Re: Duplicate SVG attribute

[Jean Abou Samra]

Le 24/12/2022 à 15:38, Paolo Prete a écrit :
> Hello,
>
> I'm noting that the generated SVG of this...:
>
> {
>      \once \override DynamicText.output-attributes.val = "x"
>      \once \override DynamicText.output-attributes.val = "y"
>      c'\mf
> }
>
> ... creates the following SVG tag, with a duplicate "val" attribute:
> <g val="y" val="x">


I've opened https://gitlab.com/lilypond/lilypond/-/issues/6494

It is unusual, though, to do two \once \override for the same grob 
property at the same time.

I would say it's reasonable to expect this to work when the overrides 
are together in sequential music ({ ... }) like this, but it is 
preferable not to do it in parallel music like

\version "2.25.0"

\new Voice <<
  { \once \override NoteHead.color = red c' }
  { \once \override NoteHead.color = blue d' }
>>


because in more complex cases, it gets tricky for LilyPond to determine 
which override is meant to have precedence.


> Is this a bug? Also note that the duplication doesn't happen if I use
> "\override" instead of "\once \override".
>
> In case of a bug, is there a workaround for it (I mean: is there a way to
> call \once \override multiple times for the same attribute without
> duplicating it)?


\version "2.25.0"

#(define* (my-uniq-list lst #:optional (cmp equal?))
  (reverse!
   (fold (lambda (x acc)
           (if (null? acc)
               (list x)
               (if (cmp x (car acc))
                   acc
                   (cons x acc))))
         '() lst) '()))


#(set! (@@ (lily output-svg) stencil-dispatch-alist)
       (assq-set!
        (@@ (lily output-svg) stencil-dispatch-alist)
        'start-group-node
        (lambda (attrs)
          (format
           #f
           "<g ~a>\n"
           (string-concatenate
            (map (match-lambda
                   ((attr . val) (ly:format " ~a=\"~a\"" attr val)))
                 (my-uniq-list
                  (sort attrs
                        (comparator-from-key car (comparator-from-key 
symbol->string string<?)))
                  (comparator-from-key car eq?))))))))

{
    \once \override DynamicText.output-attributes.val = "x"
    \once \override DynamicText.output-attributes.val = "y"
    c'\mf
}



With the usual warning when using @@ that you should not expect any sort 
of stability for this kind of code.

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