Re: Scheme predicative types

[Aaron Hill]

On 2020-09-17 1:14 am, Lukas-Fabian Moser wrote:
> Hi Aaron,
> > > 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.
> That's what I hoped would happen. :-) If I understand it correctly,
> both the genius and the drawback of your solution lies in having the
> user write an explicit "\tempo" in order to invoke the parser's
> hard-wired way of dealing with the possible syntax variants, right?

That is correct.  In this situation, though, it might be too much to ask 
of the user.


> > %%%%
> > \version "2.20.0"
> >
> > #(define (tempo? arg)
> >   (and (ly:music? arg)
> >        (not (null? (extract-typed-music arg 'tempo-change-event)))))
>
> Maybe I'm overly cautious, but I'd prefer this to be more specific so
> that it accepts only actual "\tempo ..."-constructs. For example:
>
> #(use-modules (scm display-lily)) #(define (tempo? arg)   (and
> (ly:music? arg)        (string-startswith (music->lily-string test)
> "\\tempo")))
>
> This is probably an awful waste of computing effort, but I liked it
> better than checking for the kind-of special internal structure of a
> music expression containing only a \tempo (namely, a sequential-music
> containing both a TempoChangeEvent and a PropertySet specced to
> context Score.)

Note that \tempo produces different music depending on whether or not 
there is a tempo-unit and metronome-count specified.  That is why I 
wrote the code the way I did, to keep a consistent interface despite the 
internal differences.  It could also be possible someone would want to 
reuse an existing music expression that sets a tempo but does other 
things, so tightening down the interface as you suggest could be overly 
restrictive.

The only addition I would recommend would be to add an informative 
warning (or error) should the provided music contain more than one 
\tempo command as that would be somewhat ambiguous.


-- Aaron Hill