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From: "Paul Mensonides" <leavings@comcast.net>
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Subject: Re: Teaching new tricks to an old dog (C++ -->Ada)
Date: Sun, 27 Mar 2005 22:48:07 -0800
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Date: 2005-03-27T22:48:07-08:00
List-Id: <comp.lang.ada>

Hyman Rosen wrote:
> Jerry Coffin wrote:
>> I see. So what is it that (you incorrectly believe) the C
>> preprocessor can do that a Lisp macro can't do?
>
> Can Lisp macros manipulate the text of the macro arguments?
> For example,
>
>     #define cat1(a, b) a ## b
>     #define cat2(a, b) cat1(a, b)
>
>     #define A x
>     #define B y
>     cat1(A, B) // produces AB
>     cat2(A, B) // produces xy

Lisp macros cannot produce partial syntax either, whereas C/C++ macros can (e.g. 
on macro can create a '{' and another separate invocation can create the closing 
'}').  In the general case (obviously) Lisp (and Scheme) macros are far more 
powerful than C/C++ macros.

As an aside, given suitable input, it is possible to create a compiler or 
interpreter with the C/C++ preprocessor--with a relatively small number of 
macros.

Jerry is correct to say that they do similar things--they parametize code with 
code.  The main differences is that in Lisp (etc.) their are syntactic 
constraints (which is both good and bad in various scenarios) and that Lisp 
executes in an environment (even when compiled) that can interpret code created 
at runtime.

Regards,
Paul Mensonides