编程与软件工程aiai.ed.ac.uk
CLOS 简明指南（1998）
A Brief Guide to CLOS (1998)
2015 — 2018，它一次次回到首页
黑点是上首页（≥ 50 分）的那次投稿，空心点是没冲上去的投稿，点越大分数越高。
导读
这篇 1998 年的短文简要介绍 Common Lisp Object System（CLOS），涵盖类、泛型函数、方法以及方法组合等核心机制。它更像一份面向已有 Lisp 或面向对象背景读者的快速导览，帮助理解 CLOS 与主流 OO 语言不同的组织方式。
为什么经久不衰：它篇幅短小却抓住了 CLOS 最独特的部分——泛型函数与方法组合，让有 OO 经验的读者重新思考方法分派和继承的边界。
HN 评论精选
CLOS 很强大，但作者在生产中并未真正需要其高级特性，怀疑普通 OO 已够用。
我在高中学习了 Common Lisp 和“够用”的 CLOS，被它的能力震撼，整个职业生涯都希望能参与一个用上 CLOS 各种有趣特性的复杂 CL 系统，但一直没遇到（我 80% 在 Ruby，20% 在 Clojure）。奇怪的是，我也没真正渴望过 AOP、多方法、MOP 等。是我做过的系统太无聊，还是 Ruby 的 OO 和库生态已经够用？我很想听听资深 Lisp 程序员在生产中使用 CLOS 的经验。
tsm查看英文原文
I learned Common Lisp and "enough" CLOS in high school, was blown away by CLOS's power†, and have spent the entirety of my professional career wishing that I could have a job working on a complex system in CL that used all of CLOS's interesting features. This has not happened (I've been 80% in Ruby-land and 20% doing Clojure), but what's weird is that I haven't even had situations where I really yearned for AOP, multi-methods, the MOP, etc. I can't decide if the systems I've worked in have just been too boring (they certainly solved the business problem though) or if Ruby's object-orientation (and library ecosystem) is good enough that I just don't need the full power of CLOS. (Designing Clojure programs is a totally separate kettle of fish.) I'd be interested to hear from more seasoned Lispers about their experience with CLOS in production. † I took an FP class during my first semester of college. The professor wrote the book himself, and had an extended sidebar comparing the design considerations of OO systems (easy to add another class; tedious to reopen n subclasses to add another method to each one) and FP systems (easy to add another function dispatching on type; tedious to reopen n functions and make them dispatch on a new type). I pointed out that CLOS handled both situations equally well, and in the final edition of the book he tossed in a footnote saying as much and thanking me.
通过 CLOS 的显式机制能更深刻地理解 OO 方法分派与继承的可塑性和通用性。
强烈推荐没试过 CLOS 的 OO 程序员玩一玩 CLOS（或 Guile 的 GOOPS、Racket 的 Swindle 这类 Scheme 方言仿制品）。你可能整天处理方法分派和继承这些 OO 概念，但在 CLOS 中它们被显式地摆出来，与底层 Lisp 分离，能帮你内化这些概念，看清它们多么任意、可塑，以及按 CLOS 那样宽泛解释时多么通用（日常语言的 OO 系统堵死了很多路）。
CamTin查看英文原文
I highly recommend playing with CLOS (or one of the Scheme flavored knockoffs like GOOPS in Guile or Swindle in Racket) for anyone who hasn't but works in OO languages (probably most working programmers). You probably spend all day dealing with OO ideas like method dispatch and inheritance, but to see them laid out explicitly in CLOS, as distinct from the underlying Lisp language, really helps you to internalize them and how arbitrary and malleable they are, as well as how astoundingly general they are when interpreted as broadly as CLOS does (the object systems in workaday languages cut off a lot of the avenues open to systems defined in terms of CLOS). You can get a similar "aha" moment by looking at how Perl does OO if you already know Perl, or reading the implementations of one of its dozens of different user-implemented object systems. Perl has some language-level support for OO but it is very minimal so these object systems have to do most of the work that a Common Lisp object system would. I think the main benefit is in seeing OO implemented in a language with no built-in support for it, rather than a language built around the concept of classes and inheritance or some version of it.
方法组合器强大但容易失控，CLOS 限制为三种常用组合器是吸取历史教训。
我好奇作者会不会提方法组合器，他提了。它们非常强大、有用，也是制造意大利面代码的绝佳机会。CLOS 的组合器吸取了早期教训，只允许 :before、:after 和 :around 这些最有用、最可理解的组合器。Lisp Machine Lisp 有大量方法组合器，包括完整控制结构：:or、:and、:progn 等。它们让代码可扩展、紧凑，也完全不可预测（如果你不精确理解运行时方法层级，:or 组合会运行哪些方法？）。尽管如此，我有时到今天还怀念它们，哪怕在 C++17 代码库里。
gumby查看英文原文
I wondered if the author would mention method combinators, which he did. They are super powerful, very useful...and a marvelous opportunity for spaghetti code. Actually the CLOS combinators show learning from earlier mistakes and allow only :before, :after and :around, the most useful and the most comprehensible combinators. Lisp Machine Lisp (including the "zetalisp" variant dreamt up by Symbolics' marketing department) had an enormous panoply of method combinators including full control structures: IIRC I had cause to resort at one time or another to :or, :and and :progn combinators (the latter being somewhat like :around, I suppose, but not the same). They made the code extensible, compact, and completely unpredicatable (if you didn't understand the runtime method hierarchy precisely which combined methods would be run in an :or combination? Nevertheless I sometimes miss them to this day, even in my C++17 codebase!
CLOS 应协议优先：先定义泛型函数，再用方法和类实现协议。
从类出发、把泛型函数描述成发送消息的方式，对 CLOS 来说相当本末倒置。CLOS 的最佳用法主要是先用泛型函数定义协议，再用方法和（可选的）类实现协议。http://xach.com/lisp/jrm-clos-guide.html 有 Joe Marshall 的“Warp Speed” CLOS 指南副本。
zachbeane查看英文原文
Starting from classes and describing generic functions as a way to send messages is pretty backwards for CLOS. CLOS is best used primarily by defining a protocol with generic functions, then using methods and (optionally) classes to implement the protocol. http://xach.com/lisp/jrm-clos-guide.html has a copy of Joe Marshall's "Warp Speed" CLOS guide.
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