Quoting in macro-defining macro

Question

I'm trying to write a macro that defines some helpers for struct-of-arrays data structure (based on this snippet). Inside that macro I define another macro that helps with traversing all of the slot values in struct. The thing is I can't make double unquoting work properly. Here's the code:

(defmacro defcomponent (name-and-options &body slots)
  (setf name-and-options (ensure-list name-and-options))
  (let ((struct (first name-and-options))
        (slot-names (iter (for s in slots)
                          (collecting
                           (ematch s
                             ((or (and name (symbol)
                                       (<> _ '*)
                                       (<> _ nil))
                                  (list* name _ (plist :type _ :read-only _)))
                              name))))))
    `(progn (defstruct ,name-and-options
              ;; some task-specific stuff omitted here
              )
            (defmacro ,(symbolicate 'with- struct) (components &rest body)
              `(loop
                 ,@',(iter (for s in slot-names)
                          (appending `(for ,s across (,(symbolicate struct '- s) components))))
                 do ,@body)))))

So for instance (defcomponent buzz x y) macroexpands to

(PROGN
 (DEFSTRUCT (BUZZ)
   X Y)  ;; details omitted
 (DEFMACRO WITH-BUZZ (COMPONENTS &REST BODY)
   `(LOOP ,@'(FOR X ACROSS (BUZZ-X COMPONENTS) FOR Y ACROSS (BUZZ-Y COMPONENTS))
          DO ,@BODY))

which kinda works, but I want to access components parameter of the internal with-buzz macro, i.e. something like this

(DEFMACRO WITH-BUZZ (COMPONENTS &REST BODY)
   `(LOOP FOR X ACROSS (BUZZ-X ,COMPONENTS) FOR Y ACROSS (BUZZ-Y ,COMPONENTS)
          DO ,@BODY))

How do I possibly acheive that? I've tried a lot of the combinations of , and ,@ to no avail.

Rainer Joswig · Accepted Answer · 2019-11-09T10:59:57.893

Sometimes it helps not to work with backquote patterns. Then scope problems can be easier understood with the help of a compiler, which would warn about the usual variable scope problems.

As a slightly simplified exercise, we will write a function, which generates code. The generated code is a macro definition, which itself generates code.

(defun makeit (name slots)
  (labels ((symbolicate (pattern &rest things)
             (intern (apply #'format nil pattern things)))
           (compute-for-clauses (slots)
             (loop for s in slots
                   append (list ''for (list 'quote s)
                                ''across (list 'list
                                               (list 'quote
                                                     (symbolicate "~a-~a" name s))
                                               'components)))))

    (list 'progn
          (list 'defmacro
                (symbolicate "WITH-~a" name)
                '(components &rest body)
                (append '(list* 'loop)
                        (compute-for-clauses slots)
                        (list ''do 'body))))))

Example

CL-USER 51 > (pprint (makeit 'buzz '(x y)))

(PROGN
  (DEFMACRO WITH-BUZZ (COMPONENTS &REST BODY)
    (LIST* 'LOOP
           'FOR
           'X
           'ACROSS
           (LIST 'BUZZ-X COMPONENTS)
           'FOR
           'Y
           'ACROSS
           (LIST 'BUZZ-Y COMPONENTS)
           'DO
           BODY)))

CL-USER 52 > (eval *)
NIL

CL-USER 53 > (macroexpand-1 '(with-buzz a (+ 12) (+ 30)))
(LOOP FOR X ACROSS (BUZZ-X A) FOR Y ACROSS (BUZZ-Y A) DO (+ 12) (+ 30))
T

score 0 · Answer 2 · answered Nov 08 '19 at 16:19

All right, I've managed to do it by resorting to manual list construction + eval instead of quasiquoting, but sweet mother of god it looks so hakish.

;; skip
(defmacro ,(symbolicate 'with- struct) (components &rest body)
    (append
    '(loop)
    (eval
    `(iter (for s in ',',slot-names)
            (appending `(for ,s across (,(symbolicate ',',struct '- ,'s) ,,components)))))
    '(do)
    body))

I'll gladly accept any other answer solving the problem more idiomatically.

Quoting in macro-defining macro

2 Answers2