Slot inheritance was invented for AI and other reasoning systems -- 
and also for providing protection of polymorphic meanings. One of the 
ways of looking at it is that for many objects, the contents of the 
instance variables can be thought of as "parts". Thus, one could 
imagine a lookup strategy that would search down through the instance 
variables for ideas and info about the object.
      An example might be to deal with the question "how heavy?" the 
idea might be to look at the slot called "material" and ask the 
actual value for help. It could in turn ask its variables for help, 
etc. So this gives a dynamic way to enlarge the property space of an 
object. If you include the static superclass chain in the mix of 
searched variables, then you get regular inheritance. This is one of 
the many inheritance strategies used in CYC.
      Another view of this is to consider the polymorphic meaning of 
"print" in Smalltalk. There is no protection of the generic meaning, 
any programmer can write whatever they want in a method. However, we 
could make a subclass of class Method called Print whose instances 
are Print-Methods. These would not allow arbitrary code to be written 
in them, but might have places where programmers could add useful 
ideosyncratic information. All of these print methods would really 
try to print something. So slot classes and slot inheritance allow 
real definitions and protections of meanings. This idea also goes 
back to the early 70s.

All these were easy to do in ST-72 because each 
class/closure/instance really got passed control *before* the message 
was received.
      There were some suggestions that a "prereceive" method be 
allowed in ST-76 and -80 -- if present, a flag would be set somewhere 
efficient, and it would be called first with the actual message put 
in a place where it could be received later. This would have allowed 
all of these schemes, delegation, etc., to also be done in the later 
versions of ST. Someone on the list might want to figure out good 
ways to do these schemes in Squeak. Folks like Nathanial Scharli and 
Stephane, who are playing with metasystems, should add these kinds of 
considerations into their play.

Etc.

Cheers,

Alan

-------

At 3:32 PM +0200 5/1/02, Henrik Gedenryd wrote:
>stephane ducasse   (home) wrote:
>
>>  What was slot inheritance?
>>
>>  Stef
>
>As I understand it that was a precursor of delegation a la Lieberman and
>then Self and so on: inheritance goes via the slots instead of via the
>class.
>
>But this understanding is based on very scarce information.
>
>Henrik


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