编程与软件工程lists.warhead.org.uk
为什么 Lisp 宏很酷：一个 Perl 程序员的视角（2005）
Why Lisp macros are cool, a Perl perspective (2005)
2009 — 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
这是 Perl 社区知名开发者 Mark Jason Dominus 在 2005 年写的一封邮件，从 Perl 程序员的视角解释为什么 Lisp 的宏系统如此强大：Lisp 宏操作的是解析后的代码数据结构，因此不会像 C/C++ 那样因文本替换而轻易出错。文中通过对比源码过滤器等 Perl 中的近似方案，说明 Lisp 语法统一（代码即数据）带来的根本优势。
为什么经久不衰：它用 Perl 程序员能懂的语言讲清了 Lisp 宏“代码即数据”的本质，是理解宏与文本替换区别的经典入门，多年来反复登上 HN 首页。
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Perl 其实也能拿到解析后的代码结构，只是不如 Lisp 优雅
源码过滤器的问题在于你拿到的是字符串，要自己解析，边界情况容易出错。如果 Perl 能先帮你解析好源码、交给你一个数据结构，就弥补了最大的短板。方案一：B::Deparse；方案二：B::CallChecker。也就是说，你其实是可以访问解析器的，只是因为缺乏统一语法，不如 Lisp 那么优雅，但仍能做强大的程序变换。
bmn_查看英文原文
Source filters are bad because you get a string and parse it yourself and then get it wrong for edge cases. What if Perl already parsed a piece of source for you and handed you a data structure instead? That fixes the greatest shortcoming. Solution 1: B::Deparse <http://yapceurope.lv/ye2011/talk/3597>, <http://act.yapc.eu/gpw2014/talk/5266> Solution 2: B::CallChecker / <http://perldoc.perl.org/perlapi.html#cv_set_call_checker> In other words, you do have access to the parser. It's not as neat as in Lisp because of the lack of uniform syntax, but still allows for powerful program transformations: <http://p3rl.org/Kavorka::Manual::Signatures>
Lisp 宏也有变量捕获等罕见陷阱，但比 C 宏好得多
公平地说，“Lisp 宏从不坏”并不完全对。Common Lisp 的宏在少数微妙情况下也会出问题，比如不用 GENSYM 时变量捕获，或者调用方 FLET 了宏展开中用到的符号。不过这些问题很容易避免，实践中只要善用 GENSYM 和包机制就基本没事，比 C 宏好太多了。想深入了解可以看 Scheme 的卫生宏。
sjolsen查看英文原文
> In Lisp, source filters never break. > Never. To be perfectly fair, this is not quite true. In Common Lisp (the dialect being used here), macros can break in rare, subtle ways. The obvious happens when you don't use GENSYM: (defmacro foo (var) `(let* ((x 3)) (setf ,var x))) CL-USER> (let ((x 7)) (foo x) x) 7 CL-USER> (let ((y 7)) (foo y) y) 3 but is fairly easy to avoid. In practice, as long as you make judicious use of GENSYM and packages, the only thing you really need to worry about is someone FLETing a symbol in the package COMMON-LISP (or maybe an external symbol in your package): (defmacro bar (x y z) `(list ,x ,y ,z)) CL-USER> (let ((list '(7))) (bar 1 2 3)) (1 2 3) CL-USER> (flet ((list (a b c) (+ a b c))) (bar 1 2 3)) 6 SBCL actually yelled at me when I tried this for violating the package lock on COMMON-LISP. So in practice, it's really not much of an issue, and is certainly a hell of a lot better than C macros. If you're interested in learning more about this, look at Scheme's hygienic macros.
作者是顶级技术作家，其著作远超 Perl 范畴
作者是《Higher Order Perl》的作者，据我所知那是史上最好的编程书之一。虽然书名里有 Perl，但它其实是对函数式编程范式详尽而非学院派的介绍。另外 Perl 圈有很多优秀的书作者，他们的入门书比大多数计算机教科书教给我的计算机科学还多。
G3rn0ti查看英文原文
The author wrote „Higher Order Perl“ which — to my knowledge — is one of the best books on programming ever written. While it’s got Perl in its title it is really an exhaustive introduction to the functional programming paradigm while being refreshingly non-academic about the subject. BTW: In general, I think there were many great Perl book authors having written very insightful introductory text books. They were mostly fun too read and taught me more about computer science than most computer science text books I had access too.
解释文中 =3D 乱码是邮件编码残留，实为等号
文中奇怪的 "=3D" 字符串是因为这封邮件原本是 quoted-printable 编码，发布到网页时没被清理掉。读代码时可以放心地把 "=3D" 当作普通的等号——在这种编码里等号需要用 "=3D" 转义。这让我读的时候有点抓狂。
dinom查看英文原文
I'd like to point out that the strange occurrences of the "=3D" string are due to the fact that the message was originally a "quoted-printable" encoded email and for whatever reason this artifact wasn't removed when posted to the web. Perhaps this was pipermail's fault? Anyway, when reading the code, you're safe in assuming "=3D" is just a single plain old equal sign. In the "quoted-printable" encoding scheme the two characters following an equal sign are the hex value of the intended character. In effect, equal signs need to be escaped using the "=3D" sequence. This drove me a little nuts as I was reading and trying to code switch between the assorted languages used in the examples.
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