Re: \bookpart mystery

[Aaron Hill]

On 2018-06-04 08:54, David Sumbler wrote:
> In my efforts (largely successful) to find a way easily to control
> conditional compilation, I came across this apparent anomaly.
>
> The following two short examples work as one might expect:
>
> %%%%%
> \book {
>   \score { g }
> }
> %%%%%
>
> %%%%%
> var =
> \book {
>   \score { g }
> }
>
> \var
> %%%%%
>
> Adding a \bookpart to the first also works as expected:
>
> %%%%%
> \book {
>   \bookpart {
>     \score { g }
>   }
> }
> %%%%%
>
> But putting this into a variable thus:
>
> %%%%%
> var =
> \book {
>   \bookpart {
>     \score { g }
>   }
> }
>
> \var
> %%%%%
>
> causes parsing errors.
>
> For my better understanding of how LilyPond works, can somebody explain
> to me why this happens?
>
> David

I tried a few more permutations and found you can do this:

%%%%
  \version "2.19.81"
  varOne = \bookpart { \score { g } }
  varTwo = \book { \varOne }
  \varTwo
%%%%

But going back to your original code, given the output from lilypond...

> GNU LilyPond 2.19.81
> Processing `bookpartvar.ly'
> Parsing.../usr/local/lilypond/usr/share/lilypond/current/scm/lily-library.scm:243:5:
>  In procedure ly:book-process in expression (process-procedure book 
> paper ...):
> /usr/local/lilypond/usr/share/lilypond/current/scm/lily-library.scm:243:5:
>  Wrong type (expecting real number): #<undefined>

...here are the lines leading up to and including 243 from 
lily-library.scm:

%%%%
  (define (print-book-with book process-procedure)
    (let* ((paper (ly:parser-lookup '$defaultpaper))
           (layout (ly:parser-lookup '$defaultlayout))
           (outfile-name (get-outfile-name book)))
      (process-procedure book paper layout outfile-name)))
%%%%

It would seem there is a problem with the so-called "default paper".  
That led me to try this:

%%%%
  \version "2.19.81"
  var = \book { \bookpart { \paper {} \score { g } } }
  \var
%%%%

And that compiles.

-- Aaron Hill