bug#17394: 24.4.50; enhancement request: split `next-error-function' functionality in two

[Drew Adams <drew.adams@oracle.com>]

Enhancement request, to make the `next-error' feature, or more precisely
the buffers that offer it, more usable by other Lisp functions.

Currently, buffers, such as `occur' and `grep', that provide
`next-error' information hold the destination information for each hit
in different ways, and they provide a function for the buffer-local
value of `next-error-function' that does two things together:

1. Get the destination information for the next (or previous) hit,
   e.g. based on point in the search-hits buffer.

2. Go to that search-hit destination.

What I would like to see is a buffer-local variable whose value is a
function that does only #1.  (This var would then be used by a function
that is the value of `next-error function', to do its #1 part.)

Why?  Because instead of just going to the "next" or "previous" hit, I
want to gather all of the hits at once, to serve as candidates
(e.g. completion candidates) for another way of choosing and navigating
among them (e.g. in different orders besides "next" in the hit list).

(I sometimes want to in fact visit the target buffers during a #1
function call and pick up some text at or near the hit position to serve
as the candidate text, if what is in the error buffer does not provide
the best context for choosing.  But all I'm really asking for here is to
get the search-hit destinations (locations).)

This is easy enough to do for each particular buffer and mode such as
occur or grep separately, but each one is different.  The point of
variable `next-error-function' is to provide a common interface: just
call the value of that function to do #1 and #2 - no need to know how
the given search-hit buffer represents/maps the hit information.

When I call the function (the value of the new variable) to do #1, I
just want it to return the information for the next hit (its location).
I do not want to actually visit the hit at that time for the user,
displaying and raising the destination buffer etc.  I want to gather
the destination information for each of the hits, and in a
buffer/mode-independent way.

Essentially, I want a wrapper that provides a common interface to the
hit information that is stored in the different error buffers in
different ways.  AFAICT, there is no such feature today, but let me know
if I'm missing something obvious.  And let me know if this request is
not clear to you.


In GNU Emacs 24.4.50.1 (i686-pc-mingw32)
 of 2014-04-29 on ODIEONE
Bzr revision: 117031 monnier@iro.umontreal.ca-20140429151607-qnkgbymwfaj5ut08
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/snapshot/trunk
 --enable-checking=yes,glyphs 'CFLAGS=-O0 -g3'
 LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''