Re: The Expression problem

[Per Bothner <per@bothner.com>]

On 8/30/22 11:24, daniel szmulewicz via Kawa wrote:
> Thank you for the prompt response. This reference indeed tells me how to
> create new classes or extend existing classes. However, I was interested in
> a way to add functionality without touching existing code. For example,
> make all instances on the java.lang.String class respond to `foo`, as in
> (foo "a string") -> "bar".

I think of "extending a class" as "creating a new class that extends an
existing class".  You're asking for something a little different, which I
would call "multi-methods".  Common Lisp calls them "generic functions"
(using "generic" in a different way than Java does).  Common Lisp also
differs from Java-style classes in that CL "methods" do not "belong" to
a class, which allows for more flexibility.

Kaws does have some support for "generic functions":
https://www.gnu.org/software/kawa/Generic-procedures.html

I don't remember if the "Warning" accurately describes the current state of Kawa
and method resolution.  You can certainly solve "the Expression problem"
if you provide the methods to make-procedure in proper order (most-specific
to least-specific).  I think there is a mechanism to automatically sort
by specificity, but I don't remember - it's been a long time.

I think Common Lisp generics are problematic if you have a module system
(or library system to use R7RS terminology) based on static name resolution.
CL generics assume a generic function is global, and as each module is loaded
it dynamically adds methods to the generic procedure.  This is powerful,
but seems contrary to modern module system.  I've thought a bit about how best to
specify multi-methods in a more disciplined way.  It's not an easy problem.
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/