编程与软件工程people.csail.mit.edu
“闭包是穷人的对象，对象是穷人的闭包”（2003）
“A closure is a poor man’s object; an object is a poor man’s closure” (2003)
2010 — 2017，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
这是 2003 年 LL1 邮件列表中的一条经典讨论，围绕“闭包是穷人的对象，对象是穷人的闭包”这一说法展开，探讨闭包与对象在封装状态、抽象方式上的等价性与差异。帖子以公案式的对偶句收尾，多年来在 Hacker News 上反复引发讨论。
为什么经久不衰：它用一句话点破了闭包与对象这对核心抽象的深层同构，至今仍是理解函数式与面向对象编程关系的绝佳切入点。
HN 评论精选
真正的对象应能动态改变自身行为，按此标准 Erlang 比 Java 更面向对象
我对对象的定义是：一种 ADT，能用私有运行时状态“决定”如何响应任何交互。由此得出一个简单检验：如果某物是对象，你应该能给它发一条消息，让它此后改变自己接受哪些消息。C++/Java 的原生对象做不到这一点，但 Ruby（Object#extend）和 Erlang 可以。结论：Erlang 比 Java 更面向对象。这个定义虽与几十年来关于 OOP 的说法相悖，但它才是原始定义——Smalltalk、LambdaMOO、乃至说 JavaScript 是面向对象时指的都是它。可惜两种含义共用一个词。
derefr查看英文原文
My definition of an object is "an ADT that gets to 'decide', using private runtime state, what to do in response to any/all attempts to interact with it." Which makes for a very simple test: if something is an object, you should be able to send it a message that makes it change its mind about what messages it accepts after that. There's no obvious way to implement a behavior like this using the native "objects" in nominally-object-oriented languages like C++ or Java. (But you can do so with the closure types from these languages. Or you can look higher on the stack: threads with IPC queues, or POSIX processes, are both objects.) But implementing this behavior is obvious in Ruby; this is how basic things like Object#extend work. And implementing this behavior is also obvious, oddly enough, in Erlang: http://joearms.github.io/2013/11/21/My-favorite-erlang-program.html In conclusion, Erlang is more object-oriented than Java. ;) --- And yeah, I know, this definition of OOP-ness sort of flies in the face of decades of things people have said about OOP. Objects, under this definition of OOP, don't obey any of the SOLID principles. They're slippery; unpredictable; "alive." But this was the original definition! It's the one Smalltalk fits; it's the one LambdaMOO fits; it's what is meant by referring to Javascript as object-oriented. It's a shame we have this collision between meanings; having two separate words for "OOP like Smalltalk" and "OOP like Java" would have saved language-designers all a world of arguments from people who try to propose extensions to a language in one category, assuming it's in the other.
闭包以代码为中心，对象以数据结构为中心
我喜欢在 Scheme 里构建对象系统。闭包的方式像是专注于代码而不太在意底层数据结构；对象的方式则是关注数据结构，代码退居其次。
jfoutz查看英文原文
I enjoyed building object systems in scheme. The closure way is kind of like focusing on the code without worrying to much about the underlying data structure. The object way is worrying about the data structure and the code is secondary.
闭包可共享重叠的变量引用，这是对象做不到的，二者并非真正等价
多个闭包可以重叠引用同一组变量，而对象做不到这一点，所以两者并不那么相似。只有最简化的闭包用法（每个闭包对应一组独立的变量槽）才与对象封装相匹配。
white-flame查看英文原文
Multiple closures can overlap in their referencing of some set of variables. Objects can't really do this, so they're not that similar. Only the most simplistic application of closures (one unique set of instantiated variable slots per closure) matches object encapsulation.
闭包隐式捕获状态，让探索抽象的成本更低
对象与闭包的一个区别是：对象需要显式传入状态，而闭包可以从上下文捕获状态。这很强大，因为让你能低成本地探索不同抽象。用 Lisp 编程时我常常发现用 Java 这类没有闭包的语言注意不到的抽象。比如一个按钮就是一个闭包，它只支持一种操作：点击。
asimjalis查看英文原文
One difference between objects and closures is that objects require explicitly passing state into them, while closures can capture state from their context. This is powerful because it lets you explore different abstractions cheaply with closures. Programming in Lisp I frequently stumble upon abstractions that I would not have noticed if I was programming in Java or some other closure-less language. For example, a button is just a closure. It supports exactly one operation: click.
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