Lisp-在宏中使用`和`list` [英] Lisp- Usage of `'`and `list` in macros

问题描述

我正在尝试学习Elisp，因此我正在阅读Elisp的GNU手册.到目前为止，所有内容都很容易理解，但是当我阅读宏部分时，遇到了一些我很难理解的事情.我也真的找不到适当的解释:

I am trying to learn Elisp, so I am reading the GNU Manual for Elisp. Everything so far is easy to understand, however when I read to the section of macro I encountered something I have a hard time understanding. I couldn't really find an adequate explanation neither:

例如，一个简单的宏程序将变量加1:

For example, a simple macro program that increments the variable by 1:

(defmacro inc (var)
   (list 'setq var (list '1+ var)))

我不确定为什么在 setq 和 1 + 前面有'符号?这是否会使它们成为文字元素列表?(例如，包含三个元素的列表(setq var(1+ var))

I am not sure why there is ' symbol in front of setq and 1+ ? Won't this make them to a list of literal elements? (For example a list containing three elements (setq var (1+ var))

为什么我不能写:

; this seems more reasonable to me
(defmacro inc (var)
   (setq var (1+ var))

我不太确定 list 在这里的工作方式，在这里使用 list 对我来说似乎很奇怪.有人可以向我解释吗?

I am not really sure how list works here and it seems strange to me using list here. Can someone explain to me?

推荐答案

这不会使它们成为文字元素列表吗?(例如，包含三个元素的列表(setq var(1+ var))

确切地说是这样(如果我们将实际参数替换为 var ).

Precisely so (if we substitute the actual argument for var).

宏会生成/返回 code .

这是扩展"宏的阶段，通常发生在elisp文件的字节编译期间.

This is the "expansion" phase of the macro, which typically takes place during byte-compilation of the elisp file.

就该字节编译的.elc文件的内容而言，在.el源文件中使用这两种方法之间没有什么区别:

So as far as the contents of that byte-compiled .elc file are concerned, there's no difference between you having used either of these things in your .el source file:

• (inc foo)
• (setq foo(1+ foo))

即在这两种情况下，要进行字节编译的代码都是(setq foo(1+ foo))

I.e. In both cases the code which gets byte-compiled is (setq foo (1+ foo))

在字节编译之外，当.el文件被 load 加载时，宏可能会被扩展；否则，在需要评估扩展的代码之前，应按需进行扩展(但是您应该始终假设扩展完全独立于后续评估而发生.

Outside of byte-compilation, a macro might be expanded when the .el file is loaded or, failing that, on-demand right before the expanded code needs to be evaluated (but you should always assume the expansion happens completely independently of the subsequent evaluation).

这篇关于Lisp-在宏中使用`和`list`的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！