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2.2.2 Scheme function usage
Scheme functions can be called pretty much anywhere where a Scheme
expression starting with # can be written. You call a scheme
function by writing its name preceded by \, followed by its
arguments. Once an optional argument predicate does not match an
argument, Lilypond skips this and all following optional arguments,
replacing them with their specified default, and ‘backs up’ the
argument that did not match to the place of the next mandatory argument.
Since the backed up argument needs to go somewhere, optional arguments
are not actually considered optional unless followed by a mandatory
argument.
There is one exception: if you write \default in the place of an
optional argument, this and all following optional arguments are skipped
and replaced by their default. This works even when no mandatory
argument follows since \default does not need to get backed up.
The mark and key commands make use of that trick to
provide their default behavior when just followed by \default.
Apart from places where a Scheme value is required, there are a few
places where # expressions are currently accepted and evaluated
for their side effects but otherwise ignored. Mostly those are the
places where an assignment would be acceptable as well.
Since it is a bad idea to return values that can be misinterpreted in some context, you should use normal scheme functions only for those cases where you always return a useful value, and use void scheme functions (see section Void scheme functions) otherwise.
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