Re: Scheme predicative types

[Aaron Hill]

On 2020-09-16 12:09 pm, Lukas-Fabian Moser wrote:
> I'm sure more knowledgeable people will be able to provide more
> insightful answers, but for what it's worth: Looking at
> lily/parser.yy, I see
>
> tempo_event:
>     TEMPO steno_duration '=' tempo_range    {
>         $$ = MAKE_SYNTAX (tempo, @$, SCM_EOL, $2, $4);
>     }
>     | TEMPO text steno_duration '=' tempo_range    {
>         $$ = MAKE_SYNTAX (tempo, @$, $2, $3, $5);
>     }
>     | TEMPO text {
>         $$ = MAKE_SYNTAX (tempo, @$, $2);
>     } %prec ':'
>     ;
>
> which I take to mean: The three forms
>
>  * \tempo 4 = 96
>  * \tempo Crazy 4 = 260-270
>  * \tempo "Sluggishly slow"
>
> are hardcoded as variants into the parser. My guess is that this might
> be hard (or impossible) to accomplish in a music function.

You just need to be a little creative.  Consider:

%%%%
\version "2.20.0"

#(define (tempo? arg)
  (and (ly:music? arg)
       (not (null? (extract-typed-music arg 'tempo-change-event)))))

doSomethingWithATempo =
#(define-void-function
  (tempo)
  (tempo?)
  (set! tempo (first (extract-typed-music tempo 'tempo-change-event)))
  (let ((tempo-unit (ly:prob-property tempo 'tempo-unit #f))
        (metronome-count (ly:prob-property tempo 'metronome-count #f))
        (text (ly:prob-property tempo 'text #f)))
    (format #t "\nTempo: ~s"
      (list (cons 'tempo-unit tempo-unit)
            (cons 'metronome-count metronome-count)
            (cons 'text text)))))

\doSomethingWithATempo \tempo 4 = 60
\doSomethingWithATempo \tempo "Text" 4 = 60
\doSomethingWithATempo \tempo "Text"
%%%%

====
GNU LilyPond 2.20.0
Processing `tempo-function.ly'
Parsing...
Tempo: ((tempo-unit . #<Duration 4 >) (metronome-count . 60) (text . 
#f))
Tempo: ((tempo-unit . #<Duration 4 >) (metronome-count . 60) (text . 
"Text"))
Tempo: ((tempo-unit . #f) (metronome-count . #f) (text . "Text"))
Success: compilation successfully completed
====


-- Aaron Hill