Re: Iterators in Scheme?

[Niols]

Hello,

On 07/08/2020 00:18, David Kastrup wrote:
> Niols <niols@niols.fr> writes:
> > I have not managed to override the "\repeat" music function.
>
> It isn't a music function but a "reserved" word in LilyPond.

I know. Yet there is a "repeat" scheme function defined in 
scm/ly-syntax-constructors.ly that I tried to redefine (by recompiling 
LilyPond) and successfully managed to make it do what I wanted. I guess 
the lambda it contains gets passed to a table of some kind that 
associates reserved words to behaviours, but I could not locate such a 
table.

> > I suspect there is something I do not understand in the order in which
> > things are executed in LilyPond.
> >
> > I have however found a solution based on overriding "make-music". I
> > replace "make-music" by a wrapper around it that adds the callback
> > from the snippet to all 'VoltaRepeatedMusic and leaves the rest
> > alone. This is actually just a few lines:
> >
> >      #(define the-make-music make-music)
> >
> >      #(define (make-music-wrapped name . music-properties)
> >        (let ((music (apply the-make-music (cons name music-properties))))
> >         (if (equal? name 'VoltaRepeatedMusic)
> >          (ly:music-set-property! music 'elements-callback new-volta-set))
> >         music))
> >
> >      #(set! make-music make-music-wrapped)
> >
> > It might not be the cleanest, but that is all I have, and that seems
> > to work.
>
> It would be easier to do
>
> music-descriptions.VoltaRepeatedMusic.elements-callback = ...
>
> A cursory glance would suggest that might work (though its effect would
> not be limited to one session).

Great idea! By reading scm/define-music-types.scm,I saw that 
music-descriptions is fed to a hash table. I thus managed this with:

    #(let ((props (hashq-ref music-name-to-property-table 
'VoltaRepeatedMusic)))
      (assoc-set! props 'elements-callback new-volta-set))

> You could also try to redefine the make-volta-set function.

That, I did not manage to make work. I suppose that the music-properties 
list and, later, the music-name-to-property-table only store the 
contents of make-volta-set; overriding it later is thus of no use.

> > > 2. Maybe there is a way to define a function that will be ran before
> > > processing on any music? But this I have no idea if this is
> > > possible, or how to do it, and my research has proven ineffective so
> > > far.
>
> There are the scorification hooks.

That sounds really interesting, whatever that is, also for other hacks 
around LilyPond. Would you happen to have any pointer for that?

> > > 3. Maybe there is a way to override the "make-volta-set" callback
> > > instead of creating a new one? Such that then, in LilyPond's normal
> > > execution, it uses the new definition and not and old one that then
> > > needs to be replaced?
> >
> > For 2. and 3., I still have no idea as this is far beyond my
> > understanding of the internals of LilyPond. I am not really looking
> > into it though.
>
> Not a matter of internals.
>
> #(define original-make-volta-set make-volta-set)
>
> (define (make-volta-set ...) ...)
>
> Possibly
>
> (set! make-volta-set (lambda (...) ...))
>
> but I think that the define probably works.

I suppose they work to override make-volta-set. As said earlier, though, 
I did not manage to get the expected result from this. I suppose the 
lambda is already fed to the music-name-to-property-table. Overriding 
this thus does not give much useful things.

Thank you very much for you help!
— Niols