[pro] The Best Examples of "Code is Data"

[Teemu Likonen]

* 2010-09-05 14:24 (+0200), Kazimir Majorinc wrote:

> Imagine that someone invited you to write the presentation "Five best
> CL macros ... I seen" or "Five best CL macros ... I wrote." What would
> you chose and why?

Just regular syntactic abstractions here. I wanted to execute a series
of regular expression replacements for a string:

    (with-regexp-series STRING
      ("[^a-z0-9._-]" "_")      ;forms are ("regexp" "replacement")
      ("[_-]*\\.+[_-]*" ".")
      ("_*-+_*" "-")
      ("([._-])\\1+" "\\1")
      ("^_+(.)" "\\1")
      ("(.)_+$" "\\1"))

The macros:

    (defmacro with-value-passing ((var expression) &body body)
      `(let ((,var ,expression))
         ,@(loop for top-level in body
              collect `(setf ,var ,top-level))))

    (defmacro with-regexp-series (string &rest clauses)
      (let ((value (gensym)))
        `(with-value-passing (,value ,string)
           ,@(loop for clause in clauses
                collect `(ppcre:regex-replace-all ,(first clause) ,value
                                                  ,(second clause))))))

WITH-VALUE-PASSING is a general-purpose macro which binds a new variable
(VAR) and gives it the value of EXPRESSION. Then every top-level form in
the macro's body is evaluated and each time the value is assigned to the
VAR. It's just a way to run series of commands that depend on previous
command's return value. I think it's more readable than deeply nested
function calls.