Re: Setting default arguments for music function?

[Jean Abou Samra]

On 01/02/2023 01:23, Ahanu Banerjee wrote:
> Is it possible to have one of the arguments rely on a property of another 
> argument? 
> 
> In my example, I want the default value for "parenColor" to be the same as 
> the color of the "parenItem":
> 
> \version "2.24"
> \language "english" 
> altParen = #(define-music-function
>      (parenColor parenSize parenItem)
>      ( (color? "black") (number? -4) ly:music?)
>    #{
>      \tweak Parentheses.font-size #parenSize \tweak Parentheses.color 
> #parenColor \parenthesize #parenItem
>    #})
> { c \altParen -\tweak color "green" \upbow }


Well, the color you want to access isn't a property of parentItem.
parenItem is just a bit of music. Rather, it is a property of the
grob that will eventually be caused by the music parentItem. So,
in the music function, the color is not available. However, what
you can do is writing a callback which runs waay later in the process,
and can access it. Cf.

https://extending-lilypond.readthedocs.io/en/latest/extending/backend.html#understanding-callbacks

\version "2.24.0"

\language "english" 

#(define (color-from-host grob)
   (ly:grob-property (ly:grob-object grob 'sticky-host) 'color))

altParen = #(define-music-function
     (parenColor parenSize parenItem)
     ( (color? color-from-host) (number? -4) ly:music?)
   #{
     \tweak Parentheses.font-size #parenSize \tweak Parentheses.color 
#parenColor \parenthesize #parenItem
   #})
{ c \altParen -\tweak color "green" \upbow }



Actually, because this is a common pattern for grobs like parentheses
which attach to another grob, there is a shortcut:

\version "2.24.0"

\language "english" 

altParen = #(define-music-function
     (parenColor parenSize parenItem)
     ( (color? (sticky-grob-interface::inherit-property 'color)) (number? -4) 
ly:music?)
   #{
     \tweak Parentheses.font-size #parenSize \tweak Parentheses.color 
#parenColor \parenthesize #parenItem
   #})
{ c \altParen -\tweak color "green" \upbow }



Jean

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