如何实现推宏? [英] How could I implement the push macro?
问题描述
有人可以帮助我了解如何将push实施为宏吗?下面的朴素版本会对位置表单进行两次评估，然后在评估元素表单之前进行评估:
(defmacro my-push (element place)
`(setf ,place (cons ,element ,place)))
但是，如果我尝试按以下方式解决此问题，那么我setf-放错了位置:
(defmacro my-push (element place)
(let ((el-sym (gensym))
(place-sym (gensym)))
`(let ((,el-sym ,element)
(,place-sym ,place))
(setf ,place-sym (cons ,el-sym ,place-sym)))))
CL-USER> (defparameter *list* '(0 1 2 3))
*LIST*
CL-USER> (my-push 'hi *list*)
(HI 0 1 2 3)
CL-USER> *list*
(0 1 2 3)
如何在不进行两次评估的情况下setf正确的位置?
做到这一点似乎要复杂一些.例如，SBCL 1.0.58中push的代码为:
(defmacro-mundanely push (obj place &environment env)
#!+sb-doc
"Takes an object and a location holding a list. Conses the object onto
the list, returning the modified list. OBJ is evaluated before PLACE."
(multiple-value-bind (dummies vals newval setter getter)
(sb!xc:get-setf-expansion place env)
(let ((g (gensym)))
`(let* ((,g ,obj)
,@(mapcar #'list dummies vals)
(,(car newval) (cons ,g ,getter))
,@(cdr newval))
,setter))))
因此，阅读 get-setf-expansion 上的文档似乎很有用. /p>
记录下来，生成的代码看起来很不错:
插入符号:
(push 1 symbol)
扩展为
(LET* ((#:G906 1) (#:NEW905 (CONS #:G906 SYMBOL)))
(SETQ SYMBOL #:NEW905))
插入支持SETF的功能(假设symbol指向列表列表):
(push 1 (first symbol))
扩展为
(LET* ((#:G909 1)
(#:SYMBOL908 SYMBOL)
(#:NEW907 (CONS #:G909 (FIRST #:SYMBOL908))))
(SB-KERNEL:%RPLACA #:SYMBOL908 #:NEW907))
因此，除非您花一些时间研究setf， setf扩展和公司，这看起来很不可思议(即使在研究了它们之后，看起来仍然如此). OnLisp 中的通用变量"一章也可能有用.
提示:如果您编译自己的SBCL(不是那么难)，则将--fancy参数传递给make.sh.这样，您将能够快速查看SBCL内的函数/宏的定义(例如，在Emacs + SLIME内使用 M-.).显然，不要删除这些源(可以在install.sh之后运行clean.sh，以节省90％的空间).
Can someone help me understand how push can be implemented as a macro? The naive version below evaluates the place form twice, and does so before evaluating the element form:
(defmacro my-push (element place)
`(setf ,place (cons ,element ,place)))
But if I try to fix this as below then I'm setf-ing the wrong place:
(defmacro my-push (element place)
(let ((el-sym (gensym))
(place-sym (gensym)))
`(let ((,el-sym ,element)
(,place-sym ,place))
(setf ,place-sym (cons ,el-sym ,place-sym)))))
CL-USER> (defparameter *list* '(0 1 2 3))
*LIST*
CL-USER> (my-push 'hi *list*)
(HI 0 1 2 3)
CL-USER> *list*
(0 1 2 3)
How can I setf the correct place without evaluating twice?
Doing this right seems to be a little more complicated. For instance, the code for push in SBCL 1.0.58 is:
(defmacro-mundanely push (obj place &environment env)
#!+sb-doc
"Takes an object and a location holding a list. Conses the object onto
the list, returning the modified list. OBJ is evaluated before PLACE."
(multiple-value-bind (dummies vals newval setter getter)
(sb!xc:get-setf-expansion place env)
(let ((g (gensym)))
`(let* ((,g ,obj)
,@(mapcar #'list dummies vals)
(,(car newval) (cons ,g ,getter))
,@(cdr newval))
,setter))))
So reading the documentation on get-setf-expansion seems to be useful.
For the record, the generated code looks quite nice:
Pushing into a symbol:
(push 1 symbol)
expands into
(LET* ((#:G906 1) (#:NEW905 (CONS #:G906 SYMBOL)))
(SETQ SYMBOL #:NEW905))
Pushing into a SETF-able function (assuming symbol points to a list of lists):
(push 1 (first symbol))
expands into
(LET* ((#:G909 1)
(#:SYMBOL908 SYMBOL)
(#:NEW907 (CONS #:G909 (FIRST #:SYMBOL908))))
(SB-KERNEL:%RPLACA #:SYMBOL908 #:NEW907))
So unless you take some time to study setf, setf expansions and company, this looks rather arcane (it may still look so even after studying them). The 'Generalized Variables' chapter in OnLisp may be useful too.
Hint: if you compile your own SBCL (not that hard), pass the --fancy argument to make.sh. This way you'll be able to quickly see the definitions of functions/macros inside SBCL (for instance, with M-. inside Emacs+SLIME). Obviously, don't delete those sources (you can run clean.sh after install.sh, to save 90% of the space).
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