Re: Optional arguments for scheme functions

[Fr. Samuel Springuel]

> On 26 Apr, 2020, at 1:01 PM, David Kastrup <address@hidden> wrote:
> 
> Predicates used in argument parsing have to be "primary" in that they
> have to deliver a result for anything thrown at them.
> 

Can that be noted in the docs 
(http://lilypond.org/doc/v2.19/Documentation/extending/scheme-function-definitions)?
  For a scheme novice (like myself) that distinction isn’t expected as the docs 
just point to the full list of predicates 
(http://lilypond.org/doc/v2.19/Documentation/notation/predefined-type-predicates)
 without noting that only primary ones will work in this context.

Further, in that full list of predicates, the first two sections are clearly 
labeled as primary and secondary, but what about the other three?  Clearly 
index? and ly:music? are primary (since they work in this context) so I suspect 
that all LilyPond scheme predicates (where index? appears) and LilyPond 
exported predicates (where ly:music? appears) are primary, but it would be 
nicer if that was clearly stated.  Further, what about the Guile predicate?


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Fr. Samuel, OSB
(R. Padraic Springuel)
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