问题描述
以下 with-gensyms 实现,旨在对类似 let 的参数列表进行操作,在嵌套的 let 中生成一个额外的逗号:
(defmacro with-gensyms (vars &rest forms)
`(let,(loop for var in vars
collect (if (listp var)
`(,(car var) (gensym))
`(,var (gensym))))
`(let,(loop for genvar in vars
collect (if (listp genvar)
`(,(car genvar),(car (cdr genvar)))
`(,genvar nil))),@forms)))
宏展开示例:
(with-gensyms ((var1 1) (var2 2) (testvar "testname") the-number-zero))
(LET ((VAR1 (GENSYM))
(VAR2 (GENSYM))
(TEStvar (GENSYM))
(THE-NUMBER-ZERO (GENSYM)))
`(LET,((VAR1 1) (VAR2 2) (TEStvar "testname") (THE-NUMBER-ZERO NIL))))
在嵌套 let 的参数列表之前有一个额外的逗号,我不清楚为什么会这样。
目标输出如下:
(let ((var1 (gensym)) (var2 (gensym)) (testvar (gensym)) (the-number-zero (gensym)))
`(let ((,var1 1) (,var2 2) (,testvar "testname") (,the-number-zero nil))))
任何帮助都会很好!
解决方法
这值得更好的解释,而不仅仅是一个注释。有些人可能会对此感到困惑。您必须跟踪哪些括号已打开,哪些已关闭。将任何不靠在同一行上的括号对排列起来,这样更容易解释。
(defmacro with-gensyms (vars &rest forms)
;; v-- this backticked-parenthesis has been opened here.
`(let,(loop for var in vars
collect (if (listp var)
`(,(car var) (gensym))
`(,var (gensym))
)
)
;; ^-- but it hasn't been closed yet.
;; so we now have to unquote for that one...
;; v-- as well as this new one.
`(let,(loop for genvar in vars
collect (if (listp genvar)
`(,(car genvar),(car (cdr genvar)))
`(,genvar nil)
)
),@forms
) ;; second backtick closes here.
) ;; first backtick closes here.
)