Re: TimeSignature with note in denominator

[Aaron Hill]

On 2021-11-12 5:57 pm, Kieren MacMillan wrote:
> so I again (perhaps naïvely) hope that
>     \time \note-denom 3/4
> and
>     \time \note-denom #'(3 . 4)
> could both work.

As David said, those are precisely the same thing.  A fraction is one of 
the primitive tokens our lexer presents to the parser.  This becomes a 
number pair in the Scheme world.  So for all intents, it is simply too 
late in the process to distinguish 3/4 and #'(3 . 4).

The point of the helper I proposed is that it makes it clear that the 
user is to provide a number followed by a duration.  In this way, there 
is no confusion (for the user or the program), although it means you 
cannot use a slash to separate the fraction as one might want to do.

It should be possible for the helper to accept a single fraction as 
well, so both of these usages could work:

%%%%
\time \note-denom 6/8   ;; slash means simple numeric fraction
\time \note-denom 2 4.  ;; no slash means scalar and duration
%%%%

Consider the following function signature:

%%%%
#(define (fraction-or-duration? arg) (or (fraction? arg) (ly:duration? 
arg)))

note-denom =
#(define-scheme-function
  (scalar fraction-or-duration)
  ((number? 1) fraction-or-duration?)
  ; ...
)
%%%%

But is \note-denom doing anything useful for the simple case?  Users 
would probably just want to say \time 6/8 directly and cut out the 
middle man.  The only need for something like \note-denom would be in 
the 2/4. scenario where we cannot, at the moment, input 2/4. as a 
fraction and need some syntactic wrapping.


> Follow-up question: Is there any reason it can’t be a prefixed helper
> function? i.e.
>
>    \note-denom \time #'(3 . 4)
>
> which takes the time-signature-music as its input, and outputs
> adjusted time-signature music?

Functions can accept ly:music? and you can drill down to determine if it 
contains TimeSignatureMusic and modify its properties.  The only issue 
is that currently \time (and TimeSignatureMusic) require the denominator 
to be a number.  You can convert 2/4. to its logical equivalent of 6/8 
but key information is lost.  \note-denom cannot magically recover the 
original intention to be able to render the signature as the user 
wanted.


-- Aaron Hill