Writing a ++ macro in Common Lisp

Question

I've been attempting to write a Lisp macro that would perfom the equivalent of ++ in other programming languages for semantic reasons. I've attempted to do this in several different ways, but none of them seem to work, and all are accepted by the interpreter, so I don't know if I have the correct syntax or not. My idea of how this would be defined would be

(defmacro ++ (variable)
  (incf variable))

but this gives me a SIMPLE-TYPE-ERROR when trying to use it. What would make it work?

Answer 1

Remember that a macro returns an expression to be evaluated. In order to do this, you have to backquote:

(defmacro ++ (variable)
   `(incf ,variable))

Answer 2

Both of the previous answers work, but they give you a macro that you call as

(++ varname)

instead of varname++ or ++varname, which I suspect you want. I don't know if you can actually get the former, but for the latter, you can do a read macro. Since it's two characters, a dispatch macro is probably best. Untested, since I don't have a handy running lisp, but something like:

(defun plusplus-reader (stream subchar arg)
   (declare (ignore subchar arg))
   (list 'incf (read stream t nil t)))
(set-dispatch-macro-character #\+ #\+ #'plusplus-reader)

should make ++var actually read as (incf var).