bug#70622: [PATCH] New window parameter 'cursor-type'

[Eli Zaretskii <eliz@gnu.org>]

> From: Eshel Yaron <me@eshelyaron.com>
> Cc: rudalics@gmx.at,  70622@debbugs.gnu.org
> Date: Mon, 29 Apr 2024 10:18:33 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > I still don't think I understand the difficultly.  The "window
> > parameter not set" can be detected by testing whether cursor-type is
> > an element in the alist returned by window-parameters, couldn't it?
> 
> Perhaps I'm missing something, but I think it's not so simple.  We
> can't/shouldn't interpret a value of nil differently from no alist entry
> at all, since there's no way to remove alist entries (in order to unset
> the window parameter) from lisp, right?

Why is the ability to remove important?

> > It is confusing to have several parameters and variables with a
> > similar semantics that each require special quirks to get the same
> > effect.  We should try to make the forms of the values of such
> > variables and parameters be uniform.
> 
> Agreed.  If we could use exactly the same format for the values of the
> variable and window parameter that would be best.  But AFAIU we need
> some encoding (special quirk) when translating from variable values (VV)
> to window parameter values (WPV), due to the competing interpretations
> for nil.  The first encoding I proposed is simple and uniform: cons to
> go from VV to WPV, car to go the other way.  The alternative encoding I
> proposed below, following mode-line-format, is trivial for most VV, but
> not uniform, because one needs to translate nil to 'none when going from
> VV to WPV.
> 
> >> Alternatively, we can use the exact same format for the window
> >> parameter as we do for the variable, except we interpret a window
> >> parameter nil value as "window parameter unset", not "no cursor", and
> >> instead add another value for the window parameter, 'none, that'll
> >> correspond to the nil value of the variable (so window parameter 'none
> >> would say not to show the cursor).
> >
> > That's possible, but doesn't the absence of cursor-type parameter from
> > window-parameters already allow to solve that?
> 
> See above.  Please let me know if I'm mistaken about the possibility of
> differentiating a value of nil from the absence of an alist entry.
> 
> >> This is in line with what we have
> >> for the mode-line-format variable and window parameter, for example.
> >
> > Where do we use this convention for window parameters?
> 
> See "(elisp) Window Parameters":
> 
> ‘mode-line-format’
>      This parameter replaces the value of the buffer-local variable
>      ‘mode-line-format’ (*note Mode Line Basics::) of this window's
>      buffer whenever this window is displayed.  The symbol ‘none’ means
>      to suppress display of a mode line for this window.
> 
> Here the window parameter has the same set of possible values as the
> variable, except the window parameter also has the value 'none, which
> corresponds to the variable's nil value.  The nil value of the window
> parameter means "window parameter not set".

If this is unavoidable (which I'm not yet sure), we can use this.  But
let's first hear from Martin, maybe there are better ideas.