10.15 LilyPond scoping

The LilyPond language has a concept of scoping, i.e., you can do:

foo = 1

#(begin
   (display (+ foo 2)))

with \paper, \midi and \header being nested scope inside the .ly file-level scope. foo = 1 is translated in to a scheme variable definition.

This implemented using modules, with each scope being an anonymous module that imports its enclosing scope’s module.

LilyPond’s core, loaded from .scm files, is usually placed in the lily module, outside the .ly level. In the case of

lilypond a.ly b.ly

we want to reuse the built-in definitions, without changes effected in user-level a.ly leaking into the processing of b.ly.

The user-accessible definition commands have to take care to avoid memory leaks that could occur when running multiple files. All information belonging to user-defined commands and markups is stored in a manner that allows it to be garbage-collected when the module is dispersed, either by being stored module-locally, or in weak hash tables.