Re: Set a temporary tempo change

[Aaron Hill]

On 2019-11-18 6:24 am, David Kastrup wrote:
> Aaron Hill <address@hidden> writes:
> > Not sure if this is really the right way to do things:
>
> It isn't.  It maintains the "stack" in a global variable rather than
> some context property, meaning that when several iterations interlock
> (like with tempo being changed in several contexts or in polyrhythmic
> situations), chaos will ensue.

Ah... you're no fun.  To misquote Willy Wonka, "A little [chaos] now and 
then is relished by the wisest men."  (:

But seriously, I was imagining only the simplest use case.  So, here's 
yet again another take:

%%%%
\version "2.19.83"

#(begin
  (set-object-property! 'tempoStack 'translation-type? list?)
  (set-object-property! 'tempoStack 'translation-doc
    "A stack to keep track of previous tempi.")
  (set! all-translation-properties
    (cons 'tempoStack all-translation-properties)))

pushTempo = #(define-music-function () ()
  (define (pushTempoHelper ctx)
    (let ((tempoStack (ly:context-property ctx 'tempoStack '()))
          (tempo (ly:context-property ctx 'tempoWholesPerMinute #f)))
      (set! tempoStack (cons tempo tempoStack))
      (ly:context-set-property! ctx 'tempoStack tempoStack)))
  #{ \context Score \applyContext $pushTempoHelper #})
popTempo = #(define-music-function () ()
  (define (popTempoHelper ctx)
    (let ((tempoStack (ly:context-property ctx 'tempoStack '())))
      (if (null? tempoStack)
        (ly:input-warning (*location*)
          "Unable to pop a tempo. Did you forget to \\pushTempo?")
        (let ((tempo (car tempoStack)))
          (set! tempoStack (cdr tempoStack))
          (ly:context-set-property! ctx 'tempoStack tempoStack)
          (if tempo
            (ly:context-set-property! ctx 'tempoWholesPerMinute tempo)
            (ly:context-unset-property ctx 'tempoWholesPerMinute))))))
  #{ \context Score \applyContext $popTempoHelper #})

\score {
  {
    \markLengthOn
    \tempo "I. Default"
    { b'4 4 4 4 } \bar "||"
    \pushTempo \tempo "II. Andante" 4=90
    { b'4 4 4 4 } \bar "||"
    \pushTempo \tempo "III. Allegro" 4=140
    { b'4 4 4 4 } \bar "||"
    \popTempo \tempo "Tempo II"
    { b'4 4 4 4 } \bar "||"
    \popTempo \tempo "Tempo I"
    { b'4 4 4 4 } \bar "||"
    \popTempo \tempo "Tempo 0?!"
    { b'4 4 4 4 } \bar "|."
  }
  \layout {} \midi {}
}
%%%%

I tried to follow the pattern in define-context-properties.scm, but 
there is no documentation in LM, NR, Extending nor Internals on this 
topic.  David, would you kindly let me know if I am overlooking 
something?


-- Aaron Hill