Re: Scheme global variables in Lilypond

[Nick Payne]

On 01/01/13 20:21, David Kastrup wrote:
> Nick Payne <address@hidden> writes:
>
> > In the following lilypond code, why is the value of the scheme
> > variable dirn not being changed by the set! command in the updown
> > function? The updown function is called on the first line of
> > beginStringNum,
> No, it isn't.
>
> > beginStringNum = #(define-event-function (parser location strng)
> >    (number?)
> >    #{
> >      \tweak direction #updown
> It is _referenced_ on the first line of beginStringNum, and the function
> reference is put into the direction field of the grob eventually created
> by the event created by \startTextSpan, and when this direction field
> gets _referenced_ eventually, updown is finally called.
>
> [...]
>
> >      \startTextSpan
> >    #}
When I add \displayLilyMusic to some simplified code

%==============
\version "2.17.6"

#(define dirn 1)

#(define (updown grob)
  (let ((ud (ly:event-property (event-cause grob) 'direction)))
    (if (eq? -1 ud)
      (begin
        (set! dirn DOWN)
        DOWN)
      UP)))

beginStringNum = #(define-event-function (parser location strng)
  (number?)
  #{
    \tweak direction #updown
    \tweak bound-details.right.text \markup {
      \draw-line #(cons 0 (/ dirn -2)) }
    \startTextSpan
  #}
)
endStringNum = \stopTextSpan

\relative c' {
  \displayLilyMusic { c4_\beginStringNum 5 c c c\endStringNum }
}
%==============

I get this shown on the console:

{ c-\tweak direction #updown -\tweak #t.(bound-details right text) 
#'(#<procedure line-markup (layout props args)> ((#<procedure 
draw-line-markup (layout props dest)> (0 . -1/2)))) _\startTextSpan c c 
c\stopTextSpan }

Does that mean that the hardcoded initial value of the dirn variable is 
being substituted in the second tweak before the updown function is 
called? That would explain what I'm seeing.