> In essence, I think it would be very useful if user defined pointcuts 
> can
> filter on bound values just like the system defined pointcuts for this,
> target, and args. Is it too hard to implement this? Or do people think 
> not
> allowing this is genuinely the right behavior? I find the asymmetry
> troubling, and the existing support for modularly doing this kludgely.
> Ramnivas reminded me that Adrian blogged about this a year ago to say
> essentially that this is the expected behavior (see
> http://www.aspectprogrammer.org/blogs/adrian/2004/05/effective_aspec.html).
> 
> 
> 
> Appendix (complete code sample)
> public abstract aspect Base {
>     public pointcut requestExecution(RequestContext requestContext) :
>         execution(* RequestContext.execute(..)) && 
> this(requestContext);
> 
>     public pointcut derivedRequestExecution() : 
>         requestExecution(DerivedRequestContext);
> 
>     private pointcut derivedRequestExecutionParam(RequestContext
> requestContext) : 
>         requestExecution(requestContext) && if(requestContext 
> instanceof
> DerivedRequestContext);
> 
>    public pointcut derivedRequestExecutionOk() :
> derivedRequestExecutionParam(*);
> 
> }
> 
>     abstract class RequestContext {
>         public abstract Object execute();
>     }
> 
>     abstract class DerivedRequestContext extends RequestContext {
>     }
> 
> Ron Bodkin
> Chief Technology Officer
> New Aspects of Software
> w: (415) 824-4690
> 
> 
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