[lispm]

This is the view of a compiled execution. Lisp can also be interpreted from source as data. Then macro-expansion time is interleaved with execution time. Each use of a macro form may trigger a macro expansion.

Remember, a Lisp interpreter interprets Lisp source as data. Not byte code. Unlike Python, Java, Smalltalk, ... which all have popular implementations which compile to byte code and execute that byte code in a byte code interpreter, aka virtual machine.

Let's use a Lisp interpreter, here from LispWorks:

We define a primitive MY-IF macro. It expands into a simple IF use. But the macro will also count the number of macro expansions.

    CL-USER 46 > (defparameter *myif-counter* 0)
    *MYIF-COUNTER*

    CL-USER 47 > (defmacro my-if (c a b)
                   (incf *myif-counter*)
                   `(if ,c ,a ,b))
    MY-IF

LispWorks can trace macros, too.

    CL-USER 48 > (trace my-if)
    (MY-IF)

Now a simple function which uses our macro:

    CL-USER 49 > (defun fac (n)
                   (my-if (= 1 n)
                          1
                          (* n (fac (1- n)))))
    FAC

Now we use it and we will see the trace information for the macro use: you see the incoming form and the result form.

    CL-USER 50 > (fac 2)
    0 MY-IF > ...
      >> COMPILER::FORM        : (MY-IF (= 1 N) 1 (* N (FAC (1- N))))
      >> COMPILER::ENVIRONMENT : #<Augmented Environment venv NIL fenv ((#:FUNCTOR-MARKER . #<COMPILER::FLET-INFO (# # # #)>)) benv NIL tenv NIL decl NIL>
    0 MY-IF < ...
      << VALUE-0 : (IF (= 1 N) 1 (* N (FAC (1- N))))
    0 MY-IF > ...
      >> COMPILER::FORM        : (MY-IF (= 1 N) 1 (* N (FAC (1- N))))
      >> COMPILER::ENVIRONMENT : #<Augmented Environment venv (#<Venv 275415194600  N>) fenv ((#:SOURCE-LEVEL-ENVIRONMENT-MARKER . #<COMPILER::FLET-INFO (NIL . #)>) (#:FUNCTOR-MARKER . #<COMPILER::FLET-INFO (# # # #)>)) benv NIL tenv NIL decl NIL>
    0 MY-IF < ...
      << VALUE-0 : (IF (= 1 N) 1 (* N (FAC (1- N))))
    0 MY-IF > ...
      >> COMPILER::FORM        : (MY-IF (= 1 N) 1 (* N (FAC (1- N))))
      >> COMPILER::ENVIRONMENT : #<Augmented Environment venv NIL fenv ((#:FUNCTOR-MARKER . #<COMPILER::FLET-INFO (# # # #)>)) benv NIL tenv NIL decl NIL>
    0 MY-IF < ...
      << VALUE-0 : (IF (= 1 N) 1 (* N (FAC (1- N))))
    0 MY-IF > ...
      >> COMPILER::FORM        : (MY-IF (= 1 N) 1 (* N (FAC (1- N))))
      >> COMPILER::ENVIRONMENT : #<Augmented Environment venv (#<Venv 275416002360  N>) fenv ((#:SOURCE-LEVEL-ENVIRONMENT-MARKER . #<COMPILER::FLET-INFO (NIL . #)>) (#:FUNCTOR-MARKER . #<COMPILER::FLET-INFO (# # # #)>)) benv NIL tenv NIL decl NIL>
    0 MY-IF < ...
      << VALUE-0 : (IF (= 1 N) 1 (* N (FAC (1- N))))
    2

Let's see how often our macro function has been used to expand code:

    CL-USER 51 > *myif-counter*
    4