2.3.2 Music function usage

With regard to handling the argument list, music functions don’t differ from Scheme functions, Scheme function usage.

A ‘music function’ has to return an expression matching the predicate ly:music?. This makes music function calls suitable as arguments of type ly:music? for another music function call.

When using a music function call in other contexts, the context may cause further semantic restrictions.

• At the top level in a music expression a post-event is not accepted.
• When a music function (as opposed to an event function) returns an expression of type post-event, LilyPond requires one of the named direction indicators (-, ^, and _) in order to properly integrate the post-event produced by the music function call into the surrounding expression.
• As a chord constituent. The returned expression must be of a rhythmic-event type, most likely a NoteEvent.

‘Polymorphic’ functions, like \tweak, can be applied to post-events, chord constituent and top level music expressions.