Re: 2.25.2 optional fine endings

[Jean Abou Samra]

Le lundi 13 mars 2023 à 13:04 -0700, Paul Scott a écrit :

I've read your wonderful scheme documentation.  I'm still struggling to make sense of Aaron's simple code
\fixed c' {
   c1
   \set Score.repeatCommands = #((volta ,voltaFine))    c4 d c2 \bar "|."    \set Score.repeatCommands = #((volta #f) (volta ,voltaCont))
   f4 g a b
   \set Score.repeatCommands = #'((volta #f))
}

I'm extremely familiar with a number of programming languages including Forth.  If one of you could say more about what the line:
   \set Score.repeatCommands = #`((volta #f) (volta ,voltaCont))
does and how it relates with

   \set Score.repeatCommands = #(list(list 'volta voltaCont))

I might be able to go farther.   

TIA for any more help  with this,

The format of repeatCommands is a list where each element is itself a list. The first element of each sublist (element of the main list) is a "command", namely a symbol, which can be volta, start-repeat or end-repeat.

• If the command is volta, LilyPond expects one remaining element in the sublist, which can be

  • a markup, to start a volta bracket and print this text under it (note that a simple string is also a valid markup),
  • #f (the boolean false), to end a volta bracket previously started with the first form.

• If the command is start-repeat, it prints a start repeat bar line.

• If the command is end-repeat, it prints an end repeat bar line.

The _expression_

#(list (list 'volta voltaCont))

uses the list function to create a list with one element inside. That element is (list 'volta voltaCont), which is itself a list with two elements, 'volta and voltaCont. The first element is a quoted symbol, so it's not evaluated and remains as a symbol. The second element is also a symbol but without a quote. Since symbols are not self-evaluating, but evaluate by looking up variables, this reads the value of the voltaCont variable. So you have

list
| list element 1:
|    list
|       list element 1: the symbol volta
|       list element 2: the markup contained in the variable voltaCont

which matches the form for a repeatCommands with one command, "volta", starting a volta bracket with the text contained in the variable "voltaCont".

The _expression_

#`((volta #f) (volta ,voltaCont))

is a quasiquote. Since a quasiquote is “almost a quote”, it means that Scheme looks literally at what _expression_ you have entered, rather than evaluating it. Because of the parentheses, it's a list. That list contains two sublists. The first one is (volta #f). Again, it's quoted, so it's not evaluated. (If it were evaluated, it would look up the function “volta” and apply it to the boolean #f.) That list is made of the symbol volta and the boolean #f.

The second sublist is (volta ,voltaCont). Again, it's quoted, so not evaluated. The first element is volta, a symbol. The second element has an unquote on it (the comma), so it does get evaluated. It's a symbol, voltaCont, and symbols evaluate by looking up variables, so it gets the value of voltaCont and inserts that markup into the sublist.

The result is

list
| list element 1:
|   list:
|    | list element 1: the symbol volta
|    | list element 2: the boolean #f
| list element 2:
|   list:
|    | list element 1: the symbol volta
|    | list element 2: the markup contained in the variable voltaCont

and it matches the format of repeatCommands for two commands: ending a previously started volta (the first element) and starting a new one (the second element).

Did it help to read it very verbosely like this?

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