request: better error message

[Thomas Morley]

Hi all,

I just tried to to update/review and modify an old score. I've got an
assertion failure.

With released 2.19.65:
/home/gub/NewGub/gub/target/linux-64/src/lilypond-git.sv.gnu.org--lilypond.git-release-unstable/lily/grob-property.cc:242:
scm_unused_struct* Grob::try_callback_on_alist(scm_unused_struct**,
SCM, SCM): Assertion `((((scm_t_bits) (0? (*(SCM*)0=((value))):
(value))) == ((scm_t_bits) (0? (*(SCM*)0=(((SCM) ((((4)) << 8) +
scm_tc8_flag)))): ((SCM) ((((4)) << 8) + scm_tc8_flag)))))) ||
(((scm_t_bits) (0? (*(SCM*)0=(value)): value)) == ((scm_t_bits) (0?
(*(SCM*)0=(marker)): marker)))' failed.
Aborted (core dumped)

With a build from recent master:
/home/hermann/lilypond-git/lily/grob-property.cc:242:
scm_unused_struct* Grob::try_callback_on_alist(scm_unused_struct**,
SCM, SCM): Assertion `scm_is_null (value) || scm_is_eq (value,
marker)' failed.

It took my quite a while and was going on my nerves before I found the
culprit in my file with a lot of included functions from other files,
etc, etc. The above error-messages didn't help in any way.
Could we please put out a meaningful message for the problem triggered
with (stripped down):

\version "2.19.65"

{
  \override Fingering #'stencil =
    #(lambda (grob)
      (ly:grob-set-property! grob 'stencil
        (grob-interpret-markup grob #{ \markup \normal-text "foo" #})))
  a''-1
}

I doubt many people would have been able to read the error and
directly point to the faulty coding.


Thanks,
  Harm