How to pass duration to lilypond function

Question

I defined a simple function:

disdn =
#(define-music-function
     (d)
     (ly:duration?)
   #{
    <dis ais dis g>#d
   #})

I am calling it as follows:

\disdn #8

I am getting an error:

wrong type for argument 1.  Expecting duration, found 8

What is the correct syntax for passing a duration? Please provide documentation reference as well.

Answer 1

You need to call your function either like this

\disdn 8

or like this

\disdn #(ly:make-duration 3)

When you write #8 you tell Lilypond to use the guile object 8, which is a number. But if you write 8 the parser will try to properly interpret this and will interpret this as duration.

For your code to work though you’d need to write

disdn =
#(define-music-function
     (d)
     (ly:duration?)
   #{
    <dis ais dis g>$d
   #})

Check the documentation for the difference between # and $: https://lilypond.org/doc/v2.24/Documentation/extending/lilypond-scheme-syntax

Basically # will mean the expression is lexed and then evaluated later by the parser. Meanwhile $ will mean that the expression is evaluated while it is being lexed, and the parser will parse the already evaluated expression. So writing

<...>#d

the parser will see <...> followed by a scheme expression and thus first evaluates <...> with the previous duration and then evaluates the scheme expression. But if you do

<dis ais dis g>$d

the parser will see <...> followed by a duration, and will thus evaluate this with the given duration.