bug#34150: 26.1; Document filtering with `isearch-filter-predicate' in Elisp manual

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 21 Jan 2019 10:40:36 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: 34150@debbugs.gnu.org
> 
> > > But it and its consequences are not necessarily
> > > obvious - especially to a user searching, as opposed
> > > to a programmer writing a filter predicate.
> > 
> > This is not a user-level facility, so the user perspective is not
> > relevant.
> 
> The "user perspective" is always relevant for Emacs.
> We are all users.
> 
> Searching is a user-level facility.  If a search
> imposes/provides filtering, that certainly affects
> user-visible behavior.  Users should understand
> that behavior.
> 
> Anyone imposing such filtering on users should
> consider its effects on users, including the
> considerations raised by this bug report.  That
> starts with making the behavior and consequences
> clear to filter implementors.

All true, but the filtering itself is not on the user level: the
_results_ of the filtering are.  So the behavior on the user level
should be described and understood by users in higher-level terms.
For example, with the default filtering, the behavior should be
described in terms of searching inside invisible text, not in terms of
filtering out some of the hits; the latter is just the implementation,
not the user-level behavior.

Therefore, the details of how filtering of matches works, and how to
write a filter, is not user-level information.  If you re-read what
you wrote above, you will see that your arguments are all consistent
with what I said, they don;'t contradict that in any way.