Getting started with music functions

[Stephan Schöll]

Hi all

When working with larger scores / several voices/instruments, I would
like to "loop through every single instrument" to produce the
\book-blocks in order to avoid duplicate lilypond code.

Let's take the following MVE:
--- MVE START ---

\version "2.19.83"

notesI = \relative c' {
  c4 d e f
}

notesII = \relative c {
  e4 f g b
}

generateBook =
#(define-music-function
     (parser location notes)
     (ly:music?)
   #{
     \book {
       \score {
         $notes
         \layout {}
       }
     }
   #})

\generateBook \notesI

\generateBook \notesII

--- MVE END ---

Lilypond exits with error:
.../document.ly:24:1: error: music function cannot return ##<Book>
\generateBook \notesI

.../document.ly:26:1: error: music function cannot return ##<Book>
\generateBook \notesII

It looks as if the "output" (return value) of a function can only be of
type "music", not "score", "book" aso, which would disappoint me. Am I
right? Or is there a way to define/change the type of the return value?

What I expect from the MVE is two PDFs, one with a notesI score, one
with a notesII score. I have been inspired to do so by the rehearsalMidi
function generated by Frescobaldi. I further intend to extend the music
function with additional parameters.

I'm currently on Win10 and prefer working with lilypond alone - without
shell scripting, Python, etc.

What I have already studied:
NR, chapter 5.6 and sub-chapters
Extending, chapter 2.3 and sub-chapters
Urs Liskas blogposts on music functions (1-4, 2014)

TIA&Regards
Steff