Thank you so much!
> On Apr 3, 2023, at 8:52 PM, Jean Abou Samra <[email protected]> wrote:
>
> Le lundi 03 avril 2023 à 20:12 -0400, Nate Whetsell a écrit :
>
>> Because this engraver doesn’t mention staffs at all, I’m not sure how this
>> is happening or how to fix it, and I’m hoping I’m missing something simple.
>> Any help would be greatly appreciated! A small test program follows.
>>
> I see
>
> #(lambda (context)
> (let ((span '())
> (stub '())
> (event-drul '(() . ())))
> ^^^^^^^^^^^
> then
>
> `((listeners
> (frame-event .
> ,(lambda (engraver event)
> (if (= START (ly:event-property event 'span-direction))
> (set-car! event-drul event)
> ^^^^^^^^^^^^^^^^^^^^^^^^^^
> (set-cdr! event-drul event)))))
> ^^^^^^^^^^^^^^^^^^^^^^^^^^
> This is Bad (™). Quoting is a more subtle concept than many people think. For
> example, many beginner/intermediate and even advanced Scheme programmers do
> not realize that
>
> $ LC_ALL=C.UTF-8 ~/lilies/2.25.3/bin/lilypond scheme-sandbox
> GNU LilyPond 2.25.3 (running Guile 2.2)
> Processing
> `/home/jean/lilies/2.25.3/share/lilypond/2.25.3/ly/scheme-sandbox.ly'
> Parsing...
> GNU Guile 2.2.7
> Copyright (C) 1995-2019 Free Software Foundation, Inc.
>
> Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
> This program is free software, and you are welcome to redistribute it
> under certain conditions; type `,show c' for details.
>
> Enter `,help' for help.
> scheme@(#{ g123}#)> (define (f) (list 'a 'b 'c))
> scheme@(#{ g123}#)> (eq? (f) (f))
> $1 = #f
> scheme@(#{ g123}#)> (define (f2) '(a b c))
> scheme@(#{ g123}#)> (eq? (f2) (f2))
> $2 = #t
> When you quote something, you always get the same thing, as per eq?, i.e.,
> identity. Therefore, the two engraver instances are sharing the same pair of
> events, which is obviously troublesome.
>
> By the way, you can debug Scheme code by adding #(ly:set-option
> 'compile-scheme-code) (see this
> <https://lilypond.org/doc/v2.25/Documentation/extending/debugging-scheme-code>;
> caveat: does not work on Windows right now). If you do this, you will get a
> Guile error about attempting to mutate a literal pair.
>
> If you read section 3.4 of the R5RS
> <https://conservatory.scheme.org/schemers/Documents/Standards/R5RS/r5rs.pdf>,
> you can find
>
> “In many systems it is desirable for constants (i.e. the val- ues of literal
> expressions) to reside in read-only-memory. To express this, it is convenient
> to imagine that every object that denotes locations is associated with a flag
> telling whether that object is mutable or immutable. In such systems literal
> constants and the strings returned by symbol->string are immutable objects,
> while all objects created by the other procedures listed in this report are
> mutable. It is an error to attempt to store a new value into a location that
> is denoted by an immutable object.”
>
> Note that “it is an error” means the program is invalid, not that the
> implementation is required to detect the error. Guile does it for
> byte-compiled code (when you use compile-scheme-code). It unfortunately
> doesn't for interpreted code, as in the default mode in LilyPond.
>
> The fix here is to change '(() . ()) to (cons '() '()) to create a fresh pair
> for each engraver.
>
> Jean
>
