What can you do in LISP that you can't do in Python

[Nonexistence]

"Terry Reedy" <reedy37 at home.com> writes:

> "Nonexistence" <huaiyuan at rac3.wam.umd.edu> wrote in message
> news:c6x1yprz2u4.fsf at rac3.wam.umd.edu...
> 
> ...
> >   (DEFMACRO NEW-STATE (TAG)
> >       `(PROGN (IF TRACING
> >                   (PUSH (CONS STATE (COPY-LIST REGISTERS))
> >                         STATE-HISTORY))
> >               (GO ,TAG)))
> >
> ...
> 
> What I know about Lisp macros is what I have gleaned from this thread,
> which isn't too much yet.  How is the above different from writing and
> calling a function.  IE
> 
> def new_state(tag):
>   if tracing: store_tracing_info()
>   goto(tag)
> 
> new_state(tag)

(Disclaimer: I am not the original author of the example, so I may be
 misinterpretating his intention.)

Notice the variables TRACING, STATE, REGISTERS, and STATE-HISTORY above;
they're not global variables, but "captured" local variables.  If you have
multiple instances of state machine (each properly setup in its own lexical
environment), the macro version of NEW-STATE above won't have the problem
of clobbering variables; i.e., each instance can have its own version of
STATE-HISTORY.

Variable capturing is usually to be avoided when creating macro; it
violates referential transparency.  But I guess it is justifiable here
because the above is supposed to be internal debugging code.

- huaiyuan