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Re: Is it possible for a macro to expand to nothing?
From: |
Pascal J. Bourguignon |
Subject: |
Re: Is it possible for a macro to expand to nothing? |
Date: |
Tue, 24 Nov 2009 10:42:50 +0100 |
User-agent: |
Gnus/5.1008 (Gnus v5.10.8) Emacs/22.3 (darwin) |
Alan Mackenzie <acm@muc.de> writes:
> Pascal J. Bourguignon <pjb@informatimago.com> wrote:
>> "Drew Adams" <drew.adams@oracle.com> writes:
>
>> This is the problem! Macros shouldn't return a _list_, they should
>> return a _form_. If you write a macro that returns a list, or you use
>> it so that it returns a list, that is not a valid form, then it is not
>> good style, even if you catch up.
>
> Is that right? I think you should be required to justify this assertion
> of "good style". If that "good style" really is good style, then the
> whole of cc-langs.el (which uses intense macro magic to generate data
> structures with both compile-time and run-time behaviour) is attrocious
> style.
If that was the case, yes, I would think so. Macros are designed to
generate code, not other data. If you are generating general data,
then using functions will be easier and clearer.
But cc-langs.el only defines four macros and they all generate
perfectly good lisp code.
> Fact is, though, it allows a simple tabular writing of constants
> which vary between C, C++, java, .... Kudos to Martin Stjernholm, who
> wrote it.
Unfortunately, most of emacs lisp code is bad code. Functions one
kilometer long, chained with one or more others one kilometer long.
Copy-and-pasted chunks instead of abstracting it away. Etc.
I cannot say that I've read a significant percentage of it, but the
code I had to peek at in my 20 years of use of emacs, I cannot say I
was impressionned by its good quality...
Now of course, I had a peek at code that had bugs or missing features
in the first place. Perhaps the good quality emacs lisp code I hadn't
a peek at because it worked well enough so I didn't need to.
>> Because it is a better style. It avoids abusing the ifdef macro.
>
> Where does this notion of "abuse" come from? What is its rationale?
> (This is a genuine question.)
The general contract of a macro is that it returns valid forms.
In all fairness, ifdef does return valid forms, when provided valid
forms as argument.
(defmacro ifdef (expr &rest body)
(and (eval expr) `(progn ,@body)))
(ifdef t (setq bar 2))
--> 2
It's only when provided invalid argu-ment that it returns an invalid
form:
(ifdef t ((setq bar 2)))
--> Debugger entered--Lisp error: (invalid-function (setq bar 2))
The fact that such a macro call embedded in another form building form
that processes it properly doesn't mean that it is not bad style: it
has to do something special to the result of ifdef to make it work.
If you extract that ifdef call to run it at the repl, it just cannot
work.
--
__Pascal Bourguignon__
- RE: Is it possible for a macro to expand to nothing?, (continued)
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/23
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/23
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/23
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?,
Pascal J. Bourguignon <=
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Jeff Clough, 2009/11/25
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/26
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/26
- Re: Is it possible for a macro to expand to nothing?, Lennart Borgman, 2009/11/26
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/26
- Re: Is it possible for a macro to expand to nothing?, Lennart Borgman, 2009/11/26