Re: POSIX vs. eval

[Eric Blake]

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According to Gary V. Vaughan on 12/29/2006 1:27 PM:
> Hi Eric,

Hi Gary, and happy new year,

> 
> On 28 Dec 2006, at 14:41, Eric Blake wrote:
>> The current wording of the m4 eval macro forbids implementations from
>> providing the , and  ?: operators as extensions, even though these
>> operators could reasonably be implemented (all of the other forbidden
>> operators involve C language constructs that are not available with
>> strict numerical evaluations).  The BSD implementation of M4 already
>> provides ?:.
> 
> Agreed.  As long as extensions are labeled as such for each
> implementation, the onus is on the user to decide whether to
> use the extension and lock their code in, or ignore the extensions
> in exchange for having portable code.
> 
> I think the only change in the POSIX wording would be to remove
> the forbid aspect, and instead list all the required operators.

Here's the aardvark I finally submitted:
https://www.opengroup.org/sophocles/show_mail.tpl?CALLER=index.tpl&source=L&listname=austin-review-l&id=2250

In preparing the aardvark, I also realized that it may be worth adding the
following GNU extension in m4 2.0.  Currently, if eval encounters a word
(ie. macro name), it is a syntax error.  But we could check if the
definition of that macro is a number, and thus support variables; once
variables are supported, we can then also support assignment operators.
Thus "define(a,1)ifelse(eval(`a++'))a" would expand to "2", similar to the
shell construct "a=1; : $((a++)); echo $a".

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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