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From: Stefan Monnier <monnier@iro.umontreal.ca>
Newsgroups: comp.lang.lisp
Subject: Re: Local/temporary methods in CLOS
Date: Tue, 11 Mar 2025 18:02:05 -0400
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>> I need some way to define "local" methods, which can override
>> temporarily "global" methods and obey some kind of dynamic scoping rule.
>
> I feel that your requirements may be somewhere in the locus of
> Pastal Costanza's work, like ContextL.
>
> https://github.com/pcostanza/contextl
>
> Also see Costanza's paper "Dynamically Scoped Functions as the
> Essence of AOP" [2003].

Thanks!  That's a nice solution, and the underlying implementation
technique makes a lot of sense (it refines the manual solution I've
been playing with).

>> Assuming I'm not the first to feel such a need, what are usual
>> approaches to try and provide that kind of behavior?
>
> Since in Common Lisp we don't have dynamically scoped functions, only
> variables, the straightforward thing, without using anyone's framework,
> would be to use special variables to hold certain functions of interest,
> and funcall them. (That can be wrapped or macroed over.)
>
> (defun overridable-fun (&rest args)
>   (apply *overridable-fun* args))
>
> (let ((*overridable-fun* (lambda (...) ...)))
>   (overridable-fun 42))

For dynamically-scoped local functions, that's indeed what I typically
use (or alternatively, I simply pass that function explicitly down the
stack), but my question is for the case where I want to do it for
methods, i.e. for "part" of a function (I guess I could simulate it by
overriding with a function that tests the args' type and delegates to the
old function, but then I get a kind of "linear-time" dispatch if there
are N local methods, which I'd rather avoid).


        Stefan