Classes with several, mostly unused, attributes

[Abhijit Mahabal]

Consider a class (e.g., the hypothetical Geometry::Triangle) that can 
have several attributes (side1, side2, side3, angle1, ang_bisector1, 
side_bisector,  altitude1 and so forth), most of which will not be 
needed for most instances of Geometry::Triangle.

I know how this can be done in P5. Using the layout "Hash" things are 
easy. How can P6 deal with this, without allocating too much memory to 
things that'd never get used? The layout "P6Hash" could come to the 
rescue, but there is still the issue of syntax:

what exactly does C<has $.bisector1> do? IIRC, in the P6Opaque layout, 
every instance of the class would have space the size of a PMC allocated 
for it. This behavior is not needed for P6Hash, and it should just leave 
attributes alone until they are assigned to (where defaults are also 
"assigns").

In which case, maybe for that layout we can get away without declaring 
all attributes, perhaps? (Since the declaration does nothing except help 
the parser).

I was thinking whether we could do something like this:
class Triangle is layout<P6Hash> does autovivify{
  method calculate_bisectors { $.bisector1 = ...;
                               # $.bisector1 autovivifies
                             }
}
where it is an error without the autovivify, and only P6Hash supports 
autovivification.

--abhijit