为什么在此宏定义中需要@符号? [英] Why is the @ sign needed in this macro definition?

问题描述

在下面的时候是宏:

(defmacro when (condition &rest body)
  `(if ,condition (progn ,@body)))

为什么有一个at符号?

Why is the there an at sign?

推荐答案

通过进行一些实验很容易看到差异

It's very easy to see the difference by making a little experiment

> (let ((x '(1 2 3 4))) `(this is an example ,x of expansion))
(THIS IS AN EXAMPLE (1 2 3 4) OF EXPANSION)

> (let ((x '(1 2 3 4))) `(this is an example ,@x of expansion))
(THIS IS AN EXAMPLE 1 2 3 4 OF EXPANSION)

如您所见，使用,@会将列表的元素直接放置在扩展中.取而代之的是将列表放置在扩展中.

As you can see the use of ,@ will place the elements of the list directly inside in the expansion. Without you get instead the list placed in the expansion.

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