编程与软件工程github.com
为 C 语言引入 Lisp 宏
Lisp macros for C
2014 — 2022，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
cmacro 是一个为 C 语言实现类 Lisp 宏系统的开源项目，让开发者能在 C 中编写强大的宏来扩展语言。项目作者认为，没有宏的语言只是工具，而有宏的语言则像建筑材料，可以被塑造成你的应用。该项目在 2014、2017 和 2022 年多次登上 Hacker News 首页，引发了关于宏系统本质的深入讨论。
为什么经久不衰：它触及一个经久不衰的话题：如何把 Lisp 式的元编程能力带进 C 这样的底层语言，每次重提都能引发关于宏系统设计本质的讨论。
HN 评论精选
真正的 Lisp 宏的关键在于宏与普通代码共享同一语言环境，可自由互相调用
评判任何“宏”系统的好问题是：假设你已用该语言写了一个有用的函数 foo，之后想写宏 bar，你能在写 bar 时使用 foo 吗？如果答案是“不能”，那你就没有真正的类 Lisp 宏。Lisp 宏的强大之处不在于能改写抽象语法树——那用解析器和模板语言也能做到——而在于“写代码”和“写宏”之间没有真正的分界，全都是代码，函数和宏交织在一起共同塑造语言。相比之下，在 ClojureScript 里写宏时，你得时刻考虑某个函数是在 Clojure 世界还是 ClojureScript 世界，需要的话还得移植过去。
stevelosh查看英文原文
A good question for any "macro" systems like this is something like the following: 1. Suppose you've written a function 'foo' in your language that does something useful (e.g.: partitions a sequence, validates a map, or pretty much anything really). 2. Some time later you want to write a macro 'bar'. 3. Can you use function 'foo' when writing 'bar'? If the answer to 3 is "no" then you don't really have Lisp-like macros. This is what makes Lisp macros so powerful. It's not just that you have a way to mangle abstract syntax trees. If that's all you want then yeah, you can write a parser and template language to do it, like this thing or sweet js, but it's not the same. Lisp macros are beautiful because there's no real divide between "writing code" and "writing macros". It's all just code. You don't have to worry about which things are available in code-land and which are available in macro-land because they're the same place. There's just "the language" which you extend and mold with functions and macros woven together as necessary. Contrast this with a language without real Lisp-like macros, like Clojurescript. If I want to write the 'bar' macro in Clojurescript I need to think "wait 'foo' is a Clojurescript function so now I need to port it back into Clojure land before I can use it in this macro because macros in Clojurescript live in Clojure-land not Clojurescript-land". I need to think about this for everything I call inside a macro. (Admittedly the situation isn't as bad in Clojurescript because it's fairly close to Clojure in syntax and it's possible to cross-compile code so it lives in both "lands", but the ugly divide is still there.) Common Lisp, Clojure, Scheme, Wisp, Julia, etc have Lisp-like macros. Sweet JS, Clojurescript, C, etc don't.
推荐 Terra：一个用 Lua 实现类似元编程能力的相关项目
类似的项目可以看看 Terra，它是“Lua 的底层对应物”，基于 Lua，但拥有通过 LLVM JIT 编译成机器码的底层函数，支持从底层函数调用 Lua 宏，也可以用 Lua 代码编写复杂的高层框架。项目仍在开发中，有些部分还不完善，但非常有意思。
tomp查看英文原文
For something similar, take a look at Terra [1], the "low-level counterpart to Lua". It's based on Lua, but has "low-level" functions that are JIT-compiled to machine code using LLVM. It supports calling Lua macros from low-level functions, or you can use Lua code to write complex high-level frameworks (e.g. Go-like object system [2], Java-like object system [3]). It's still in development, so there are parts of the system that don't work very well (e.g. global variables in compiled executables cannot be statically initialized), but it's a very interesting project. [1] http://terralang.org/ [2] https://github.com/zdevito/terra/blob/master/tests/lib/golike.t [3] https://github.com/zdevito/terra/blob/master/tests/lib/javalike.t
能否集成 Clang 工具链是决定其实用性的关键
如果能把它集成进 Clang 流水线，让基于 Clang 的自动补全和错误检测能正确处理宏展开后的源码，就能轻松融入现有项目工作流。否则由于缺乏工具链支持，可用性会受限。Clang 已经对 C 预处理器做了这件事，不过那可能容易得多，因为预处理器不是图灵完备的。
terhechte查看英文原文
It would be great to be able to integrate this into the Clang pipeline so that Clang based auto completion and error detection would not trip over it but correctly evaluate the resulting source and use that. Then, it would be easy to integrate this into an existing project workflow. Otherwise it would limit usability due to a lack in tooling support, I'd guess. Clang already does that for the C preprocessor, although that is probably much easier to do since it is not Turing complete [1] [1] By definition. Though some people throw crazy things at it: http://stackoverflow.com/questions/3136686/is-the-c99-preprocessor-turing-complete
宏能给 C 带来便利，但也担心可读性，Nim 或许是更好的选择
这是给 C 注入新生命力的有趣方式，我已经能想到几个强大宏能派上用场、消除痛点的地方。但另一方面，我不确定是否想读别人写的、混杂着高层构造的底层代码。也许 Nim 更适合这个目的？
veli_joza查看英文原文
This is very interesting way to breathe new life into C. I can already think of few places where powerful macros would come in handy and eliminate some pains. On the other hand, I'm not sure if I'd want to read other people's low-level code sprinkled with high-level constructs. Perhaps Nim would serve this purpose better?
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