Re: Set a temporary tempo change

[Thomas Morley]

Am Mo., 18. Nov. 2019 um 16:55 Uhr schrieb Aaron Hill
<address@hidden>:
>
> 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 () ()

Hi Aaron,

why wrap it into a music-function?
Only advantage seems to avoid a toplevel-definiton. Is it really an advantage?

Same for popTempo.


Cheers,
  Harm

>    (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?

Well, I'm not David ;)
You omitted throwing the error about redefining existing ones...
Feels safer to have it, but that's just me.

Cheers,
  Harm