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();
    }

Lisp宏代表了几乎在任何大型编程项目中都会出现的一种模式。最终，在一个大的程序中，你会有一段代码，你会意识到，如果你写一个程序，把源代码输出为文本，然后你就可以粘贴进去，这会更简单，更不容易出错。

在Python中，对象有两个方法__repr__和__str__。__str__只是人类可读的表示。__repr__返回一个有效的Python代码表示，也就是说，可以作为有效的Python输入解释器。通过这种方式，您可以创建一些小的Python代码片段，生成可以粘贴到实际源代码中的有效代码。

在Lisp中，整个过程由宏系统形式化。当然，它允许您为语法创建扩展并执行各种奇特的事情，但它的实际用处可以从上面总结出来。当然，Lisp宏系统允许您使用整个语言的全部功能来操作这些“片段”是有帮助的。

想想在C或c++中可以用宏和模板做什么。它们是管理重复代码的非常有用的工具，但它们在相当严重的方面受到限制。

有限的宏/模板语法限制了它们的使用。例如，不能编写扩展为类或函数以外内容的模板。宏和模板不容易维护内部数据。 C和c++复杂且不规则的语法使得编写非常通用的宏非常困难。

Lisp和Lisp宏解决了这些问题。

Lisp宏是用Lisp编写的。您拥有Lisp的全部功能来编写宏。 Lisp有一个非常规则的语法。

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所需的所有模板。或者是《现代c++设计》等(优秀)书籍中的所有疯狂技巧，尽管语言已经标准化了10年，但这些技巧仍然很难调试，而且(在实践中)无法在真实的编译器之间移植。如果用于元编程的语言与用于编程的语言相同，那么所有这些问题都消失了!

Lisp宏允许您决定何时(如果有的话)对任何部分或表达式求值。举个简单的例子，想想C语言:

expr1 && expr2 && expr3 ...

它说的是:计算expr1，并且，如果它是正确的，计算expr2，等等。

现在试着把这个&&变成一个函数…没错，你不能。像这样调用:

and(expr1, expr2, expr3)

将在产生答案之前评估所有三个表达式，而不管expr1是否为假!

使用lisp宏，你可以编写如下代码:

(defmacro && (expr1 &rest exprs)
    `(if ,expr1                     ;` Warning: I have not tested
         (&& ,@exprs)               ;   this and might be wrong!
         nil))

现在你有一个&&，你可以像函数一样调用它，它不会计算你传递给它的任何表单，除非它们都为真。

要了解这是如何有用的，请进行对比:

(&& (very-cheap-operation)
    (very-expensive-operation)
    (operation-with-serious-side-effects))

and:

and(very_cheap_operation(),
    very_expensive_operation(),
    operation_with_serious_side_effects());

你可以用宏做的其他事情是创建新的关键字和/或迷你语言(例如，查看(loop…)宏)，将其他语言集成到lisp中，例如，你可以编写一个宏，让你这样说:

(setvar *rows* (sql select count(*)
                      from some-table
                     where column1 = "Yes"
                       and column2 like "some%string%")

这还没有涉及到Reader宏。

希望这能有所帮助。

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