Re: Macro to prepend element to list

[Jean Abou Samra <jean@abou-samra.fr>]

Le 20/03/2021 à 19:10, Linus Björnstam a écrit :

> I see!
>
> To be honest, this seems like a guile-1.8ism... I don't think such code would work for guile2 though 3. Why? Because your are mutating a pointer local to your procedure, not the actual data pointed to by the music property You are modifying the pointer to the list returned by the call, not the list where it is stored.
>
> I don't know enough lilypond, but a more standard way of doing that would be something like (ly:music-property-set! m 'articulations (cons  (make-music 'ArticulationEvent 'articulation-type "staccato") (ly:music-property m 'articulations))) (or a shorthand thereof).
>
> More verbose, sure, but also standard scheme...

Well, not sure what you mean exactly, but ly:music-property is defined 
with a setter:

(define-public ly:music-property
   (make-procedure-with-setter ly:music-property
                               ly:music-set-property!))

which makes the code equivalent to (ly:music-set-property! m 
'articulations (cons ... (ly:music-property m 'articulations))), doesn't 
it?

> This could of course be abstracted away in some nicer way, like something I just pulled out of my posterior without much prior knowledge of lilypond (except for a fingering chart for bassoon I maintain):
>
> (update! (select '(note-event 'artuculations)) (lambda (p) (cons (make-music ...) m)))
>
>   This could be done with higher order functions and passing lambdas around without having to rely on macros...

Indeed, we could do things along these lines:

\version "2.23.1"

#(define (ly:music-transform-property! music prop func)
    (ly:music-set-property! music
                            prop
                            (func (ly:music-property music prop))))

addStaccato =
#(define-music-function (music) (ly:music?)
    (map-some-music
      (lambda (m)
        (if (music-is-of-type? m 'note-event)
            (ly:music-transform-property!
              m
              'articulations
              (lambda (elts)
                (cons (make-music 'ArticulationEvent 'articulation-type 
"staccato")
                      elts))))
        #f)
      music))

\addStaccato { c'4 d' e'8 f' g' a' }

It is more verbose for prepending elements, but much more versatile, and 
'cute' from (srfi srfi-26) can make the call more compact as well.

Best regards,
Jean