Re: Iterators in Scheme?

[Niols]

Hi David,

On 29/07/2020 15:58, David Kastrup wrote:
> Markup expressions are converted into stencils at a comparatively late
> point of time.

So that means I could create the markups for the several bar numbers 
while I see them, and these markups would use a variable that contains 
the bar number of the end of the repeat, and by the time the markups are 
actually interpreted, I can have filled the variable with the right value?

> >  From what I understand, however, there are two passes: first the
> > iteration and then the engraving.
>
> No, that's wrong.  It's just a single pass.  Iterators are strongly tied
> to the sequence of music expressions in the source code (and only
> persist while such an expression is being interpreted) while engravers
> are tied to contexts and persist as long as the respective context
> lives.  Iterators are driven by the music expressions and the advancing
> time while engravers are driven by stream events announced in contexts.
>
> But there is no separate pass involved.

Oh, OK, my bad. Thank you very much for the explanation!

> > For the problem of repeats, I read the code of the
> > Volta_repeat_iterator. In particular, it seems to read the
> > "repeat-count" property:
> >
> >      rep_count_ = scm_to_int (get_property (get_music (), "repeat-count"));
> >
> > but only uses it to detect the "first time" and the other times. The
> > first time:
> >
> >      if (first_time_)
> >        {
> >          add_repeat_command (ly_symbol2scm ("start-repeat"));
> >          first_time_ = false;
> >        }
> >
> > it adds a "start-repeat" command. As far as I understand, that means
> > that the engraving then only gets the repeat commands, which is
> > actually not enough information for what I am trying to achieve.
> >
> > I am really motivated to read code and do my own research and write my
> > own Scheme code, but I do not really know where to look and if I
> > understood stuff correctly. If you had any pointer to understand more
> > (or better) about what I understood, and to achieve this in the end, I
> > would be very grateful.
> >
> > In any case, I am already very grateful for the time you took to read
> > my message/s and answer me. Sorry for the long text.
>
> I thought we had several options for bar numbering in connection with
> repeats but don't know about the details.  There may be a more
> specialised solution for your use case.

The only option of bar numbering with respect to repeat that I know of 
is with respect to the alternatives.

https://lilypond.org/doc/v2.18/Documentation/internals/bar_005fnumber_005fengraver

The Bar_number_engraver reads the alternativeNumberingStyle property 
that decides whether the numbers should keep increase, or be the same in 
all alternatives but with a letter, or just be the same in all alternatives.

If there is an other option, I have never found it in all the time I've 
crawled in the documentation.

> Then there also is the recording-group-emulate function which you can
> use for causing a complete pass of iteration just for event gathering
> purposes.  It might be feasible to use that for collecting repeat count
> information in advance.

So one can run the whole thing a first time, possibly with engravers 
gathering information, and then a second time with the acquired data. 
That sounds interesting. I am not sure about the repeat count 
information though.

Am I right to say the the music properties are accessible from iterators 
but not engravers? And thus that, unless an iterator puts the repeat 
count music property somewhere where an engraver can get it, I have no 
way of knowing how many times the repeat goes?

Thanks again for your time!

Best,
— Niols