Question

I want to do a macro in common lisp which is supposed to take in one of its arguments a list made of slots and strings. Here is the prototype :

(defclass time-info ()
  ((name :initarg name)
   (calls :initarg calls)
   (second :initarg second)
   (consing :initarg consing)
   (gc-run-time :initarg gc-run-time)))

(defun print-table (output arg-list time-info-list) ())

The idea is to print a table based on the arg-list which defines its structure. Here is an example of a call to the function:

(print-table *trace-output*
             '("|" name "||" calls "|" second "\")
             my-time-info-list)

This print a table in ascII on the trace output. The problem, is that I don’t know how to explicitely get the elements of the list to use them in the different parts of my macro.

I have no idea how to do this yet, but I’m sure it can be done. Maybe you can help me 🙂

Answer 1

I would base this on format. The idea is to build a format string
from your arg-list.

I define a helper function for that:

(defun make-format-string-and-args (arg-list)
  (let ((symbols ()))
    (values (apply #'concatenate 'string
                   (mapcar (lambda (arg)
                             (ctypecase arg
                               (string 
                                (cl-ppcre:regex-replace-all "~" arg "~~"))
                               (symbol
                                (push arg symbols)
                                "~a")))
                           arg-list))
            (nreverse symbols))))

Note that ~ must be doubled in format strings in order to escape them.

The printing macro itself then just produces a mapcar of format:

(defmacro print-table (stream arg-list time-info-list)
  (let ((time-info (gensym)))
    (multiple-value-bind (format-string arguments)
        (make-format-string-and-args arg-list)
      `(mapcar (lambda (,time-info)
                 (format ,stream ,format-string
                         ,@(mapcar (lambda (arg)
                                     (list arg time-info))
                                   arguments)))
               ,time-info-list)))

You can then call it like this:

(print-table *trace-output*
             ("|" name "||" calls "|" second "\\")
             my-time-info-list)

Please note the following errors in your code:

• You need to escape \ in strings.

• Second is already a function name exported from the common-lisp
  package. You should not clobber that with a generic function.