Re: Macro to prepend element to list

[Jean Abou Samra]

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