It certainly makes sense to use multiple arguments instead of a list for a music function that applies an override to a breakable item like a clef, since there are only two pieces to consider. However, the function is designed to apply to spanners as well, and these of course may have more than two pieces (e.g., SystemStartBar). I know it’s possible to have music functions with optional arguments, but are they able to accept an indeterminate number of arguments, one for each potential spanner sibling?
Regarding optional arguments: I’ve been experimenting, and I’ve hit several stumbling blocks. In the following (very contrived) example, I expect that both key signatures will be green. Both are when all arguments are present. When the optional argument is omitted, both should again be green because (scheme? arg2) still returns #t. Instead, both key signatures disappear. I don’t understand this behavior.
#(define-music-function (parser location property arg1 arg2)
(symbol? scheme? (scheme?))
(display (scheme? arg2)) ; returns #t even if arg2 omitted
\once \override Staff.KeySignature $property =
(if (eq? (ly:item-break-dir grob) LEFT)
(if (scheme? arg2) arg1)))
\test #'color #green #red
%% key signatures disappear if following line used:
%\test #'color #green
\key cis \major
My second quandary is the fact that (scheme? arg2) returns #t when arg2 is omitted, so it appears useless as a criterion for whether an optional argument is there or not.. (I’ve chosen scheme? as my predicate so the generalized function can accept a wide variety of data types.)
Am I doing something wrong here? Any guidance you could give me would be much appreciated.