Re: Naming _another_ lacking puzzle piece

[Phil Holmes]

----- Original Message ----- 
From: "David Kastrup" <address@hidden>
To: "Trevor Daniels" <address@hidden>
Cc: <address@hidden>
Sent: Saturday, October 13, 2012 11:25 AM
Subject: Re: Naming _another_ lacking puzzle piece


> "Trevor Daniels" <address@hidden> writes:
>
> > David Kastrup wrote Saturday, October 13, 2012 1:01 AM
> > > In ly/music-functions-init.ly I see code like the following:
> > >
> > > crossStaff =
> > > #(define-music-function (parser location notes) (ly:music?)
> > >  (_i "Create cross-staff stems")
> > >  #{
> > >  \override Stem #'cross-staff = #cross-staff-connect
> > >  \override Flag #'style = #'no-flag
> > >  $notes
> > >  \revert Stem #'cross-staff
> > >  \revert Flag #'style
> > > #})
> > >
> > > There is a problem with that: in terms of stack operations, \override
> > > and \revert are not opposing pairs: \override is pop+push (so that
> > > multiple overrides in a row don't accrue cruft), \revert is pop.  So the
> > > net effect of this sequence is "pop", while it should be neutral.
> >
> > What would be the effect of this?  The implication is that something
> > goes wrong, but bugs in this area have never been reported, AFAIK.
> >
> > I understand stacks, pop and push, but I'm clearly missing something
> > here.  Perhaps it's due to my unfamiliarity with the structure of the
> > stack(s).
>
> Each context has its own stack, so popping too much only makes a
> difference if the stack actually was non-empty to start with.
>
> Check out
>
> \new Staff \with { autoBeaming = ##f }
> { \override Flag #'style = #'mensural c8 c \crossStaff c c c c c c }
>
> After the crossStaff, the flag style has reverted to normal again rather
> than mensural.
>
> --
> David Kastrup


Surely this points to the pop operation in \override as being at fault?  If 
\override was simply push, rather than pop-push then the code above would 
seem to work as intended.

The further implication of \override being pop-push would seem to be that if 
you do something like (pseudo code):

{\override Flag style = mensural c8 c c c \override Flag color = red c c c 
c}

Then you'd revert the mensural style when you change the color?  That would 
be wholly unexpected to me.

--
Phil Holmes