svn commit: r298247 - head/sbin/fdisk_pc98

[John Baldwin]

On Thursday, April 21, 2016 04:01:17 PM Bruce Evans wrote:
> On Wed, 20 Apr 2016, John Baldwin wrote:
> 
> > On Wednesday, April 20, 2016 01:06:38 PM Bruce Evans wrote:
> >> On Wed, 20 Apr 2016, Marcelo Araujo wrote:
> >>
> >>> 2016-04-20 0:16 GMT+08:00 John Baldwin <jhb at freebsd.org>:
> >>>
> >>>> On Tuesday, April 19, 2016 04:46:13 AM Marcelo Araujo wrote:
> >>>>> Author: araujo
> >>>>> Date: Tue Apr 19 04:46:13 2016
> >>>>> New Revision: 298247
> >>>>> URL: https://svnweb.freebsd.org/changeset/base/298247
> >>>>>
> >>>>> Log:
> >>>>>   Remove redundant parenthesis.
> >>>>>
> >>>>>   Submitted by:       pfg
> >>>>>   MFC after:  2 weeks.
> >>
> >> I don't realling like churnging to the nonstandard nitems().  Use
> >> of the nonstandard <sys/param.h> is bad enough.
> >
> > I think it's not that bad from a readability standpoint.  Other languages
> > have fairly concise syntax for 'for-each' loops and this provides a
> > closer variant of that for statically sized arrays.  TAILQ_FOREACH() is
> > still nicer of course.  One could imagine doing some sort of
> > ARRAY_FOREACH() that was:
> 
> Ugh, I really realling (sic) don't like the FOREACH macros.  The FOREACH
> macros add syntactic salt.  The queue macros were bad enough before they
> had FOREACH.  Arrays don't start with such nastiness.
> 
> > #define	ARRAY_FOREACH(p, array)    \
> > 	for (size_t __i = 0, (p) = &(array)[0]; __i < nitems((array)); __i++, (p)++)
> 
> This only works in the not very usual case of a simple loop from the start
> to the end.  Even the name EACH becomes wrong if the range is anything else.
> For full nastiness, add a few hundred FORFOO macros to handle multi-
> dimensional arrays with different access methods, array slices, sentinels
> and other terminating conditions, etc.

Actually, combined with break this is a common case.  The fairly widespread
use of the queue FOREACH macros (and most of the uses of nitems() in the
tree) are a testament to this.

> > Perhaps better is this:
> >
> > #define	ARRAY_FOREACH(p, array)	\
> > 	for ((p) = &(array)[0]; (p) < &(array)[nitems((array))]; (p)++)
> >
> > (No need for __i)
> 
> But hiding the indexes forces the old C programming idiom of using
> pointers for everything.  A mere few hundred FORFOO macros won't do.
> For just 1-dimensional FOREACH, you need 3 versions to expose p, i or
> both p and i.

If someone wanted 'i' they could maintain it on their own through the loop.
In practice, 'foreach' is a common idiom in other languages and is used to
mean 'for each item in container'.  The 'short' syntax always implies a full
iteration (though you could use break to bail early).  For example, in
python:

	for item in list:
		# do stuff

Or in C++11:

	for (p : array) {
	}

In C++11 I think this only works for std::array rather than a plain C
array, though compared to a fixed-size array for which nitems() works,
std::array is equivalent.  Both of these work for dynamically sized
containers for which our queue macros are the equivalent.

-- 
John Baldwin