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Re: Is it possible for a macro to expand to nothing?


From: Thierry Volpiatto
Subject: Re: Is it possible for a macro to expand to nothing?
Date: Sat, 28 Nov 2009 13:57:44 +0100
User-agent: Gnus/5.110011 (No Gnus v0.11) Emacs/23.1.50 (gnu/linux)

address@hidden (Pascal J. Bourguignon) writes:

> Alan Mackenzie <address@hidden> writes:
>
>> Pascal J. Bourguignon <address@hidden> wrote:
>>> Tim X <address@hidden> writes:
>>
>>>> On the other hand, Alan's arguments also have merit. If a macro can be
>>>> useful in generating something other than a form that can be
>>>> evaluated, such as a data structure and can do so in a way that is
>>>> cleaner/simpler or just easier to understand than doing the same using
>>>> functions, then it would make sense. His examples from the C modes
>>>> seem to be a case in point.
>>
>>
>>> Perhaps Alan's problem with functions comes from the confusion between
>>> backquote and macros.  Since backquote (and , and ,@) are often used in
>>> macros, some people believe they can be used only in macros, and that
>>> they ARE what macros are.
>>
>> Er, do I actually have a problem with functions?  But no, I don't suffer
>> that particular confusion between backquotes and macros, and have indeed
>> used backquote when there hasn't been a macro within zeptoparsecs.
>>
>>> Far from it!  I don't know any language more orthogonal than lisp.
>>
>> Maybe not, but even lisp only gets to about 89 degrees.  It is missing an
>> operator which does the same as ,@ outside backquote.  This is one of the
>> lacks which makes it so difficult to write an equivalent of C's #if.
>
> Yes, emacs lisp is missing reader macros.  In Common Lisp there's #+,
> #- and #., and reader macros may return 0 object.

A little note about #., it have emacs equivalent `eval-when-compile':

,----[ That is the CL version ]
|  -- Special Form: eval-when-compile forms...
|      The FORMS are evaluated at compile-time; at execution time, this
|      form acts like a quoted constant of the resulting value.  Used at
|      top-level, `eval-when-compile' is just like `eval-when (compile
|      eval)'.  In other contexts, `eval-when-compile' allows code to be
|      evaluated once at compile-time for efficiency or other reasons.
| 
|      This form is similar to the `#.' syntax of true Common Lisp.
`----

I don't know the #- and #+ macros, what are they for?

>
>>> Backquote can be used in a function to build a s-exp (including part of
>>> a form) as it can be used anywhere.
>>
>>> Therefore it is really not easier to use macros to generate parts of a
>>> form than function.
>>
>> It is if you need "side effects", like c-lang-defconst and friends do.
>
> Well, I'm not sure about emacs, but in the case of Common Lisp, you
> have to be careful with side effects in macros, because it is not
> specified how many times, and when, macro functions are called.
>
> I'd say that with emacs you at least have the same problem if you load
> and/or eval the contents of a file and then you byte-compile it, the
> macros may be executed twice.
>
> So that's one more difficulty you have to cater to with macros, that
> you don't have with functions.
>
>
>> [ Stuff read and appreciated.  My brain's not up to it so early in the
>> morning].
>>
>>> If you wanted to use macros, in addition to the complexity of having
>>> to use macroexpand to use it, you would have the difficulty of passing
>>> the parameters, since a macro gets it's parameters from the source
>>> form.  In the case of a function, you have the choice to quote or not
>>> to quote the parameters, with macros they're always quoted for you.
>>
>> In the case of a function, you've GOT to quote, which can get very
>> tedious in some circumstances.  That's probably the reason that Martin
>> Stjernholm wrote c-lang-const etc. as macros.
>
> Read again my example, I hadn't to quote the arguments to the function
> because they weren't the literal arguments, but variables containing
> the arguments, because the function was called from another function
> or macro (macro in this case) who got them as parameter.
>
> If you had to forward arguments to a macro, it would be much more
> complicated. Then you would have to use macroexpand.  

-- 
A + Thierry Volpiatto
Location: Saint-Cyr-Sur-Mer - France





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