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Re: clear parameters

daemon@ATHENA.MIT.EDU (Tue Nov 16 19:13:34 1993 )

Date: Tue, 16 Nov 1993 16:03:19 -0800
From: dougo@pure.com (Doug Orleans)
To: coldstuff@MIT.EDU
In-Reply-To: <9311162304.AA00973@gac.edu>

 > Obviously there are ways to avoid this particular example.  Still, I
 > really don't like the assumption that because a child's value is equal
 > to the parent's value, the two are somehow linked.

Basically, there are two notions of `default value':

    1) I want the child's property to always be the default, which is
       chosen by the parent and changes dynamically;

    2) I want the child's property to be fixed at x, where x was the
       `factory default' given by the parent when the child was created.

MOO deals with this by having 1) be a clear property, and 2) be a
property that was explicitly set to the default (foo.bar = foo.bar).
FMPL deals with this by doing property lookup dynamically at runtime;
when you reference the child's property, if it's not defined directly
on the child, it looks up the parent tree until it finds the
definition, but when you assign a value to the property, it makes a
new binding on the child.  (This behavior happens because there's
really no difference between `verbs' and `properties' in FMPL, just
`bindings', so there's only one lookup mechanism.)  These seem to be
semantically equivalent, with the only difference being that FMPL
doesn't store a `clear property' marker on the child -- which is a win
in most cases, because then an empty object takes virtually no space,
instead of the same amount of space as its parent's properties; the
only downside I can see is that if you try to reference a property
that isn't defined, you have to traverse the whole tree to find this

I dunno how Cold does it; from the remarks Greg has made it I'm
guessing that properties (data members, slots, whatever you want to
call them) aren't inherited at all except at creation time, which
means that it's like MOO before it had clear properties.  I guess you
can simulate clear properties with methods that lookup the parent tree
manually if the property is equal to a certain out-of-band value (e.g.
'clear), but it seems like it would be nicer if there were some
in-server way of doing this.