python and macros (again) [Was: python3: 'where' keyword]

[michele.simionato at gmail.com]

Paul Rubin wrote:
> How about macros? Some pretty horrible things have been done in C
> programs with the C preprocessor. But there's a movememnt afloat to
> add hygienic macros to Python. Got any thoughts about that?

"Movement" seems quite an exaggeration. Maybe 2-3 people made some
experiments, but nobody within the core Python developers seems
to be willing to advocate the introduction of macros.

> Why should you care whether the output of a macro is ugly or not,
> if no human is ever going to look at it?

But at least some human eye have to look at it!
<rethorical-question>
Did you ever debug a macro?
</rethorical-question>

More seriuosly, I have given some thought to the issue, and I am very
much against the introduction of macros in Python.

Here are a few reasons:

1. I like defmacro in Lisp. Problem is, Lisp is an s-expression based
language, Python is not. We cannot implement defmacro in Python and
even if we could, if would be too ugly to be used (at least, IMHO).

2. One could proposed hygienic pattern-matching macros in Python,
similar to
Scheme syntax-rules macros. Again, it is not obvious how to
implement pattern-matching in Python in a non-butt-ugly way. Plus,
I feel hygienic macros quite limited and not worth the effort.

3. We would add to Python the learning curve of macros and their
subtilities and we do not want it.

4. Macros would complicate a lot Python module system.

5. We have Guido providing a good syntax for us all, why we should be
fiddling with it? More seriously, if some verbosity is recognized
in the language (think to the "raison d'etre" of decorators, for
instance) I very much prefer to wait for Guido to take care of
that, once and for all, than having 100 different custom made
solutions based on macros.

I am sure I could find other reasons if I think a bit more, but these
should suffice for the moment ;)

What I would be interested in is a Lisp/Scheme implementation
compiling to Python bytecode, but I am not aware of any project
in that direction.


Michele Simionato

P.S. some pointers for people interested on the topic:

http://logix.livelogix.com/ (a Python-like language with macros)
https://sourceforge.net/projects/pymac/ (an experimental
Dylan-inspired macro system for Python)