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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