Supervisors as factories *and* registries

[Garrett Smith]

I've gotten into the habit of using supervisors as factories and
really like the pattern. I think it's clean to delegate process life
cycle to supervisors, since that's what they're there for. So far so
good, with one big exception...

As factories, supervisors are also a logical candidate as registries.
supervisor:which_children, for example, is a brute force way to get at
the processes under supervision.

Bit I need a finer grained interface to supervised children. I wish
supervisors exposed state the way gen_server does, in which case I
could maintain an index (for example) that would let me access
children more efficiently. (This seems like a conspicuously missing
feature, but there's probably a good reason for it.)

Is there a recommended pattern for this sort of thing? To summarize
what I'm looking for:

- Wrap calls to supervisor:start_child in a factory function for processes
- Provide a lookup interface to supervised children that doesn't rely
on a linear scan of supervisor:which_children(Sup)

I could obviously maintain a separate process as a registry, but I'd
prefer to avoid the sync/link/cleanup overhead associated with that.
I'd be okay with hacking supervisor (create a supervisor2?), but I
haven't explored that option.

Garrett