Re: [Zope] Acquisition Algebra; interaction of containment and acquisition is confusing

2001-01-26 Thread Evan Simpson

From: Fred Yankowski [EMAIL PROTECTED]
 for the very last case, "a.b.c.x".  I just can't follow why the
 equivalent expression isn't

 x.__of__(a).__of__(c.__of__(b.__of__(a)))

 rather than the more complex answer given:

 x.__of__(a).__of__(b.__of__(a)).__of__(c.__of__(b.__of__(a)))

You can expand any access path into an acquisition expression using the
following 3-1/2 rules:

1. Given an unwrapped object 'x', x.child = (child o x)
2. Given a wrapper (self o parent),
  a. (self o parent).child = self.child o (self o parent) if 'child' is
found in 'self'.
  a. (self o parent).child = parent.child o (self o parent) if 'child' is
found in 'parent'.
3. Reduce ((a o b) o (b o c)) to (a o (b o c)) as soon as it appears.

So, a.b = (b o a).  a.b.c = (b o a).c = (b.c o (b o a)) = (c o (b o a)).
Finally, a.b.c.x = (c o (b o a)).x = ((b o a).x o (c o (b o a))) =
   ((a.x o (b o a)) o (c o (b o a))) = (((x o a) o (b o a)) o (c o (b o a)))

 When I run the test cases given in the document I see that the latter
 does match the behavior, but I find that baffling.  In particular, why
 is the effective search order x-a-b-c rather than x-a-c-b?  It almost
 looks like the effective search order could be described as "up
 through the containment heirarchy, then down through the remaining
 acquired path", but I'm not at all sure if that's a valid
 generalization.

Almost.  It's "up through the containment hierarchy, then the rest of it
somehow".  Trying to control or predict the exact search order for any but
the simplest acquisition trees is a dangerous game.  You can read it off
directly from the algebra form, as in (((x o a) o (b o a)) o (c o (b o a)))
= x, a, b, c (ignoring duplicates), but it's unlikely to be useful, as you
saw.

Cheers,

Evan @ digicool  4-am


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Re: [Zope] Acquisition Algebra; interaction of containment and acquisition is confusing

2001-01-26 Thread Fred Yankowski

Thank you for describing the transformation rules in detail.  With those
I can mechanically create the same acquisition expression that you get. 
Working some further examples with this knowledge [and how can I use
this knowledge to make mo' money?], I find that given a containment tree
like this:

  a -- b -- c
   \-- x -- y

the access path a.b.c.x.y results in a search order of (y, x, a, b, c)

Evan Simpson wrote:
 Trying to control or predict the exact search order for any but
 the simplest acquisition trees is a dangerous game.  You can read it off
 directly from the algebra form, as in (((x o a) o (b o a)) o (c o (b o a)))
 = x, a, b, c (ignoring duplicates), but it's unlikely to be useful, as you
 saw.

Now, does that bother anyone besides me?  Since acquisition is intrinsic
and ubiquitous in Zope, shouldn't we be concerned that it is hard to
control or predict?

-- 
Fred Yankowski   [EMAIL PROTECTED]  tel: +1.630.879.1312
Principal Consultant www.OntoSys.com   fax: +1.630.879.1370
OntoSys, Inc 38W242 Deerpath Rd, Batavia, IL 60510, USA

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Re: [Zope] Acquisition Algebra; interaction of containment and acquisition is confusing

2001-01-26 Thread Evan Simpson

From: Fred Yankowski [EMAIL PROTECTED]
 Now, does that bother anyone besides me?  Since acquisition is intrinsic
 and ubiquitous in Zope, shouldn't we be concerned that it is hard to
 control or predict?

Keep in mind that it is only the *order after containment* that has this
problem. For instance, schemes to "skin" a subfolder by changing access
paths should ensure that there is always exactly one "skin" implementation
in the acquisition path.  It is tempting to provide a "default skin" in the
containment path, but then you can't override it.

Cheers,

Evan @ digicool  4-am


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