Re: CLOS-style MOP for Guile ?

[Christian Lynbech <chl@tbit.dk>]

If I understand Michaels concern, the problem is this:

When adding a method to a generic function, the generic function is
destructively updated, so to speak.

So in the current implementation, when module B imports a generic from
module and adds a method, the generic function is changed not only in
module B but also in module A, which could be a surprise for users
of the function inside module A.

The safest approach to importing and updating a generic function would
then be something like this: in module B, define a *fresh* generic
function which then, each time it is invoked, will have to compute the
actual set of specializers by looking them up in both A and B in order
to derive the correct method to call.

Apart from the fact that this does not sound overly efficient, I am
not quite sure that this is really what one wants. If such a scheme
was implemented, it meant that there was no way for module B to
intentionally change the (global) behaviour of the generic function
originating from A.

An alternative to the restrictive approach of preventing B from doing
global updates, would be to allow it and instead use specifications on
the module A detailing what should and should be done to the generic
functions provided by A.

The idea is of course not my own. Kiczales and Lamping has written a
rather nice paper on how to specify OO libraries. It is called:

      Issues in the Design and Specification of Class Libraries

and can be found under the name of documenting-oo-programs.ps.Z
somewhere at their ftp site, whose address I do not remember but which
I guess is the Xerox PARC one.


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Christian Lynbech          | Telebit Communications A/S                       
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Hit the philistines three times over the head with the Elisp reference manual.
                                        - petonic@hal.com (Michael A. Petonic)