On Thu, Sep 9, 2010 at 3:48 AM, Laurent PETIT <laurent.pe...@gmail.com> wrote:
> Hello,
>
> 2010/9/9 Sean Corfield <seancorfi...@gmail.com>:
>> On Wed, Sep 8, 2010 at 7:28 AM, CuppoJava <patrickli_2...@hotmail.com> wrote:
>>> I found the easiest way to introduce macros is just to introduce them
>>> as small syntactic sugaring. For example, getting rid of the explicit
>>> (fn [] ...) for macros like (with-open file ...).
>>
>> Interesting. I don't see any real difference between macros and C
>> preprocessor stuff and C++ templates at a conceptual level. I think
>> Clojure macros are much cleaner, but essentially they are similar. So
>> in the Java world, generics (templates) are not yet widely used
>> outside the libraries and maybe that's why Java devs find macros hard
>> to comprehend?
>
> I think that even at the conceptual level, the differences are big:
>
> a. C/C++ is a "pre-processor". It does a first pass on the code.
> Only at the end is the C/C++ compiler invoked. In Lisps, there is
> still this "first pass/second pass" thing, but it's at a waay finer
> granularity level: the top level form. At the end of the evaluation of
> each top level form, a new macro may have been defined and can be
> called immediately by the next top level form. So not only is the
> "set" of macros not closed in Lisp (and in C/C++, to some extent, it's
> also not closed, even if rather limited), but it can be expanded
> during the compilation of the "program".
Of course the real difference is that in Lisp macros you are working
directly on the AST, where in C/C++ macros you're working at the
source level. My understanding of the C/C++ preprocessor is that it
more or less does a string substitution, which *may* lead to a syntax
errors (you often see #define X(y) do { y } while (0); to get around
different limitations), or undesired results, not to mention
unintended variable capture, etc, etc, etc.
The fact that Lisp macros actually operate on the AST means that Lisp
macros can make *changes* to the AST (insert things, remove things,
rearrange things), and *not* just substitute FOO for BAR. This is a
hell of a lot more powerful.
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