Re: Help for \beamExceptions

[Simon Albrecht]

Am 02.09.2015 um 21:35 schrieb Richard Shann:
> On Wed, 2015-09-02 at 20:13 +0200, David Kastrup wrote:
> > > And there is a comment somewhere about using | to separate
> > something,
> > > I guess bars with the same \time signature (or, indeed different
> > > ones).
> > No, it's always for exactly one time signature, but at the time it is
> > called, LilyPond has no idea just how long a bar is and can't count it
> > off itself.  So you need to separate rule patterns by |.
> Ah, in that case I need a way of building up the list of beamExceptions,
> so the user can define beaming rules for all the time signature changes
> they use in the score. My first attempt was this:
> 8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><
> beamExceptions_first =   \beamExceptions { \time 4/4  e'8 f'8[ a'8 b'8  a'8] 
> f'8[ e'8 e'8] }
> beamExceptions_second =  \beamExceptions { \time 6/8  e'8 f'8[ b'8  a'8] f'8[ 
> e'8] }
>
> \layout {
> \set beamExceptions = #(append beamExceptions_first beamExceptions_second)
> }
> 8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><8><
>
> but using \displayScheme I see that this results in
>
> (make-music
>    'ContextSpeccedMusic
>    'context-type
>    'Bottom
>    'element
>    (make-music
>      'PropertySet
>      'value
>      (list (list (quote end) (list 1/8 5 3))
>            (list (quote end) (list 1/8 4 2)))
>      'symbol
>      'beamExceptions))
>
> Which I guess means that this is not creating a beaming rule specific to
> those time signatures - can I augment the alist returned by
> \beamExceptions to make it into a rule specific to the time signature
> concerned?
Any value of beamExceptions only applies to one time signature, which is 
the one currently in effect.
>   I guess I would know the answer to this if I could find out
> what the alist holds initially, but my feeble attempt to elicit this via
> \void \displayScheme #beamExceptions
> gave me
>
> #<Music function #<procedure #f (music)>>
>
> which is the "music" function rather than the alist - how come these two
> things appear to have the same name?
Well, that’s by design :-)
The default values of beamExceptions for each time signature are defined 
in scm/time-signature-settings.scm – also the best resource for examples 
in dealing with the ‘internal’ Scheme data structure of beamExceptions, 
which is hard to grasp :-)

Yours, Simon