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   RE: [xml-dev] [OT] Difference between an extensible versus an evolvable

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Yes.  So one has to pay attention to what layer or level of a system is 'evolving' or 'extending'.
The interesting bit about the implementation is that its evolution independent of the interface
is closed.  If changes in the interface don't change it (and shouldn't), then what is the source
of its evolution?  Programmer?  Internal measurements?  On the other hand, the improvement
in its speed does propagate to the containing system (it speeds up by some number determined
by for instance, the number of calls to it).   This then is be propagated to the system within
which that one is couched (think of a race condition).    The fun of modeling these is in setting
up coupled conditions that create novel behaviors.  That is emergence.  It isn't spooky behavior;
just tedious to audit and a reason to be suspicious of data mining systems.
At the heart of these concepts is the notion of system and environment in which the environment
is an evolvable system based on the activities of evolvable agents within it.  That is the basic concept 
of feedback-mediated adaptation which may be random or directed.   Nota bene:  boundaryless
systems never are because a system without boundaries goes to entropy at light speed.  This
has seldom stopped any consulting group in the last twenty five years from making money
on the concept though.  Now they will say 'permeable' and that is correct. 
Bottom up?  Maybe not.  It actually works in both directions which is something the so-called
'hippies' and 'straights' had to finally cope with and maybe the 'open sourcers' and
'proprietaries' will as well when the environment forces them both to evolve. ;-)

From: Chris Angus [mailto:chris.angus@btinternet.com]
I think that these definitions work well in terms of an interface.  However, if one looks at an implementation X of an interface B, the implementation might be said to 'evolve' if it changes without any change to the interface B.  Such 'evolution' might be to improve performance in some respect, with better algorithms surviving and poorer algorithms being killed off.


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