Root-relative namespace keys

[Patrick R. Michaud]

On Tue, Jan 06, 2009 at 04:44:47PM -0800, Allison Randal wrote:
>> Similarly, to check if an object isa ['Foo';'Bar'] from that other
>> hll (and we don't already know that the ['Foo';'Bar'] class exists):
>>
>>     $P99 = get_root_namespace ['hll';'Foo';'Bar']
>>     if null $P99 goto not_isa
>>     $I0 = isa $P0, $P99
>>     goto done
>>   not_isa:
>>     $I0 = 0
>>   done:
>
> If the other hll namespace doesn't exist, then we can guarantee that the  
> given object isn't from that class. That is, passing in a null PMC to  
> 'isa' should always return false. (Checking...) Okay, yes, a null PMC  
> returns false for 'isa' so you could just do:
>
>   $P99 = get_root_namespace ['hll';'Foo';'Bar']
>   $I0 = isa $P0, $P99

When I try this in current trunk (r35084), I get:

  $ cat x.pir
  .sub 'main'
      $P0 = box 3
      $P99 = get_root_namespace ['parrot';'Foo';'Bar']
      $I0 = isa $P0, $P99
      say $I0
  .end

  $ ./parrot x.pir
  Null PMC access in isa_pmc()
  current instr.: 'main' pc 6 (x.pir:4)
  $

More to the point, if the 'isa' opcode manages to accept a null PMC
in the way you describe (i.e., without throwing an exception), it 
will be the *first* such opcode I've encountered that does so.  
If we're going to change 'isa' to automatically return false when
given a null PMC as its last argument, then there are some other opcodes
where I'd like us to consider applying the same rule -- i.e., returning
false or something more graceful than throwing an exception.

> [...]
> I'll propose alternatives in a separate email.

I look forward to this.

Pm