Re: skylines and custom-code

[Kevin Barry]

Hi Harm,

> Ok, another 5 days passed working allday on the problem, progress: zero.
> I give up.
> 
> Some insights:
> (1)
> It would have been helpful to understand how Stems fit into a Beam,
> then I could use this knowledge to probably do similar with glissando
> and Stems.
> I was not able to figure it out.
> (2)
> Obviously the Stems needs to be positioned after Beam (or Glissando) is done.
> An unpure-pure-container looked promising.
> Though, I was not able to code even the simplest via native scheme.
> This leads to the conclusion that unpure-pure-containers are not
> accessible/customizable for users like me.
> At least unless much more working examples are done in the docs.

I haven't had time to try and understand your issue, but having worked
on problems with Stems and Beams in the past I think it's an area of the
code that could be improved, because there is an effective circular
dependency:
- Stems check Beams in order to determine their direction/position etc
- Beams check where the first and last noteheads are - compared to the
  whole system - to try and figure out on which side they should be on
  or if the beam should be kneed, etc
- Checking where noteheads are in relation to the whole system triggers
  lots of other grob calculations. If any of these calculations check
  the Beam or the Stem you get a circular dependency (I fixed at least
  one regression in this area where DynamicText was triggering the
  circular dependency).
- If you try to dive into all the consequences of the code that
  calculates a Stem's direction it is almost impossible to understand.
  It's a long chain of pure/unpure calls - maybe 50 stack frames of just
  that stuff (IIRC); completely impenetrable (for me at any rate)
- In my opinion, the fix is to find a way to figure out notehead
  positions without calculating everything else, then Beams, then Stems

I doubt that is any help to you, but perhaps someone else will read this
and know more.

Kevin