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Re: Better way to interpret $N as dollars, not macro argument?

From: Daniel Goldman
Subject: Re: Better way to interpret $N as dollars, not macro argument?
Date: Sat, 16 Aug 2014 11:10:09 -0700
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0

On 8/15/2014 12:07 PM, Eric Blake wrote:

Side note: it's a bad idea to use m4_define without quoting the first
argument.  If the define gets called more than once, then you end up
defining a completely different macro name (namely, whatever the
expansion of the original macro name was).  You should ALWAYS quote the
first argument to define.

I'm well aware of that strong recommendation. I appreciate your bringing it up. To not get off-topic on this post, and to give me time to think about this more before going out on a limb (which might break), it's best for me to hold off responding to the side note.

Alas, such is life.  Another approach that I have personally used is to
delegate to helper functions that concatenate the results into the
desired output.  Instead of:

m4_define([[[foo]]], [[[[[[$]]]1 is $1]]])

I do:

m4_define([[[cat]]], [[[$1$2]]])
m4_define([[[foo]]], [[[cat($, [[[1 is $1]]])]]])

which still requires special casing $, but a little less thought on how
to add creative use of quoting.

Thanks for the suggestion. I do something similar, but not for the dollar sign. Here is an example.

m4_define(M4_JAM, $1$2)
m4_define(CHT_Def_Val_001, [[[ M4_JAM(CHT_Def_Val_, RUN_ID_001) ]]])

Maybe I should quote $1$2 like you do, it wouldn't hurt. It works fine unquoted, because it always "jams" together parts of C names, so at least never any commas to protect. But if I used [[[$1$2]]] it would be more general purpose.

Anyway, my question: Is there a better way to tell m4 not to interpret
$N as "dollar amount"? Based on the manual, the answer seems "no". But I
thought I would ask, hoping maybe some other way...

Sadly, this is one area where M4 is locked into historical warts.  At
one point, there was an M5 language written, where you could use $$
inside any macro definition to have the expansion of that macro use one
less $ instead of treating $ as a possible parameter lead-in (thus,
m4_define([[[foo]]], [[[$1 $$1]]]) and called as foo(a) would expand to
'a $1').  It might be interesting if someone were to write a patch to
add an opt-in setting to m4 to copy from M5's dollar quoting mechanism
(of course, it must remain off by default for back-compat).  But I don't
know of anyone currently trying to write such a patch.


(Hmm, I know at one point I was able to read a pdf about M5 at no
charge, but can't seem to find a URL for it via a quick search today)

I saw something on m5, when I was researching and learning about m4. The m5 stuff seemed old and dead, similar to what you found dated from 1991. :(

Personally, instead of trying to implement $$, I think a better priority might be to get named macros working. [[[$1 $$1]]] IS a lot easier to read than [[[$1 [[[$]]]1]]] (IMO), but maybe the extra effort to add $$ is not worth the modest readability benefit, especially as `$1 $$1' and `$1 `$'1' seem closer in readability (IMO).


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