Top-level Bindings

Like most programming languages Scheme has the concept of scope, which basically means that names are visible and invisible in/from certain places.

Top-level bindings are variable definitions outside of any expression, somewhere in the input files. Bindings created on top-level are globally visible in the LilyPond files, both from and in included files as well. But of course they are only visible after they have been created, and it is an error to refer to them earlier in the input file. There is a notable difference that will be discussed in the context of procedure definitions.

In the chapter about including Scheme in LilyPond we have already seen top-level bindings (or “global variables” as they are often named in other languages). As demonstrated there bindings can be created in LilyPond and in Scheme syntax, and both produce equivalent bindings

% This is written in a LilyPond file
variableA = "Hello, I'm defined in LilyPond"
#(define variableB "And I'm defined in Scheme")

Now we have bound two strings to the variable names variableA and variableB. Structurally the bindings are equivalent and both variables can equally be referred to using Scheme and LilyPond syntax (\variableA vs. #variableA). The only difference is in the rules for naming the variables, as one will be parsed by LilyPond and the other by Scheme/Guile. LilyPond is rather restricted with regard to identifier naming while Scheme more or less allows everything. TODO: Reference to LilyPond naming options (including advanced options) For example the following definition is perfectly valid in Scheme, while the LilyPond definition would fail:

#(define f9!^¡ "What is this?")

f9!^¡ = "What is this?"

Such a variable will not be available through LilyPond's backslash syntax, but you can always access the actual Scheme value using the # hash sign:

{
  c'1 \mark #f9!^¡
}

Special Scope in Scheme Modules

As said earlier ariables defined in LilyPond files this way are globally visible also from or in included files. However, defined in Scheme modules there are different rules of visibility or “scope”. For now you are far from writing Scheme modules, so I'm only mentioning the fact.

By default, when you bind a variable in a Scheme file using (define), it is only visible from within the same module, respectively file. In order to make it available from outside one has to use (define-public) instead. This is an important method to hide elements that should not be accessed directly from outside.

Additionally there is the concept of (define-session-public), which does make an element publicly visible, but only for the current compilation out of a set of multiple files that are being processed.


Last update: January 31, 2020