Re: how to define a boolean?

[Marc Hohl]

Mark Polesky schrieb:
> Marc Hohl wrote:
>
>   
> > Thank you! At least I can play with this ...
> >     
> > > The callback is a little trickier for me for the moment.
> > > I'll get back to you.
> > >  
> > >       
> > Great. I am awaiting your response.
> >     
>
> Man, this stuff is tricky. Here's the best I can do for right
> now. I commented out the third line of myfunc; I wasn't sure
> how it made sense. Note that a music-function must return
> music. So you can only use ly:parser-define! *before* the
> music block. I couldn't get the callback to talk with the
> parser, so I made a callback that accepts the boolean value
> from within the music block itself.
>   
Thanks again for your help, Mark!
> One caveat: whenever the value of a parser variable is
> modified (anywhere), it affects everything below, and is
> not confined to any local scope. You can \include a \score
> from another directory entirely, and the included score
> will use the modified value.
>
> Will this work for you?
> - Mark
>   
Not yet, sorry. I played around a bit with your solution, but it doesn't fit
perfectly. But at least, I gain more and more insight in lily's internals...
(just a bit of comfort ;-)

I need a music-function which (aside from other tasks it fulfils) sets a 
boolean
to #t; moreover, a customized stencil callback looks if this value is 
set to #t
and performs a special action depending of the value, then the callback 
/itself/
has to set the boolean to #f.

Is this possible?

As I can see from now, giving mybool as an argument won't work in this case,
because any change won't be recognized until a next \override.

Marc


> \version "2.13.4-1"
>
> #(define mybool #f)
>
> myfunc =
> #(define-music-function (parser location note) (ly:music?)
>   ;(ly:parser-define! parser 'mybool #t)
>   (let* ((mybool (ly:parser-lookup parser 'mybool))
>          (color  (if mybool red blue)))
>     #{
>       \once \override  NoteHead #'color = $color
>       $note
>     #}))
>
> #(define-public (custom-callback mybool)
>   (lambda (grob)
>     (if mybool green yellow)))
>
> \relative c'' {
>   #(set! mybool #t)
>   \myfunc c
>
>   % ugh... dummy scheme object to cope with parser-lookahead.
>   % Notice what happens when it's commented out.
>   #0
>
>   #(set! mybool #f)
>   \myfunc c
>
>   \override NoteHead #'color = #(custom-callback mybool)
>     c c
>
>   % if you change the parser variable, you have to call the \override
>   % again - the callback arg doesn't get updated on-the-fly
>   #(set! mybool #t)
>   \override NoteHead #'color = #(custom-callback mybool)
>     c c
> }
>
>
>
>       
>
>