2012/10/13 David Kastrup <d...@gnu.org>:
>
> 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.
>
> In the Scheme layer, we have the distinction between
> make-grob-property-set (corresponding to user command \override, pop+push)
> make-grob-property-override (not available as user command, just push)
> make-grob-property-revert (corresponding to user command \revert, just pop)
>
> So we need something like
>
> crossStaff =
> #(define-music-function (parser location notes) (ly:music?)
> (_i "Create cross-staff stems")
> #{
> \temporary\override Stem #'cross-staff = #cross-staff-connect
> \temporary\override Flag #'style = #'no-flag
> $notes
> \revert Stem #'cross-staff
> \revert Flag #'style
> #})
>
> and, as a corrollary, \temporary\whatever and \undo\whatever will be a
> proper push/pop pair.
>
> Now I find \temporary a fitting name. It is just a bit cumbersome to
> type. \interim is hardly shorter, \adhoc seems a bit less descriptive.
>
> Opinions? It might also be possible to do
>
> \temporary { \override Stem #'cross-staff = #cross-staff-connect
> \override Flag #'style = #'no-flag }
> $notes
>
> and have this apply matching push/pop pairs around $notes. It certainly
> would at least cater for the most common need of a non-first-popping
> push. Perhaps we don't need both (at the current point of time, we
> don't have either, and the naming is inconsistent with
> make-grob-property-*), but as the crossStaff example shows, we need at
> least one of those options.
as a programmer I want a single push and wouldn't mind if pop+push
were two operations; I can't tell what a musician wants.
p
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