Re: `ly:context-property` and empty alist

[Jean Abou Samra]

Le 10/07/2022 à 14:32, Valentin Petzel a écrit :
> Hello Werner,
>
> the problem here is not really ly:context-property, but the C-implementation
> Context::internal_get_property, which uses SCM_EOL as default value. So
> Lilypond’s guile interface cannot know if a NULL is actually set or if the
> context property is not set. It would be possible to check before whether the
> context in question has this property set. Sadly we do not have a scheme
> handle on the here_defined macro, but only on the where defined macro.
>
> Still we can use something like
> (eq? (ly:context-property-where-defined context sym) context)
> to check if a symbol is defined in a specific context. So we could use something
> like:
>
> #(define (context-has-defined? context sym)
>     (eq? (ly:context-property-where-defined context sym) context))


That differs in semantics from ly:context-property not just with the 
handling of '(). Normally, context properties are inherited. If the 
property is not defined in the context, it is read from its parent, or 
the parent of the parent, etc. How about this instead:

#(define* (context-strict-property context property #:optional (default 
'()))
   (if (null? (ly:context-property-where-defined context property))
       default
       (ly:context-property context property)))

Best,
Jean