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1.2.8 Internal music representation
Internally, music is represented as a Scheme list. The list contains various elements that affect the printed output. Parsing is the process of converting music from the LilyPond input representation to the internal Scheme representation.
When a music expression is parsed, it is converted into a set of Scheme music objects. The defining property of a music object is that it takes up time. The time it takes up is called its duration. Durations are expressed as a rational number that measures the length of the music object in whole notes.
A music object has three kinds of types:
-
music name: Each music expression has a name. For example, a note
leads to a
NoteEvent, and
\simultaneousleads to a SimultaneousMusic. A list of all expressions available is in the Internals Reference manual, under Music expressions. -
‘type’ or interface: Each music name has several ‘types’ or
interfaces, for example, a note is an
event, but it is also anote-event, arhythmic-event, and amelodic-event. All classes of music are listed in the Internals Reference, under Music classes. -
C++ object: Each music object is represented by an object of the C++
class
Music.
The actual information of a music expression is stored in properties.
For example, a
NoteEvent has pitch and
duration properties that store the pitch and duration of that
note. A list of all properties available can be found in the
Internals Reference, under
Music properties.
A compound music expression is a music object that contains other
music objects in its properties. A list of objects can be stored in
the elements property of a music object, or a single ‘child’
music object in the element property. For example,
SequentialMusic has its children in elements,
and
GraceMusic has its single argument in
element. The body of a repeat is stored in the element
property of
RepeatedMusic, and the alternatives in
elements.
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