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Re: Scheme predicative types

From: Aaron Hill
Subject: Re: Scheme predicative types
Date: Thu, 17 Sep 2020 01:30:25 -0700
User-agent: Roundcube Webmail/1.4.2

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)

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

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