Re: Iterators in Scheme?

[Niols]

Hi David,

On 29/07/2020 15:16, David Kastrup wrote:
> Niols <niols@niols.fr> writes:
> > Must I understand that it is not possible to write iterators in Scheme
> > and that they have to be written in C++?
>
> Yes.

Thank you; at least I stop wondering about that.

> > Must I then understand that, if I wanted to write iterators for a
> > personal use, I would have to recompile LilyPond myself?
>
> Yes, or reuse one of the existing ones.  Some are comparatively generic.

I would love that as it would allow to only have to write some Scheme. 
But I am not sure this can be achieved. See below.

> What do you want to achieve?

This is actually follow-up research from my side after a previous e-mail 
to this list. I would like to print bar numbers that are "repeat aware", 
that is such that if bars 9 to 12 repeat twice, then the bar numbers go

    1, 2, …, 9, …, 12, 17, 18, etc.

with a jump after 12 that corresponds to the bars 13 to 16 that are 
actually 9 to 12. Even better, I would like to print the two bar numbers 
for these bars (9/13, 10/14, etc.)

I have tried hacking around the current bar number with an engraver that 
would recognize repeat commands, putting the current bar number in a 
stack whenever reaching a "start-repeat" and popping it when reaching an 
"end-repeat". This works, but:

- does not seem very clean,

- does not allow to write several bar numbers because we can't know yet 
how long the repeat is going to be,

- does not recognize repeats that are for more than twice.

From what I understand, however, there are two passes: first the 
iteration and then the engraving. Besides, the iteration gets access to 
all the information of the score, and thus also the number of repeats 
that is put by the user (N in "\repeat volta N"), while the engraving 
only get the events generated by the iteration.

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.

Best,
— Niols