Lilypond Scheme Substitution function not using its arguments properly?

Question

I would like to create a Lilypond music function that allows me to express this MWE:

{ \override Score.SpacingSpanner.base-shortest-duration = #(ly:make-moment 1/30) a b c }

as

\spn #1 #30 { a b c }

I tried with with this:

\version "2.22.1"

 spn =
#(define-music-function
  (beg end notes)
  (number? number? ly:music?)
  #{
 
 { \override Score.SpacingSpanner.base-shortest-duration = #(ly:make-moment #(cons beg end)) #notes }
    
  #})

\spn #1 #30 { a b c }

\relative c''
    { \override Score.SpacingSpanner.base-shortest-duration = #(ly:make-moment 1/30) a b c }

which does not produces same output as the MWE.

I get programming error: Unknown type predicate.

I suspect I am expressing the ly:make-moment argument improperly.

What am I doing wrong?

Answer 1

"unknown type predicate" would be an error with LilyPond's internals when typechecking the arguments of ly:make-moment because the complaint should be that the argument of ly:make-moment is not a rational number. If this persists into the latest version, report a problem to the bug reporting list.

Of course your code also is faulty: you need to write (/ beg end) where you write (cons beg end) because the 1/30 you replace is not a LilyPond fraction 1/30 (which would be represented by (1 . 30) indeed) but a Scheme exact rational number 1/30. Fixing that will help with your problem.

But you should get a proper error message rather than LilyPond not being able to tell you what it is complaining about.