宏扩展可以包含(声明...)表达式吗? [英] Can macro expansion contain (declare ...) expressions?

问题描述

Common Lisp Hyperspec指出宏形式不能扩展成声明；声明表达式必须作为它们所引用的形式的实际子表达式出现."

The Common Lisp Hyperspec states "Macro forms cannot expand into declarations; declare expressions must appear as actual subexpressions of the form to which they refer."

我对扩展为"的含义感到困惑.诸如此类的宏由于明显的原因而无法正常工作:

I'm confused on the meaning of "expand into". A macro such as the following won't work for obvious reasons:

(defmacro optimize-fully ()
    `(declare (optimize (speed 3) (safety 0))))

但是，如果宏扩展仅包含一个(declare ...)表达式怎么办?

But what if the macro expansion merely contains a (declare ...) expression?

(defmacro defun-optimized (name (&rest lambda-list) &rest body)
    `(defun ,name ,lambda-list
        (declare (optimize (speed 3) (safety 0)))
        ,@body))

(defun-optimized foobar (a b)
    (* a b))

这是否违反规范?我使用的CL实施SBCL并没有抱怨，实际上，上面的宏似乎完全按照预期的方式工作.有什么作用?

Is this a violation of the spec? The CL implementation I use, SBCL, does not complain, and in fact, the macro above seems to work precisely as expected. What gives?

推荐答案

第一个示例正是它所禁止的.您不能将像这样的代码与类似这样的东西组合在一起:

Your first example is exactly what it's prohibiting. You couldn't have code like that combined with something like this:

(defun optimized (a b)
  (optimize-fully)
  (+ a b))

不过，有时我会看到这样的代码:

I sometimes see code like this, though:

(defvar *optimization-settings* '(optimize (speed 3) (safety 0)))

(defun foo (a b)
  (declare #.*optimization-settings*)
  ...)

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