Aaron Hill <[email protected]> writes:
> 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:
# does no processing of its own, $ creates a copy when given music.
> %%%%
> \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?
No, it's that \notes is identical to $notes (apart from the syntax) in
that it creates a copy. So whenever you write \something and do
something with it, whatever you do with it will not affect the original
stuff stored in the Scheme variable something . Basically the idea is
that you get a copy whenever you reference a variable, but the music
passed around in expressions and from one function to another does not
get copied unless you need more than one version of it.
--
David Kastrup
