Re: region-active-p

["Stephen J. Turnbull" <turnbull@sk.tsukuba.ac.jp>]

Richard Stallman writes:

 >      > If there is no extension, just the mark set,
 >      > `zmacs-region-extent' is nil and `region-active-p'
 >      > returns nil therefor.
 > 
 >     Right.  However, if zmacs-region-extent is non-nil, it is possible
 >     that (region-beginning) equals (region-end), in which case there is
 >     (usually) no point in operating on the region.  It is that check that
 >     Richard is asking about AFAICS.
 >
 > That really puzzles me.

`region-active-p' does not check that the region has non-zero length.

My point is that whether it makes sense to operate on a region of
length zero depends on the operation, so `region-active-p' doesn't try
to decide.

Note that Andreas and I are talking at cross-purposes to some extent,
because his usage of "extent", although impeccable as English, differs
from XEmacs terminology where it denotes a specific type of
Lisp_Object, approximately an Emacs overlay.  This confused me, and
I'm probably not responding to him in a way that's useful to him or
you.

 > It seems to say that in the following XEmacs code
 > 
 >     (defun fill-paragraph-or-region (arg)
 >       "Fill the current region, if it's active; otherwise, fill the paragraph.
 >     See `fill-paragraph' and `fill-region' for more information."
 >       (interactive "*P")
 >       (if (region-active-p)
 > 	  (call-interactively 'fill-region)
 > 	(call-interactively 'fill-paragraph)))
 > 
 > with an active but empty region, region-active-p sometimes returns t
 > and sometimes returns nil.  (This is because zmacs-region-extent might
 > be nil, or might be t and empty.)
 > 
 > Do I understand correctly?

No, if the region is active but empty (by which I mean having zero
length), `region-active-p' will return non-nil and `fill-region' will
be called.

An active region exists if and only if `zmacs-region-extent' contains
an extent object.  Code that activates the region does so by setting
`zmacs-region-extent' to an extent whose endpoints are point and mark,
and whose face property is 'zmacs-region.  Deactivation is done by
setting `zmacs-region-extent' to nil.