Re: Direction operators in event-function

[Urs Liska]

Am 21.10.2018 um 16:58 schrieb David Kastrup:
> Urs Liska <address@hidden> writes:
>
> > But if I simply create two elements in the event-function the explicit
> > direction operator takes no effect:
> >
> > propagate-direction =
> > #(define-event-function (text)(markup?)
> >     #{
> >       -(
> >       -\markup #text
> >     #})
> >
> > {
> >    g'1 ^\propagate-direction "Up" g' )
> > }
> >
> > (here both the slur and the text are printed below the staff.
> >
> > Why is that?
> Multiple post events in one post event expression are temporarily
> wrapped in one PostEvents music event.  This is then what gets the
> up/down direction.  When it is unwrapped in use, the direction gets
> lost.

I see, this explains the observations.

> Arguably, dissolving a PostEvents into separate events should try
> applying some fields (tweaks, color, direction?).  Unfortunately, there
> are a lot.  Alternatively, directions and tweaks should bypass
> PostEvents and go through the enclosed events.  Either would have some
> performance impact.  Maybe just walk the property list of a PostEvents
> and copy _everything_ found there?
>
> > And more importantly: how can I achieve the goal of creating more than
> > one element in an event-function and still have the explicit
> > directions from the calling code be applied?
> >
> > And why can't I access the explicit direction operator from within the
> > event-function? I understand that the *generated* direction isn't
> > accessible at that moment, but an explicit setting should be known at
> > that point, isn't it?
> I have no idea what you are talking about.

Somehow I was afraid I didn't express myself clearly enough :-(

> What do you mean with
> "explicit direction operator" here?  What do you mean with "generated"
> direction?

With explicit I refer to a direction explicitly assigned, either thorugh 
^/_ or an \override like \slurUp etc. If I call my function with 
^\propagate-direction there is an explicit direction in place while the 
event-function is called, isn't it? If OTOH I call it with 
-\propagate-direction or without an operator the direction will only be 
generated/calculated at some later point, depending on the layout 
algorithms.
Is that correct or am I missing something?

What I would *like* to do is a function like

propagate-direction =
#(define-event-function (text)(markup?)
   #{
     -\tweak direction #UP
     -(
     -\tweak direction #UP
     -\markup #text
   #})

but instead of hard-coding #UP I'd like to calculate the values based on 
something. For example let the text have the opposite direction of the 
slur and have the slur accept the ^\_ operator. From what I can 
understand this information *should* be present at that moment, while 
I'm aware that it is *not* possible to do the same relying on the 
automatically determined direction of the slur (which is only calculated 
later).

Does that make my question clearer?

Urs

> The only direction operator I see here is clearly applied
> _after_ calling the event function.  It cannot magically affect
> expressions you create inside of the function before the function even
> returns.