Re: TimeSignature with note in denominator

[Lukas-Fabian Moser]

Hi Kieren,

> You’re not extrapolating the concept, as I have been asking people to, so I’ll 
> once again make it more explicit for you:
>
> I want the user to be able to say
>
>      \tweak style #'note-denom \time 3/4.
> or
>      \tweak style #'note-denom \time #'(3 . "4.")
> or
>      \tweak style #'note-denom \time #'(3 . {4.})
>
> or basically any other intuitive input that would allow a dotted duration for 
> the denominator, which would pass through the parser to the TimeSignature 
> formatting function(s) without “loss” (as per Aaron described it).

It's probable that I'm missing a point (since I only skimmed the 
discussion so far), but:

Currently, \time is defined (in ly/music-functions-init.ly) with the 
signature

#(define-music-function (beat-structure fraction)
   ((number-list? '()) fraction?)

Now the predicate fraction? is defined (in scm/c++.scm) as

(define-public (fraction? x)
  (and (pair? x)
       (index? (car x)) (index? (cdr x))))

to wit, a pair of non-negative exact integers.

Now the lexer turns 3/4 into such a pair: See lily/lexer.ll, by virtue 
of the lines

N        [0-9]
FRACTION    {N}+\/{N}+

which translate as "a fraction consists of one or more digits, followed 
by /, followed by one or more digits"; and the call to the procedure 
scan_fraction; just search for it in lexer.ll. (I'm deliberately giving 
you more information than you probably need, to help you navigate the 
source and see how everything fits together.)

So, \time 3/4 is converted to \time #'(3 . 4) already while lexing, 
which is very early and very "basic". It's no problem at all to make 
\time accept more general pairs: Just replace fraction? by (e.g.) pair? 
in the definition of \time, or define a suitable predicate of your own 
accepting only the "right" kind of pairs.

So, of your three examples, only the first one (\time 3/4.) definitely 
would require changes to the lexer/parser. The others are not hard to 
implement (provided I don't miss something). But: I think it would be 
desirable to be able to enter durations in LilyPond syntax, in order to, 
for example, be able to enter complicated constructs like "8.~8" from 
Werner's example. But if you want to include LilyPond syntax in an 
explicit pair argument, you need #{ #} which certainly works but makes 
for horrible syntax:

test =
#(define-void-function (test) (pair?)
   (display-scheme-music test))

\test #`(3 . ,#{ 8~8. #})
% or not much better:
\test #(cons 3 #{ 8~8. #})

Wouldn't it be easier to define an independent \kierenTime function that 
expects an integer (index?) numerator and a music (ly:music?) 
denominator? Then we could just write

\version "2.22"

kierenTime =
#(define-void-function (num den) (index? ly:music?)
   (format #t "~a\n" num)
   (display-lily-music den))

\kierenTime 3 { 8~8. }
\kierenTime 3 4.  % the dot is important :-)

I provided a very silly simple example along these lines in my mail 
replying to Werner's example. I'm not sure if you saw this, as you 
reported that only part of the discussion came through to your end.

Lukas