Re: Music function to manage a moment and beat for the Staff and the Voice

[Valentin Petzel]

Hello Vlad,

in addition to Jean’s answer it might be helpful to understand what

Staff.property

actually means. This get’s parsed to a list of the symbols 'Staff and 
'property. So you can in fact simply remain with scheme, and directly enter 
#(list scope 'property) like this

%%%%%
\version "2.24.1"

momentBeat =
#(define-music-function (scope moment beat) ((symbol? 'Staff) fraction? list?)
   #{
     \set #(list scope 'beamExceptions) = #'()
     \set #(list scope 'baseMoment) = #(ly:make-moment (/ (car moment) (cdr 
moment)))
     \set #(list scope 'beatStructure) = #beat
   #})

{
  \momentBeat Voice 1/8 3,3,2
  c'8 8 8 8 8 8 8 8
}
%%%%%

or if you enjoy abusing (quasi)quoting by doing #`(,scope property) like this

%%%%%
\version "2.24.1"

momentBeat =
#(define-music-function (scope moment beat) ((symbol? 'Staff) fraction? list?)
   #{
     \set #`(,scope beamExceptions) = #'()
     \set #`(,scope baseMoment) = #(ly:make-moment (/ (car moment) (cdr 
moment)))
     \set #`(,scope beatStructure) = #beat
   #})

{
  \momentBeat Voice 1/8 3,3,2
  c'8 8 8 8 8 8 8 8
}
%%%%%

Cheers,
Valentin

Am Samstag, 1. Juli 2023, 16:58:48 CEST schrieb Jean Abou Samra:
> Le samedi 01 juillet 2023 à 16:42 +0200, Volodymyr Prokopyuk a écrit :
> > I'm trying to define a music function as below, however I've faced
> > difficulties with parameter predicates, visibility of the Staff object,
> > and flexibility of the solution ideally without code duplication
> > 
> > **Desired music function** (the code is not working)
> > 
> > ```
> >   momentBeat = #(define-music-function (moment beat scope)
> >     (fraction? list? (what? Staff))
> >     #{
> >       \set #scope.beamExceptions = #'()
> >       \set #scope.baseMoment = #(ly:make-moment moment)
> >       \set #scope.beatStructure = #beat
> >     #})
> > ```
> > 
> > **Usage**
> > 
> > ```
> > \momentBeat 1/2 #'(1)
> > \momentBeat 1/4 1,1,1,1 Voice
> > ```
> > 
> > **Questions**
> > 
> > 
> > - It seems that Staff is not available to Guile at that moment. I tried to
> > use "Staff" string and 'Staff symbol for the scope parameter with a cond
> > expression and code replication for Staff and Voice, but I did not manage
> > to get it to work due to incorrect predicates errors
> > 
> > - How would it look like an idiomatic and working implementation of the
> > above music function?
> There are a couple problems here.
> 
> - The `#` character introduces a Scheme expression. In Scheme, dots are
> allowed in identifiers. As such, `\set #scope.beamExceptions` is trying to
> look up a variable literally called "scope.beamExceptions". You need a
> space after `#scope` to terminate the Scheme expression. - A `fraction?` is
> not a rational number but a (numerator . denominator) pair. The reason is
> that `fraction?` is the predicate for music functions like `\time`, and
> `\time 4/4` needs to be distinguished from `\time 2/2` (for example). The
> `ly:make-moment` function, on the other hand, expects a rational number. -
> If you want an optional argument to actually be optional, it must not be
> the last argument. Otherwise, you can't actually omit it, you can only
> replace it with `\default`. See
> [here](https://extending-lilypond.gitlab.io/en/extending/music.html#optiona
> l-arguments) for more details.
> 
> Here is a working version of your function:
> 
> ```
> \version "2.24.1"
> 
> momentBeat =
> #(define-music-function (scope moment beat) ((symbol? 'Staff) fraction?
> list?) #{
>      \set #scope .beamExceptions = #'()
>      \set #scope .baseMoment = #(ly:make-moment (/ (car moment) (cdr
> moment))) \set #scope .beatStructure = #beat
>    #})
> 
> {
>   \momentBeat Voice 1/8 3,3,2
>   c'8 8 8 8 8 8 8 8
> }
> ```
> 
> Best,
> 
> Jean

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