通用Lisp宏本质上扩展了代码的“语法原语”。

例如，在C语言中，switch/case结构只适用于整型，如果你想将它用于浮点数或字符串，你就只能使用嵌套的if语句和显式比较。你也不可能编写一个C宏来为你做这项工作。

但是，由于lisp宏(本质上)是一个lisp程序，它接受代码片段作为输入，并返回代码来替换宏的“调用”，因此您可以尽可能地扩展您的“原语”库，通常最终会得到一个更可读的程序。

要在C中做同样的事情，您必须编写一个自定义预处理器，它会吃掉您的初始(不完全是C)源代码，并吐出C编译器可以理解的东西。这不是一种错误的方法，但它不一定是最简单的。

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

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)

lisp宏以程序片段作为输入。这个程序片段被表示为一个数据结构，可以按照您喜欢的任何方式进行操作和转换。最后，宏输出另一个程序片段，这个片段是在运行时执行的。

c#没有宏功能，但是如果编译器将代码解析为CodeDOM树，并将其传递给一个方法，该方法将其转换为另一个CodeDOM，然后将其编译为IL，则会有等效的宏功能。

这可以用来实现“糖”语法，如for each-statement using-clause, linq select-expressions等等，作为转换为底层代码的宏。

如果Java有宏，您就可以在Java中实现Linq语法，而不需要Sun更改基本语言。

下面是在c#中实现lisp风格的宏的伪代码:

define macro "using":
    using ($type $varname = $expression) $block
into:
    $type $varname;
    try {
       $varname = $expression;
       $block;
    } finally {
       $varname.Dispose();
    }

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

我不确定我能给每个人的(优秀的)帖子添加一些见解，但是……

Lisp宏工作得很好，因为Lisp语法的本质。

Lisp是一种非常规则的语言(把所有东西都想象成一个列表);宏使您能够将数据和代码视为相同的(不需要字符串解析或其他技巧来修改lisp表达式)。将这两个特性结合起来，就有了一种非常简洁的方式来修改代码。

编辑:我想说的是Lisp是同构的，这意味着Lisp程序的数据结构是用Lisp本身编写的。

因此，您最终可以在语言之上创建自己的代码生成器，使用语言本身的所有功能(例如。在Java中，你必须破解字节码编织的方法，尽管一些框架(如AspectJ)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)。

在实践中，使用宏可以在lisp的基础上构建自己的迷你语言，而不需要学习其他语言或工具，并使用语言本身的全部功能。