lilypond-essay: Flexible architecture
Flexible architecture
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When we started, we wrote the LilyPond program entirely in the C++
programming language; the program’s functionality was set in stone by
the developers. That proved to be unsatisfactory for a number of
reasons:
• When LilyPond makes mistakes, users need to override formatting
decisions. Therefore, the user must have access to the formatting
engine. Hence, rules and settings cannot be fixed by us at
compile-time but must be accessible for users at run-time.
• Engraving is a matter of visual judgment, and therefore a matter of
taste. As knowledgeable as we are, users can disagree with our
personal decisions. Therefore, the definitions of typographical
style must also be accessible to the user.
• Finally, we continually refine the formatting algorithms, so we
need a flexible approach to rules. The C++ language forces a
certain method of grouping rules that cannot readily be applied to
formatting music notation.
These problems have been addressed by integrating an interpreter for
the Scheme programming language and rewriting parts of LilyPond in
Scheme. The current formatting architecture is built around the notion
of graphical objects, described by Scheme variables and functions. This
architecture encompasses formatting rules, typographical style and
individual formatting decisions. The user has direct access to most of
these controls.
Scheme variables control layout decisions. For example, many
graphical objects have a direction variable that encodes the choice
between up and down (or left and right). Here you see two chords, with
accents and arpeggios. In the first chord, the graphical objects have
all directions down (or left). The second chord has all directions up
(right).
[image src="" alt="[image of music]" text="image of music" ]
The process of formatting a score consists of reading and writing the
variables of graphical objects. Some variables have a preset value.
For example, the thickness of many lines – a characteristic of
typographical style – is a variable with a preset value. You are free
to alter this value, giving your score a different typographical
impression.
[image src="" alt="[image of music]" text="image of music" ]
Formatting rules are also preset variables: each object has variables
containing procedures. These procedures perform the actual formatting,
and by substituting different ones, we can change the appearance of
objects. In the following example, the rule governing which note head
objects are used to produce the note head symbol is changed during the
music fragment.
[image src="" alt="[image of music]" text="image of music" ]
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