Pattern, Abstract Factory / Factory

[Faré]

>:Pascal Bourguignon, speaking of some code being discussed

> Also, it doesn't really implement the abstract factory design pattern,
> because defclass being a macro, that specifies that the metaclass IS the
> name of the metaclass, not that it is EVALUATED to a name of a
> metaclass, you cannot create a class with a metaclass in a variable (not
> directly at least; you could do it using eval and building a defclass
> form at run-time).  Fundamentally defclass is a compilation-time
> operator, while the abstract factory design pattern is designed to
> select the concrete classes at run-time.
>
If you want to define "classes" at runtime rather than compile-time,
you may want to use a prototype object system, rather than a
class-based object system.

Yale T Scheme in 1981 had one, designed by Jonathan Rees. In Lisp,
there have been Object Lisp (1985), ABCL (1986), and more recently
Sheeple (2008), dto's CLON (that I can't find anymore), CLPOS (2015),
and possibly many more.
https://github.com/xach/sheeple
https://github.com/DrPyser/Common-Lisp-Prototype-Object-System

I've been building my own prototype object orientation system for
Gerbil Scheme, POO, and have been extremely happy using it for over a
year. Precisely because I need to generate types or classes at
runtime, since my runtime is my users' compile-time. I just added this
week multiple inheritance to it with C3 linearization, and it's pretty
cool. (I also built a similar system in nixpkgs.)

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