Defining new Scheme predicates

[Andrew Bernard]

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?

 

 

Andrew