Re: bookparts

[Jan-Peter Voigt]

Am 06.03.2018 um 13:12 schrieb David Kastrup:
> > IISC a major change to the codebase would be needed to allow something
> > like this: \bookpart { music = { c''4 } { \music } }
> > but there are a lot users who would like to use it this way.
> > I don't want to talk about scopes and coding style, but want to show
> > how I deal with it:
> > Using void-functions is a great thing because the can be called
> > anywhere. So if you want to define something inside a file that is to
> > be included inside curly brackets you can use a function like this:
> > --------------------------------------
> > \version "2.19.80"
> >
> > def = #(define-void-function (sym val)(symbol? scheme?)
> >           (ly:parser-define! sym val))
> >
> > \bookpart {
> >    \def music { c''4 } % this might be part of an included file
> >    { \music }
> > }
> It would be utterly trivial to make assignments work like that void
> function in the parser.
OK> It still would not introduce scoping, so people would be surprised that
> assignments made in one book overrode assignments in an earlier book or
> at top level.

> People would expect to set the same named variables in different books
> to different values and it would not work.
Essentially I talked about that in the second paragraph.

> A generic function like "def" is about as useful as a generic opener for
> child-proof bottle caps: the only person needing it is one you don't
> want to have it.
You may be right. But still, *sometimes* I use such a function for files 
which are included and which *might* throw an error otherwise.

> Now books and bookparts actually have a \header with associated scopes.
> Anything wrong with doing assignments in there?
That might look like this?
------------------------------
\version "2.19.80"

headerVal =
#(define-scheme-function (name)(symbol?)
   (let ((alist (ly:module->alist (ly:book-header $current-bookpart))))
     (assoc-get name alist)
     ))

\bookpart {
  \header {
    thevoice = \relative { a'4 c g f }
    thelyrics = \lyricmode { la la la la }
  }
  <<
    \new Voice = "heinz" { \headerVal thevoice }
    \new Lyrics \lyricsto "heinz" { \headerVal thelyrics }
  >>
}
----------------------------------

This might be useful and essentially lead to something. Now, I am used 
to my own toolchain and prefer delegating score- and bookpart-creation 
to music- and scheme-functions. And I really don't like using the header 
for content. (Btw. I didn't follow the whole thread so maybe nobody was 
asking for that)

Jan-Peter