# and $ (was Re: metronome-mark-alignment)

[Aaron Hill]

On 2020-01-14 2:39 pm, David Kastrup wrote:
> Ok, let me try again.
>
> # and $ differ in several respects.  # inserts Scheme constructs in
> places where LilyPond can decide how they fit into its syntax without
> looking at their value first.  In that case, evaluation of those Scheme
> expressions is done at the time the expression is being actually used
> which tends to be what one expects.  In contrast, $ can be used in
> places where the value of the expression will have an impact on just 
> how
> LilyPond will put them into context.  For example, a $ expression
> evaluating to a duration may be combined with a preceding pitch to form
> a note, while having it evaluate to another pitch in the same place 
> will
> form a separate note.
>
> This syntactic flexibility may lead to the expression getting evaluated
> at an unexpectedly early point of time, namely when LilyPond needs to
> know its type in order to decide that it does not actually combine with
> the preceding expression.

Many thanks for the clearer picture.

Would you be able to clarify how # and $ differ with regards to making 
copies of music?  Consider:

%%%%
\version "2.19.83"

notes = \fixed c' { g4 fis8 g a2 }

test = #(define-music-function (which music) (symbol? ly:music?)
  (define (red-notes music)
    (if (memq 'note-event (ly:music-property music 'types))
      (set! (ly:music-property music 'tweaks) '((color 1 0 0))))
    music)
  (case which
    ((hash) #{ {
        #music % will be red
        $music % will NOT be red
        \musicMap #red-notes #music % modifies music
        #music % will be red
        $music % will be red
      } #})
    ((dollar) #{ {
        #music % will NOT be red
        $music % will NOT be red
        \musicMap #red-notes $music % modifies a copy of music
        #music % will NOT be red
        $music % will NOT be red
      } #})))

\test hash \notes
\test dollar \notes
%%%%

The resulting behavior largely makes sense with one exception:

Why does the use of \musicMap against #music in the first \test not have 
an impact on \notes itself?  Is there an implicit copy of \notes when 
invoking the \test function that goes beyond the use of # and $ within?


-- Aaron Hill