Issue 2717: Implement \single for converting overrides to tweaks

\single, like \once, can be applied to music consisting of one or more
overrides.  It translates such an override set, for example
\easyHeadsOn, into a tweak on the following music item, so you can use
it like

{ < \single\easyHeadsOn c' g' >2 q }

inside of a chord and have it only affect a single notehead of the
chord when \once\easyHeadsOn would affect all noteheads.

In that manner, most overrides defined in ly/property-init.ly
are made available as tweaks.

diff --git a/ly/music-functions-init.ly b/ly/music-functions-init.ly
index 0fa79af3ddb63acd68e1861710c29cfa6903ccf6..26db17c5ede8ce07e9b72f78fa7d05c0f82a7cbf 100644 (file)
--- a/ly/music-functions-init.ly
+++ b/ly/music-functions-init.ly
@@ -1146,6 +1146,31 @@ shiftDurations =
     (lambda (x)
       (shift-one-duration-log x dur dots)) arg))
 
+single =
+#(define-music-function (parser location overrides music)
+   (ly:music? ly:music?)
+   (_i "Convert @var{overrides} to tweaks and apply them to @var{music}.
+This does not convert @code{\\revert}, @code{\\set} or @code{\\unset}
+and ignores nested overrides.")
+   (set! (ly:music-property music 'tweaks)
+         (fold-some-music
+          (lambda (m) (eq? (ly:music-property m 'name)
+                           'OverrideProperty))
+          (lambda (m tweaks)
+            (let ((p (cond
+                      ((ly:music-property m 'grob-property #f) => list)
+                      (else
+                       (ly:music-property m 'grob-property-path)))))
+              (if (pair? (cdr p))
+                  tweaks ;ignore nested properties
+                  (acons (cons (ly:music-property m 'symbol) ;grob name
+                               (car p)) ;grob property
+                         (ly:music-property m 'grob-value)
+                         tweaks))))
+          (ly:music-property music 'tweaks)
+          overrides))
+   music)
+
 skip =
 #(define-music-function (parser location dur) (ly:duration?)
   (_i "Skip forward by @var{dur}.")