Re: Accessing Grob properties

[David Kastrup]

Urs Liska <address@hidden> writes:

> It seems I'm now at a similar point in C++ Guile (scm_whatsoever) as I
> was with Guile Scheme at an earlier stage :-(
>
>   SCM subdivide_details =
>         scm_assq_ref (beam_settings_, ly_symbol2scm ("subdivide-details"));
>
> seems to fetch the right thing, namely something like
>     '((beam-count . metric))
>
> But then I start to get lost.
> I would expect
>
>   SCM beam_count_prop =
>         scm_assq_ref (subdivide_details, ly_symbol2scm ("beam-count"));
>
> to assign beam_count_prop a symbol "metric" - as I get when I do the
> same thing in Scheme.

It does.

> However, from there I don't get any further, and it seems I totally
> don't understand how the Scheme types are matched in C++.
> Any attempt to compare the content of beam_count_prop with a set of
> predefined values seems to fail, and even
>
>     if (scm_equal_p (ly_symbol2scm ("one"), ly_symbol2scm ("two")))
>
> returns true (using scm_equal_p equally as for the eq and eqv
> versions.

It does not return true but SCM_BOOL_F.  Which you then convert to a
true C++ boolean since it is non-zero.

You want   if (scm_is_eq (... ))
instead: predicates with name xxx_is_yyy return a C (or C++) boolean.
Predicates with name xxx_p return an SCM boolean which you need to
convert to a C++ boolean using, for example, scm_is_true.

> I would have expected that
>
>   beaming_options_.subdivided_beam_count_ =
>     (scm_eqv_p (ly_symbol2scm("one"), ly_symbol2scm("one")))
>             ? ONE
>             : (scm_eqv_p (subdiv_beam_count_prop,
> ly_symbol2scm("base-moment")))
>                 ? BASE_MOMENT
>                 : METRIC;
>
> would assing the (new) member subdivided_beam_count one out of the three
> constants, depending on the value of the property:

This isn't Emacs Lisp (which has a clever definition of nil, which in
Lisp serves both the functions of #f and '()).  SCM_BOOL_F as a C
condition is _true_ rather than false.

-- 
David Kastrup