> ``` > (let* ((bound-details (ly:grob-property grob 'bound-details)) > (left (assoc-get 'left bound-details)) > (left (assoc-set! > left > ```
If you add another red text spanner, you will see that it gets the text "foo"
in spite of \once. The reason is that you're mutating the default
'bound-details value shared by all TextSpanner grobs.
In fact, nothing guarantees that the default value is even mutable. Mutating a
constant, as created by a quote expression, is disallowed. (In recent Guile
versions, it can even lead to true "nasal demons" undefined behavior — when
you enable byte-compilation and optimizations, `(set-car! '(1 . 2) 3)` outright
segfaults. The time I looked at the bug, I couldn't understand where it was
coming from.)
Better use alist-copy here. Also, there is no need for mutating the grob in
before-line-breaking, you can use the more functional approach with
grob-transformer:
```
\version "2.24"
{
\once \override TextSpanner.bound-details =
#(grob-transformer
'bound-details
(lambda (grob orig)
(assoc-set! (alist-copy orig)
'left
(assoc-set! (alist-copy (assoc-ref orig 'left))
'text
(if (equal? red (ly:grob-property grob 'color))
"foo" "bar")))))
c'1
- \tweak color #red
\startTextSpan
c'1
\stopTextSpan
c'1\tweak color #red \startTextSpan c'1\stopTextSpan
}
```
Best,
Jean
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