Re: Building identifiers algorithmically

[Urs Liska]

Hi Trevor,

hey, that's what I just learned :-)

What you need is (ly:parser-lookup parser 'SopranoMusic) inside a music 
function.
Then create the 'SopranoMusic symbol from two strings and you're good to go.

\version "2.19.18"

SopranoMusic = {
  c' d'
}


getMusic =
#(define-music-function (parser location label1 label2)
   (string? string?)
   (ly:parser-lookup parser
     (string->symbol
      (string-append label1 label2))))

\score {
  \new Staff \getMusic "Soprano" "Music"
}

HTH
Urs

Am 21.03.2015 um 23:29 schrieb Trevor Daniels:
> LilyPond Schemers,
>
> I'm gradually getting the hang of Scheme, but I'd like some help with one 
> frustrating issue.  I'd like to build an identifier from two strings and use it 
> to reference a LilyPond variable.
>
> In other words, I have several Lily variables defined like this
>
> SopranoMusic = \relative { ... }
>
> and I'd like to reference them from Scheme code using the strings "Soprano" and 
> "Music".  The only way I've found is to build an alist of all of them using
>
> #(set! index (assoc-set! index "SopranoMusic" SopranoMusic))
>
> and then retrieve the music with
>
> #(define music (assoc-ref index (string-append "Soprano" "Music")))
>
> (In my real-use case of course some of the strings are variables.)
>
> Now while this works it seems rather clunky, so I'm wondering if there is a 
> more elegant way of doing this.  Symbols look like they might help, but so far 
> I've failed to make anything work.  I've also failed with macros, but that's 
> likely because I don't understand them yet.
>
> TIA, Trevor
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