Re: Scheme predicative types

[Lukas-Fabian Moser]

Hi Aaron,
> > > #(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.

Yes, of course you're right that your solution is way more flexible (and 
that I was too optimistic about the possible scheme-expression forms 
created by \tempo).

I think what triggered my feeling uncomfortable was just the slight 
mismatch between the name of the predicate (tempo?) and the actual tests 
it contained. Maybe some verbose thing like music-with-tempo-definition? 
would make things clearer.

Lukas