Re: Defining new Scheme predicates

[Simon Albrecht]

On 07.11.2016 01:12, Simon Albrecht wrote:
> On 07.11.2016 01:07, Andrew Bernard wrote:
> > Greetings All,
> >
> > In lilypond, if I wanted to define a Scheme predicate such as side? 
> > to check whether an argument passed matches only ’left or ’right, how 
> > would one go about it?
> >
> > Looking in lily.scm I see lists of lilypond specific predicates, but 
> > it is unclear to me how to create my own additions, and indeed, how 
> > they would be implemented (apart from the trivial function itself).
> >
> > So for example, we have a function
> >
> > someFunction =
> >
> > #(define-music-function (thing position)
> >
> > (number? side?)
> >
> > …)
> >
> > How to add such a predicate? Can you, without going under the hood?
>
> Of course you can, and quite easily:
>
> someFunction =
> #(let
>   ((side? (lambda (foo) (or (equal 'left foo) (equal 'right foo)))))
>   (define-music-function (thing position)
>         (number? side?)
>         …)
>
> TBH, I just wrote that code right here and didn’t test it – but it 
> should be fine…
Except for the missing ) at the end – but you get the idea :-)
> HTH, Simon
>
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