Re: Multiple Inheritance in Ada 9X

[odi!dlw@uunet.uu.net (Dan Weinreb)]

In article <1991Dec29.001511.8007objsys@netcom.COM> objsys@netcom.COM (Object S
ystems) writes:

   I must have missed this one, could you please give a pointer or summarize
   why CLOS's MI is more powerful?  

I think Barry summarized the important points pretty well.  The main
thing I think matters the most is that "mixin" classes can be
established in a library, and then users of the library can draw upon
them and use them together in whatever pattern in appropriate.  This
is harder or impossible to do in C++ and most non-CLOS-like approaches
that I am aware of.

				    After deciphering the semantic mess of
   how method selection is performed in CLOS, its not surprising people are
   scared of MI.  It seems to me they made a semantic rule mountain out of a
   little mole hill.  As I recall the rules are obscure (linearized) and 
   had non-intuitive special cases.

This is exactly what I mean about language theorists.  In fact, it's
not a "semantic mess".  It's well defined, and in real-life
circumstances it's simple and natural.  Many programmers who I worked
with, including me, used this stuff (actually its predecessor,
Flavors; CLOS is actually an improvement) every day, for years,
producing many, many thousands of lines of code, with no problem.

If you are a language theorist, and you work hard at it, you can
construct cases in which CLOS's behavior regarding the ordering of the
CPL is sometimes counterintuitive.  This kind of thing is rare in
practice.  It's important to theoreticians, but for real programmers
it makes no difference.  The hue and cry that has been raised about
this is completely unjustified from the point of view of getting the
job done.  Maybe you're the kind of person who never uses a hammer
under any circumstances because it is possible for the hammer head to
fly off and cause damage, and so you're waiting for the Perfect
Hammer.  But there are plenty of CLOS programmers who find it to
be a very useful tool, and they'll happily use it until a better
one comes along.