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Re: Proposed change regarding MI ambiguities

daemon@ATHENA.MIT.EDU (Fri Dec 10 21:59:59 1993 )

Date: Sat, 11 Dec 93 02:56:33 GMT
From: Andrew Wilson <Andrew.Wilson@cm.cf.ac.uk>
To: Andrew.Wilson@cm.cf.ac.uk
Cc: coldstuff@MIT.EDU

> From ghudson@MIT.EDU Sat Dec 11 02:06:43 1993
> To: Andrew Wilson <Andrew.Wilson@cm.cf.ac.uk>
> Cc: coldstuff@MIT.EDU
> Subject: Re: Proposed change regarding MI ambiguities
> Date: Fri, 10 Dec 93 20:59:27 EST
> From: Greg Hudson <ghudson@MIT.EDU>
> Content-Length: 1884
> > Absolutely not.  It is evident that the ambiguities, if any, can be
> > detected when the method is defined.  Such ambiguities are, after
> > all, merely syntactic.  There really is no reason at all for not
> > creating a @lint, or equivalent mechanism for flagging any
> > ambiguities and for refusing to construct an ambiguous lattice.
> > Force the semantics to be unambiguous and you are saved the horror
> > of run-time lattice verification.  I'd suggest that you read up on
> > the implementation of the Eiffel programming language NOW rather
> > than later.  Do it right, or don't bother.
> Two disagreements:
> (1) I disagree that run-time lattice verification is a "nightmare".
> As I said, I'm pretty sure I can do it in linear time, which is how
> long a method search is going to take in the general case anyway.
> There won't be a performance penalty relative to the current system.
> If there's some other reason why this isn't "doing it right," then I'd
> like to hear it.  Please leave the implementation details up to me,
> though.
> (2) You're not thinking in terms of a mud language.  I can't restrict
> programmers from defining methods on their objects because other
> programmers have defined methods on their objects.  Consider the
> following hierarchy:
> 	A
>     B   C
> 	D
> Suppose I own B and you own C.  Someone else owns D.  You define a
> method 'foo' on C.  Now I can't define a method 'foo' on B, I can't
> undefine your method on C because I don't own C, and I can't remove
> B as an ancestor of D because I don't own D.  But D's owner is the one
> who should have to deal with the problem, since it's D that's (after I
> define the 'foo' method) inheriting from incompatible parents.

Yep, I know how this thing breaks down.  The point is that you seem to be
confusing semantics with syntax.  The point is not 'who owns what method' or
even what the method is named, but rather 'what the method does'.  Assuming
from your lattice that A is a super-class of B and C, and D a sub-class of B
and C then there must be, of necessity, a single method on D named 'foo'.  So
B.foo may print an asterisk, and C.foo may print a plus, what should D.foo do?
The answer is 'whatever D's designer decides is good'.  So D.foo prints an
ampersand.  But implicit in the lattice semantics is the notion that D inherits
the semantics of B.foo and C.foo.

Now you can take this in a range of ways:

1)	Collapse the semantics at D by redefinition.  D.foo prints an
	ampersand.  You can define a schema like this:

		class D {
		inherit B
		inherit C
		method foo {

2)	Preserve the semantics from B and C and define additional semantics
	at D.  Rename [a SYNTACTICAL exercise] B.foo on inheritance into D to:


	Rename C.foo on inheritance into D to:


	D can now print asterisks, pluses AND ampersands.

	Indeed there's nothing to stop you from defining the following schema:

		class D {
		inherit B
			rename foo to foo_from_B
		inherit C
			rename foo to foo_from_C
		method foo {
			if (random(2))

	just for kicks.

In each case the decision for the correct behaviour of a class rests with the
class's designer, not the designer of inherited sub-classes.

[returning to G's original text]

Now, to follow your argument, suppose that at some time in the future B's
programmer decides to add a method to B called, would you believe it!...


A syntactical argument would state that this is not possible because D would
then have inherited two differently derived methods, both NAMED 'foo'.  The
lattice would be confused at D and trace'R'us.

A emantic argument would state that, sure, both B and C supplied an inheritable
method named 'foo' then D would break UNLESS:

	1)  D.foo was specified as a redefinition of both B.foo and C.foo


	2)  Either B.foo or C.foo [or both] were renamed to something else with
	D.foo's semantics being suppiled either by a complete redefinition


The true semantics of a method are defined in terms of what the USER of the
method intends.  An instance X of D isa B => X.B:foo has meaning, and will
continue to have meaning for all subclasses of D.

Jesus!  Where did all this come from?

> Also, with a disk-based db, having to check non-local conditions while
> making a local change carries severe performance penalties.

Really.  How many times a second are you going to have to check that the semantic
content of a lattice is valid before you discover that it is not.  By my
reckoning, once.  Once detected, the lattice is just plain broken and should
trace you to hell till you fix it.

> --GBH
> Remember, MUDs are a harder problem than real programming languages.