RFC -- Hygienic Macros in Python

[phil hunt]

On Fri, 8 Feb 2002 23:04:57 -0600, Jason Orendorff <jason at jorendorff.com> wrote:
>phil hunt wrote:
>> Python macros should, if they exist, have a superset of the 
>> capabilities of
>> C macros. Including the ability to be replaced by arbitrary code in the
>> program. Why arbitrary? firstly, because it is conceptually simple;
>> secondly because it doesn't restrict the programmer.
>
>C macros are quite restrictive, precisely *because* they simply
>paste tokens into your program.
>
>> Consider this code:
>>
>> defmacro incj:
>>    j = j + 1
>
>Well, I don't think that's particularly realistic code. 

Nor do I, particularly. I can't offhand think of anything where I'd
particularly want to use macros in Python, except perhaps to define my
own control structures.

But just cos i can't think of it, doesn't mean someone else won't
find it useful.

> Consider
>this instead.
>
>Jason Orendorff wrote:
>>   defmacro synchronized(lock_expr, code_block):
>>       lock = ^lock_expr
>>       lock.acquire()
>>       try:
>>           ^code_block
>>       finally:
>>           lock.release()
>
>Under a non-hygienic macro system, obviously I wouldn't write it
>like that because "lock" might clash with a local name.  Indeed if
>I have nested "synchronized" statements it's guaranteed to clash.
>So what should the code look like in a non-hygienic system?

The way i would do it, if I was implementing it, would be something
like:

defmacro synchronized(lock_expr, code_block):
   local lock
   lock = ^lock_expr
   lock.acquire()
   try:
      ^code_block
   finally:
      lock.release()      

Where the "local lock" declaration means something like: in the
current namespace create a local variable name that doesn't clash with
anything else.

In something like C++ you could just put {...} round it can define
variables within that and they'd be local to it, but you can't do 
that in Python.



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