Common Lisp中的逗号逗号 [英] comma-comma-at in Common Lisp

问题描述

我对逗号-逗号的用法感到困惑.

`,@abc

可能会说:我不知道在反引号之前进行拼接的意思是什么.永远不要再这样做."

但是以某种方式，口译员不会抱怨，而只是选择这样做

`(beg <splice stuff here and write comma in front of each of them> end)

是否与 CLHS反引号中的规则兼容?

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另请参见应扩展CHLS的"innermost backquoted形式第一"含义

解决方案

CLHS 指出:如果嵌套了反引号语法，则应首先扩展 innermost 的反引号形式.这意味着，如果连续出现多个逗号，则最左边的逗号应属于最里面的反引号."

因此，第一个逗号取消了第二个反引号的引用；然后,@会拼接到parms的内容中，因为所有引号都已撤消.取消引用不会按照您所描述的步骤逐步替换"表单-在决定如何评估所包含的表单时，逗号只会备份在带引号的表单结构中.

(let ((parms '(x y)))
  `(list 'beg ,@parms 'end))

显然是(list 'beg x y 'end)，这就是您要追求的.生成可创建所需结构作为其输出的形式.额外的外部反引号只是该语法的简写方式，与'(foo bar)是(c4>的简写方式相同).

I am confused about how comma-comma-at does what it does.

An example of use of comma-comma-at is in Is there a use for double unquote (double comma) when defining a Lisp macro?

It seems to me that

(let ((parms '(x y)))
  ``(beg ,,@parms end))

expands to or something equivalent to

`(beg ,x ,y end)

How does it expands to that?

I was thinking, if I evaluate the double backquote form, it causes the second comma to do its work, and the result is:

`(beg ,<splice stuff here> end)

and it looks like Lisp interpreter should complain and say "I don't know what it means to splice stuff before a comma" in the sense that when Lisp encounters

`,@abc

it would be likely to say "I don't know what it means to splice stuff before a backquote. Don't ever do that again."

But somehow, the interpreter does not complain and simply choose to do

`(beg <splice stuff here and write comma in front of each of them> end)

Is that compatible with rules in CLHS backquote?

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Also see CHLS "innermost backquoted form should be expanded first" meaning

解决方案

CLHS states: "If the backquote syntax is nested, the innermost backquoted form should be expanded first. This means that if several commas occur in a row, the leftmost one belongs to the innermost backquote."

So, the first comma unquotes the second backquote; the ,@ then splices in the contents of parms, as all quotes have been undone. Unquoting does not "replace" forms in steps as you describe - the commas just back up in the structure of backquoted forms when deciding how to evaluate the contained forms.

(let ((parms '(x y)))
  `(list 'beg ,@parms 'end))

obviously evaluates to (list 'beg x y 'end), which is what you are after here; producing a form that creates the desired structure as its output. The additional outer backquote is simply a shorthand for this syntax, in the same manner as '(foo bar) is (equality issues aside) a shorthand for (list 'foo 'bar).

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