我认为我从来没有见过比这个家伙解释得更好的Lisp宏:http://www.defmacro.org/ramblings/lisp.html

您将在这里找到关于lisp宏的全面辩论。

这篇文章的一个有趣的子集:

In most programming languages, syntax is complex. Macros have to take apart program syntax, analyze it, and reassemble it. They do not have access to the program's parser, so they have to depend on heuristics and best-guesses. Sometimes their cut-rate analysis is wrong, and then they break. But Lisp is different. Lisp macros do have access to the parser, and it is a really simple parser. A Lisp macro is not handed a string, but a preparsed piece of source code in the form of a list, because the source of a Lisp program is not a string; it is a list. And Lisp programs are really good at taking apart lists and putting them back together. They do this reliably, every day. Here is an extended example. Lisp has a macro, called "setf", that performs assignment. The simplest form of setf is (setf x whatever) which sets the value of the symbol "x" to the value of the expression "whatever". Lisp also has lists; you can use the "car" and "cdr" functions to get the first element of a list or the rest of the list, respectively. Now what if you want to replace the first element of a list with a new value? There is a standard function for doing that, and incredibly, its name is even worse than "car". It is "rplaca". But you do not have to remember "rplaca", because you can write (setf (car somelist) whatever) to set the car of somelist. What is really happening here is that "setf" is a macro. At compile time, it examines its arguments, and it sees that the first one has the form (car SOMETHING). It says to itself "Oh, the programmer is trying to set the car of somthing. The function to use for that is 'rplaca'." And it quietly rewrites the code in place to: (rplaca somelist whatever)

While the above all explains what macros are and even have cool examples, I think the key difference between a macro and a normal function is that LISP evaluates all the parameters first before calling the function. With a macro it's the reverse, LISP passes the parameters unevaluated to the macro. For example, if you pass (+ 1 2) to a function, the function will receive the value 3. If you pass this to a macro, it will receive a List( + 1 2). This can be used to do all kinds of incredibly useful stuff.

Adding a new control structure, e.g. loop or the deconstruction of a list Measure the time it takes to execute a function passed in. With a function the parameter would be evaluated before control is passed to the function. With the macro, you can splice your code between the start and stop of your stopwatch. The below has the exact same code in a macro and a function and the output is very different. Note: This is a contrived example and the implementation was chosen so that it is identical to better highlight the difference. (defmacro working-timer (b) (let ( (start (get-universal-time)) (result (eval b))) ;; not splicing here to keep stuff simple ((- (get-universal-time) start)))) (defun my-broken-timer (b) (let ( (start (get-universal-time)) (result (eval b))) ;; doesn't even need eval ((- (get-universal-time) start)))) (working-timer (sleep 10)) => 10 (broken-timer (sleep 10)) => 0

简而言之，宏是代码的转换。它们允许引入许多新的语法结构。例如，考虑c#中的LINQ。在lisp中，有类似的由宏实现的语言扩展(例如，内置循环构造，迭代)。宏显著地减少了代码重复。宏允许嵌入«小语言»(例如，在c#/java中可以使用xml进行配置，在lisp中可以使用宏实现同样的事情)。宏可能隐藏使用库的困难。

例如，在lisp中你可以写

(iter (for (id name) in-clsql-query "select id, name from users" on-database *users-database*)
      (format t "User with ID of ~A has name ~A.~%" id name))

这隐藏了所有数据库的东西(事务，正确的连接关闭，获取数据等)，而在c#中，这需要创建SqlConnections, SqlCommands，将SqlParameters添加到SqlCommands，在SqlDataReaders上循环，正确地关闭它们。

我从通用的lisp烹饪书中得到了这个，我认为它解释了为什么lisp宏是有用的。

宏是一段普通的Lisp代码，它对另一段假定的Lisp代码进行操作，将其翻译成(更接近于)可执行的Lisp。这听起来可能有点复杂，所以让我们举一个简单的例子。假设您想要一个版本的setq，将两个变量设置为相同的值。所以如果你写

(setq2 x y (+ z 3))

当z=8时，x和y都被设为11。(我想不出这有什么用，但这只是一个例子。)

It should be obvious that we can't define setq2 as a function. If x=50 and y=-5, this function would receive the values 50, -5, and 11; it would have no knowledge of what variables were supposed to be set. What we really want to say is, When you (the Lisp system) see (setq2 v1 v2 e), treat it as equivalent to (progn (setq v1 e) (setq v2 e)). Actually, this isn't quite right, but it will do for now. A macro allows us to do precisely this, by specifying a program for transforming the input pattern (setq2 v1 v2 e)" into the output pattern (progn ...)."

如果你觉得这很好，你可以继续读下去: http://cl-cookbook.sourceforge.net/macros.html

由于现有的答案给出了很好的具体例子来解释宏实现了什么以及如何实现的，也许它会帮助收集一些关于为什么宏功能相对于其他语言是一个重要的收获的想法;首先是这些答案，然后是其他地方的一个很棒的答案:

．.． 在C语言中，你必须编写一个自定义的预处理器(这可能是一个足够复杂的C程序)……

—瓦廷

与任何精通c++的人交谈，问他们花了多长时间来学习模板元编程(仍然不是那么强大)所需要的所有模板。

马特·柯蒂斯

．.． 在Java中，你必须破解字节码编织的方法，尽管像AspectJ这样的框架允许你使用不同的方法来做到这一点，但它基本上是一种破解。

——米格尔·平

DOLIST is similar to Perl's foreach or Python's for. Java added a similar kind of loop construct with the "enhanced" for loop in Java 1.5, as part of JSR-201. Notice what a difference macros make. A Lisp programmer who notices a common pattern in their code can write a macro to give themselves a source-level abstraction of that pattern. A Java programmer who notices the same pattern has to convince Sun that this particular abstraction is worth adding to the language. Then Sun has to publish a JSR and convene an industry-wide "expert group" to hash everything out. That process--according to Sun--takes an average of 18 months. After that, the compiler writers all have to go upgrade their compilers to support the new feature. And even once the Java programmer's favorite compiler supports the new version of Java, they probably ''still'' can't use the new feature until they're allowed to break source compatibility with older versions of Java. So an annoyance that Common Lisp programmers can resolve for themselves within five minutes plagues Java programmers for years.

——peter Seibel，在《Practical Common Lisp》中