Re: \bookpart and variables

[David Kastrup]

Werner LEMBERG <address@hidden> writes:

>>> why are variable assignments not allowed in \bookpart? [...]
>> 
>> Well, as far as I can see, assignments are also not allowed in
>> books.
>
> Hmm...
>
>> And the reason mostly would be that books and bookparts don't have
>> scope of their own (apart from book's \paper block).  So even if it
>> were allowed, it would end up just the same as a global assignment
>> outside of the book/bookpart.
>
> OK.  Even if \book and \bookpart don't provide `real' grouping (in the
> lilypond sense), they contain data between `{' and `}' – let's call
> this visual grouping.  I consider it quite natural to put stuff into
> `\bookmark { ... } ' to indicate that it belongs together.
>
>> Do you have a particular use case in mind?
>
> Yes.  I have the following structure for a collection of piano pieces
>
>   file-a.ly:  left = ...
>               right = ...
>
>   file-b.ly:  left = ...
>               right = ...
>   ...
>
> and I would like to organize it as follows.
>
>    \bookpart {
>      \include "file-a.ly"
>      \score { ... }
>    }
>
>    \bookpart {
>      \include "file-b.ly"
>      \score { ... }
>    }
>
>    ...
>
> Right now, I have to do this:
>
>    \include "file-a.ly"
>    \bookpart {
>      \score { ... }
>    }
>
>    \include "file-b.ly"
>    \bookpart {
>      \score { ... }
>    }

Well, yes.  The former arrangement suggests a locality that is not
there.  Here is an example (where I use the Scheme equivalent of
assignment):

\book {
  \bookpart {
    #(define notecolor blue)
    \score {
      { \override NoteHead.color = #(lambda (grob) notecolor)
        c1
      }
    }
  }

  \bookpart {
    #(define notecolor red)
    \score {
      { \override NoteHead.color = #(lambda (grob) notecolor)
        c1
      }
    }
  }
}

Now wouldn't you expect, assuming assignment were allowed where
#(define...) is not being used, that the bookparts would show different
colors for the noteheads?

Instead both of them have red noteheads because both use the same global
variable `notecolor' and reference it when \book starts iterating the
already assembled bookparts.

So as long as everything refers to global variables _and_ bookparts are
not iterated when they are encountered but only after the complete music
expressions in a book have been collected, we are just asking for
trouble by allowing assignment in places where the actual execution
happens at a different time than the assignments would.

-- 
David Kastrup