Issue 3918: Add \alternatingTimeSignatures

ly/music-functions-init.ly

 %%%% -*- Mode: Scheme -*-
 
 %%%% This file is part of LilyPond, the GNU music typesetter.
 %%%%
 %%%% Copyright (C) 2003--2014 Han-Wen Nienhuys <hanwen@xs4all.nl>
 %%%%                          Jan Nieuwenhuizen <janneke@gnu.org>
 %%%%
 %%%% LilyPond is free software: you can redistribute it and/or modify
 %%%% it under the terms of the GNU General Public License as published by
 %%%% the Free Software Foundation, either version 3 of the License, or
@@ skipping 109 matching lines @@
                          (assoc-get 'interfaces
                                     (assoc-get 'meta description))))
             #{
               \override #item . #property =
               #(value-for-spanner-piece arg)
             #}
             (begin
               (ly:input-warning location (_ "not a spanner name, `~a'") name)
               (make-music 'Music))))))
 
+alternatingTimeSignatures =

[Keith, 2014/05/10 23:50:11]

The function does not do what its name implies, alternate time signatures, so
you need a long explanation.

LilyPond has a more understandable way to change the *printed* time-signature,
with \override..stencil.  I suggest you make a function to handle just the part
that users might find difficult to do themselves:
fractionList = #(define-scheme-function
  (parser location timesigs) (list?)
  (lambda (grob)
     (grob-interpret-markup grob
 ...

\new Staff  \relative c' {
  \time 6/8
  \once\override Score.TimeSignature.stencil =
    \fractionList #'((6 8)(5 8)(1))
  c8 c c c c c
  \override Score.TimeSignature.stencil = ##f
  \time 5/8
  c8 c c c c
  \revert Score.TimeSignature.stencil
  \time 3/4
 ...

[uliska, 2014/05/12 07:31:37]

After some consideration I agree that it is better to make the function a little
bit more basic. This leaves the user somewhat more to type, but also leaves some
flexibility to him:

- user can decide which context to override in (e.g. if Timing has been moved)
- user can decide to make the first actual \time different from the displayed
(although I can't think that's an actual use case)

+#(define-music-function (parser location timesigs)(list?)
+   (_i "Print alternating time signatures. The argument is a Scheme list
+of lists. Each list describes one fraction to be printed.
+When the function has executed the first of the given time signatures
+will be the effective @code{\\time}, while the following are simply graphical.
+When using it you will want to use @code{\\omit Score.TimeSignature}
+before the next use of @code{\\time}. Please note that this does not
+perform any checks regarding the used time signatures, so you're
+responsible yourself to write consistent music. To return to normal
+use of time signatures use @code{\\undo \\omit Score.TimeSignature}.")
+   (let ((first-effective-timesig (cons
+                    (caar timesigs)
+                    (cadar timesigs))))
+     #{ \once\override Staff.TimeSignature.stencil =

[janek, 2014/05/10 20:49:09]

Shouldn't this override be at Score level?

[uliska, 2014/05/10 22:38:29]

Any other ideas?
I have the impression that Staff. works just as well?

[Keith, 2014/05/10 23:50:12]

Well, if there are many staves, your function changes the initial time signature
for the whole score, but prints the special indication only on one Staff, unless
the user includes the function in each staff.  (There is a context called Timing
for the general case; Timing is usually the same as Score, unless the user has
different meter in each Staff.)

[uliska, 2014/05/12 07:31:37]

That's true and a case I missed to check. So overriding in the Score context is
the correct way.
Of course this is obsolete with the principal change to the function I'm going
to do.

+        #(lambda (grob)
+           (grob-interpret-markup grob
+             #{ \markup \override #'(baseline-skip . 0)
+                \number
+                #(map (lambda (x) #{ \markup \center-column #(map number->string x) #})

[janek, 2014/05/10 20:49:08]

This assumes that the user will input lists of 2 elements.  What if the user
enters just one number, or more than two, for example \alternatingTimeSignatures
#'((6 1 8)(5)) ?  The function should either handle such input gracefully or
emit a warning/error.

[uliska, 2014/05/10 22:38:29]

Entering more than two numbers seems gracefully enough:
A column of the given numbers will be created and the first two are used for the
effective time. The result is self-explaining IMO.

But entering only one number will actually fail with an error that doesn't point
the user to the right place or cause. So I agree that this should be improved.

+                   timesigs)
+             #}))
+        \time #first-effective-timesig #}))
+
 appendToTag =
 #(define-music-function (parser location tag more music)
    (symbol? ly:music? ly:music?)
    (_i "Append @var{more} to the @code{elements} of all music
 expressions in @var{music} that are tagged with @var{tag}.")
    (music-map (lambda (m)
                 (if (memq tag (ly:music-property m 'tags))
                     (set! (ly:music-property m 'elements)
                           (append (ly:music-property m 'elements)
                                   (list more))))
@@ skipping 607 matching lines @@
                                  (if (symbol? property)
                                      (list property)
                                      property))
                          parser location
                          #:default 'Bottom #:min 3 #:max 3)))
         (if prop-path
             #{
               \override #prop-path = #(offsetter (third prop-path) offsets)
             #}
             (make-music 'Music)))))
- 
+
 omit =
 #(define-music-function (parser location item) (symbol-list-or-music?)
    (_i "Set @var{item}'s @samp{stencil} property to @code{#f},
 effectively omitting it without taking up space.
 
 If @var{item} is a symbol list of form @code{GrobName} or
 @code{Context.GrobName}, the result is an override for the grob name
 specified by it.  If @var{item} is a music expression, the result is
 the same music expression with an appropriate tweak applied to it.")
    #{ \tweak stencil ##f #item #})
@@ skipping 832 matching lines @@
 Use this if you want to have a scheme expression evaluated
 because of its side-effects, but its value ignored."))
 
 withMusicProperty =
 #(define-music-function (parser location sym val music)
    (symbol? scheme? ly:music?)
    (_i "Set @var{sym} to @var{val} in @var{music}.")
 
    (set! (ly:music-property music sym) val)
    music)