Re: Defining variables mid-stream in a music expression using tags.

[dfro]

On 5/17/23 16:30, Valentin Petzel wrote:
> Also gotta say that this is most likely not a good thing to do. Changing
> global scope variables like that can lead to confusing behaviour. Maybe if you
> can elaborate on what you want to achieve we might find a more ideomatic way to
> get there.
>
> Am Mittwoch, 17. Mai 2023, 22:24:54 CEST schrieb Valentin Petzel:
> > Hello David,
> >
> > there is some confusing going on here about how certain things are
> > evaluated. When you do
> >
> > { ... something ... }
> >
> > this is evaluated pretty much as it is parsed,  returning some music object.
> > So
> >
> > \keepWithTag #'textI \tag #'textII { #(define l "B") }
> >
> > this will first evaluate the expression { #(define l "B") } which returns an
> > empty music object and as side effect sets a variable, then it will call
> > \tag upon that music, and then call \keepWithTag on that Music.
> >
> > What you intend to do is in fact not possible with \tag. What you want to do
> > is to delay the evaluation until the point where you have the information
> > to make this decision. This can be done using a music function:
> >
> > \version "2.24.1"
> >
> > #(define l '())
> >
> > musicfn =
> > #(define-music-function (tag) (symbol?)
> >     #{
> >       #(if (equal? tag 'textI) (set! l "A"))
> >       #(if (equal? tag 'textII) (set! l "B"))
> >       c''1^\markup\l
> >       #(if (equal? tag 'textI) (set! l "C"))
> >       #(if (equal? tag 'textII) (set! l "D"))
> >       c''1^\markup\l
> >     #})
> >
> > \musicfn textI
> > \musicfn textII
> >
> > Note that here it is necessary to first create the global binding using
> > #(define ...) and then use (set! ...) to change that binding.
> >
> > Cheers,
> > Valentin
> >
> > Am Mittwoch, 17. Mai 2023, 21:58:49 CEST schrieb dfro:
> > > I want to define and change variables mid-stream in a music expression,
> > > while also using tags. Below is an example of what I am trying; but, it
> > > does not work.
> > >
> > >
> > > \version "2.24.1"
> > >
> > > \keepWithTag #'textI
> > > %\keepWithTag #'textII
> > >
> > >
> > > {
> > >
> > >     \tag #'textI { #(define l "A") }
> > >     \tag #'textII { #(define l "B") }
> > >     c''1 ^\markup \l
> > >     \tag #'textI { #(define l "C") }
> > >     \tag #'textII { #(define l "D") }
> > >     c''1 ^\markup \l
> > >
> > > }
> > >
> > >
> > > The tags are not keeping lilypond from skipping the define's that are in
> > > \tag #'textII. Is there a way to make this work?
> > >
> > > Is there a way to define variables using lilypond syntax inside a music
> > > expression?
> > >
> > >
> > > Peace,
> > >
> > > David


Valentin,

I want to be able to create a single music expression that I use in two 
staves. In that music expression I want to use custom 
define-markup-command functions that behave differently based on which 
staff they are in. Below is an example of what I have been doing.

I have been using global variables for variable changes that I want both 
staves to use, i.e. var-I, var-II. And, I have been making a music 
function for each markup function that that chooses between the staves 
using tags, and then inputs additional variables that change the 
behavior of the markup function, i.e. "staff-one-var", "staff-two-var".

I am wondering if there is a more elegant way to do this.


\version "2.24.1"

%Global variables
var-I = "var-I "
var-II = "var-II "

#(define-markup-command
  (symbol
   layout props
   var staff-variable)
  (markup? markup?)
  (interpret-markup
   layout props
   #{ \markup \concat { "Symbol " #var #staff-variable } #}
   )
  )

symbol =
#(define-music-function
  (var)
  (markup?)
  #{ -\tag #'staff-one ^\markup \symbol #var "staff-one-var"
     -\tag #'staff-two ^\markup \symbol #var "staff-two-var"
  #})


music =
{

  c1 \symbol \var-I
  c1 \symbol \var-II

}

<<
  \new Staff = "one"
  {
    \keepWithTag #'staff-one
    \relative c' {

      \textLengthOn
      \music

    }
  }
  \new Staff = "two"
  {
    \keepWithTag #'staff-two
    \relative c' {

      \textLengthOn
      \music

    }
  }
>>