bug#11378: 24.1.50; Suggestion: Let M-i in isearch cycle `search-invisible'

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

> > A major difference is that my code does not hesitate to use the
> > command to toggle the user option value.  I realize you consider
> > that a no-no.  I think that commands to toggle user options are
> > often a plus, not a minus.  (Think minor-mode variables, if that helps.
> > Or think of toggling an option via the options menu.)
> 
> Most togglable search variables should preferably have two versions
> anyway: the default value (persistent in case of `defcustom') used
> initially for every new search (and often consulted in toggling
> commands to get the initial default value) and a transient value
> used in the currently active search.
> 
> This is what `case-fold-search' and `isearch-case-fold-search'
> already do and this is a flexible approach.  I intend to
> implement the same approach for other search variables as well,
> e.g. adding `search-filter-predicates' to hold the default value
> for `isearch-filter-predicates'.  The prefix `search-' is more
> appropriate for such variables when they are used outside of isearch.
> So `isearch-filter-predicates' could be used to incrementally
> add/remove filters in isearch-mode, but to use `search-filter-predicates'
> in `query-replace', etc.

I understand that approach.  I disagree, that's all.

I believe it is good to be able to toggle the option value (what you call
the default value) on the fly (during a search) in this case, and thus
change the behavior for subsequent searches as well (until toggled again).
That is a more useful UI, IMO, in this case and in many others.

I am not sure what I think about case-fold-search.  Generally I think the
usual approach makes more sense for it.  But there is an argument to be
made for it too to "stay put" across searches.  Partly, such "sticky"
behavior requires clear feedback to the user so s?he is aware of the current
"mode" of behavior.

It is simple enough to toggle back again.  Even for a one-off search using
a different "mode" from usual, it just means two toggles instead of one.
And if you perform multiple searches in different modes (a mix: some
case-sensitive, some not),  then the sticky/modal behavior typically means
less toggling.

I still use the usual approach for case-fold-search, because (a) I am
used to it and (b) I use case-insensitive search rarely.  But in Icicles
I have a similar toggle for completion, and it is sticky/modal.  Again,
good feedback wrt the current behavior is more important for the sticky
behavior: you are not always thrown back into some default behavior.

Ideally perhaps the choice of behavior itself would be up to the user,
and perhaps even itself via a toggle.  No, I don't mean ojust a single
option value reflecting the choice, but the ability to toggle which
behavior to use for any behavior choice (toggle or cycle).

(No, I don't really expect others to agree.)