scheme function that conditionally sets several variables in \paper?

[Paul Morris]

Greetings all, 

Is there currently a way to get the scheme function below (a minimal
example) to do what it's trying to do, namely return a paper block with
several parts that are conditionally added or omitted?

I have tried various combinations of # and $ for introducing the scheme
expressions, and read about nesting \paper inside of \paper here:
http://lilypond.1069038.n5.nabble.com/Conditionally-setting-variables-in-paper-section-td149984.html#a150005

Maybe there's a way to dynamically create the paper block in scheme and
return that?  I didn't find anything on how to do that (and \displayMusic
only works on music), so it seems this is unexplored territory at the
frontiers of what's possible, or maybe I'm just missing something.

Background: this is for my "scale vertical spacing" code in openlilylib[1]
and it would be nice to only change the variables that need changing.  It
currently works, but by setting the whole group of paper variables, some are
just set to the defaults rather than new/scaled values.

[1]
https://github.com/PaulMorris/openlilylib/blob/vert-scaling-refactor/notation-snippets/scale-vertical-spacing/definitions.ily

Thanks,
-Paul


%% SNIPPET %%
\version "2.19.13"

myPaperFunction =
#(define-scheme-function (parser location arg) (list?)
   #{
     \paper {
       $(if (assq-ref arg 'system-system)
            #{ \paper { system-system-spacing = #(assq-ref arg
'system-system) } #})

       $(if (assq-ref arg 'score-system)
            #{ \paper { score-system-spacing = #(assq-ref arg 'score-system)
} #})

       $(if (assq-ref arg 'markup-system)
            #{ \paper { markup-system-spacing = #(assq-ref arg
'markup-system) } #})

       $(if (assq-ref arg 'score-markup)
            #{ \paper { score-markup-spacing = #(assq-ref arg 'score-markup)
} #})

       $(if (assq-ref arg 'markup-markup)
            #{ \paper { markup-markup-spacing = #(assq-ref arg
'markup-markup) } #})

       $(if (assq-ref arg 'top-system)
            #{ \paper { top-system-spacing = #(assq-ref arg 'top-system) }
#})

       $(if (assq-ref arg 'top-markup)
            #{ \paper { top-markup-spacing = #(assq-ref arg 'top-markup) }
#})

       $(if (assq-ref arg 'last-bottom)
            #{ \paper { last-bottom-spacing = #(assq-ref arg 'last-bottom) }
#})
     }
   #})

\myPaperFunction
#'((system-system . 2)
    (score-system . 1.5)
    ;; (markup-system . 2)
    ;; (score-markup . 2)
    ;; (markup-markup . 2)
    (top-system . 2)
    (top-markup . 3)
    (last-bottom . 2))


% EXAMPLE MUSIC
global = { \key c \major \time 4/4 }
somenotes = \repeat unfold 48 { c8[ c] }
chordNames = \chordmode { \global \repeat unfold 12 { c1:m } }
melody = \relative c'' { \global \somenotes }
verse = \lyricmode { \repeat unfold 48 { la la } }
right = \relative c'' { \global \somenotes }
left = \relative c' { \global \somenotes }
\score {
  <<
    <<
      \new ChordNames \chordNames
      \new Staff { \melody }
      \addlyrics { \verse }
    >>
    \new PianoStaff <<
      \new Staff = "right" { \right }
      \new Staff = "left" { \clef bass \left }
    >>
  >>
  \layout { }
}



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