Re: Parenthensize a horizontal group?

[Lukas-Fabian Moser]

Hi Mark,

> > parenthesizeMusic function
> This is wonderful! Thank you!
>
> This is a slight error, which I don't have the skills to fix, in that
> there is no closing paren if music consists of a single note (a
> first/last problem?). Otherwise, straight into the library!

Make that:

parenthesizeMusic =
#(define-music-function (music) (ly:music?)
  (define ((stencils which) grob)
   (let* ((stens (parentheses-item::calc-parenthesis-stencils grob))
          (sten (which stens)))
    (if (eq? which first) (list sten point-stencil) (list point-stencil 
sten))))
  (define (add-tweak event tweak)
   (ly:music-set-property! event 'tweaks
    (cons tweak (ly:music-property event 'tweaks '()))))
  (let ((evs (extract-typed-music music 'rhythmic-event)))
   (ly:music-set-property! (first evs) 'parenthesize #t)
   (if (pair? (cdr evs))
       (begin
        (ly:music-set-property! (last evs) 'parenthesize #t)
        (add-tweak (first evs) `((ParenthesesItem . stencils) . 
,(stencils first)))
        (add-tweak (last evs) `((ParenthesesItem . stencils) . 
,(stencils second))))))
  music)

Explanation: The additional (if (pair? (cdr evs)) ...) checks if there's 
more than one rhythmic event in the given music before parenthesising 
the last event and cancelling the unnecessary half of each pair of 
parentheses.

(Why this works: evs is a list of events (a b ...). Now (cdr evs) is the 
tail (b ...) of that list. This is non-empty iff it is a pair, since 
scheme lists are internally nested pairs: '(1 2 3) is '(1 . (2 . (3 . 
()))) - which is also the reason why cdr, which returns the second 
element of a pair, returns the tail of a list.)

Thanks, Aaron, for this truly wonderful function (and please forgive me 
for messing with your code).

Lukas