Macros and how to trace them

Question

The trace macro is very useful for debugging. But it comes to a halt, when used upon any macro. Like if I try to do the following :

CL-USER> (trace push)
         


        

Answer 1

Using trace on a macro seems a little odd, but it works in CLISP:

(trace push)
(defparameter *stack* '())

(defun push-xy (x y)
  (push x *stack*)
  (push y *stack*))
; 1. Trace: (push x *stack*)
; 1. Trace: push ==> (setq *stack* (cons x *stack*))
; 1. Trace: (push y *stack*)
; 1. Trace: push ==> (setq *stack* (cons y *stack*))
; ==> push-xy

The standard does not say when it should expand macros so this might happen when functions and lambdas are defined, compiled and sometimes called. Some implementations run the macro twice so you get double the output.

I never use this. I rather use macroexpand-1:

(macroexpand-1 '(push x *stack)))
; ==> (setq *stack (cons x *stack))
; ==> t

If your form returns a new form that uses macros you might want to try macroexpand instead. Its like calling macroexpand-1 over and over until there are no transformation left.