Improving inspect on class objects

[Patrick R. Michaud]

On Sun, Jan 04, 2009 at 05:00:46PM -0800, Allison Randal wrote:
> Jonathan Worthington wrote:
>> Patrick R. Michaud wrote:
>>
>>> Lastly, there's also the issue that the cloning makes inspection
>>> of classes' attributes and methods into a much more expensive  
>>> operation, and helps to explain why P6object, PCT, and Rakudo
>>> were generating far more hashes than I expected.
>
> What are you using 'inspect' for? It should be rarely used (i.e. it's  
> intended for advanced debugging, not for highly-optimized ordinary  
> program operation).

Currently it's used for two purposes:

1.  When creating a new instance of the class, it's used to identify
    (a) the list of attributes that need initialization and 
    (b) the metadata needed to initialize each attribute

2.  When defining the class itself, I'm (ab)using it to modify
    the attribute metadata, since at present there doesn't
    appear to be another interface available for doing so.

> If 'inspect' is the only way to get at some piece of data you use  
> frequently, we likely need to give you a simpler way of accessing it.

"List of attributes and metadata" is what we will want/need frequently,
as identified in (1) above.

> But, 'inspect' is definitely not the preferred way of modifying class  
> attributes. 

I can agree to this.

> The right way to add custom attribute metadata is through the  
> 'add_attribute' vtable function. If you look at:  
> docs/pdds/pdd15_object_metamodel.png you'll see it was originally  
> designed to allow user-defined metadata flags in the attribute metadata.
>
> In the vtable signature for 'add_attribute', 'type' should actually be  
> 'metadata' which can alternatively be a hash of metadata for the  
> attribute, rather than just type:
>
>   VTABLE void add_attribute(STRING *name, PMC *type) {

That would work just fine for me -- I'm guessing that the
keys in 'type' ('metadata') would end up being used as the
keys in attrib_metadata ?

> This doesn't allow for modification of metadata for an existing  
> attribute. I'm open to adding 'get_attr_*' and 'set_attr_*' vtable  
> functions to the Class PMC for modifying attribute metadata. It's  
> sensible that the 'getattribute' and 'setattribute' opcodes on a Class  
> work with attribute metadata, while on an Object they work with  
> attribute instance data.

This would be a very workable solution from my perspective.
I'll be glad to make these changes to trunk if you're willing
to commit to them.

In the 'rvar' branch I've already adjusted the inspect_str vtable 
so that it returns a shallow clone of hashes instead of a deep clone,
and I can merge that into trunk also.

Thanks!

Pm