Hello guys,

I have a question regarding the nested backquotes in macros. I wrote a macro, which creates lexical bindings for “port:ip” values:

(defun mkstr (&rest args)
  (with-output-to-string (s)
    (dolist (a args) (princ a s))))

(defun mksymb (&rest args)
  (values (intern (string-upcase (apply #'mkstr args)))))

;; my macro
(defmacro with-free-ports (start end &body body)
  (let ((range (loop for port from start to end collect (format NIL "127.0.0.1:~a" port)))
	(n 0))
    `(let ,(mapcar #'(lambda (p) `(,(mksymb "PORT-" (incf n)) ,p)) range)
       (progn ,@body))))

One sets a range of ports on localhost and these ports are bound to symbols port-1, port-2, etc…

(with-free-ports 1 3 port-1) ;; => "127.0.0.1:1"

This works fine if the start or end parameters are given as values. But if they are variables. which must be evaluated, this macro doesn’t work:

(let ((start 1))
  (with-free-ports start 3 port-1)) ;; error

In order to fix it, I made the let- bindings a part of the macro-expansion:

(defmacro with-free-ports (start end &body body)
  `(let ((range (loop for port from ,start to ,end collect (format NIL "127.0.0.1:~a" port)))
	(n 0))
     `(let ,(mapcar #'(lambda (p) `(,(mksymb "PORT-" (incf n)) ,p)) range)
	       (progn ,@body))))

but get a compilation warning that the body is never used. I assume this is because of the inner backquote.

To evaluate ,@body inside the inner backquote, I use one more comma, and the macro compiles without warnings:

(defmacro with-free-ports (start end &body body)
  `(let ((range (loop for port from ,start to ,end collect (format NIL "127.0.0.1:~a" port)))
	(n 0))
     `(let ,(mapcar #'(lambda (p) `(,(mksymb "PORT-" (incf n)) ,p)) range)
	       (progn ,,@body)))) ;; one more comma here

But it doesn’t work:

(let ((start 1))
  (with-free-ports start 3 port-1)) ;; error: port-1 is unbound

because with this ,,@body I evaluate port-1: (progn ,port-1) and this triggers the error.

I would appreciate if smbd can help me a bit and say what I am doing wrong.

Thank you.

  • xhash101OPBEnglish
    1·
    1 年前

    Thank you for your reply.

    The compiler generally will not execute the LET and then compile the WITH-FREE-PORTS form with the new binding

    Here is an example when the compiler does exactly that:

    (defmacro with-free-ports (start end &body body)
  `(list ,start ,end ,@body))

(let ((start 1)
      (end 3))      
  (with-free-ports start end NIL))

    So, why does it work?