1.1.2 Scheme variables (Extending LilyPond)

#### 1.1.2 Scheme variables

Scheme variables can have any valid Scheme value, including a Scheme procedure.

Scheme variables are created with `define`:

    guile> (define a 2)
    guile>

Scheme variables can be evaluated at the guile prompt simply by typing the variable name:

    guile> a
    2
    guile>

Scheme variables can be printed on the display by using the display function:

    guile> (display a)
    2guile>

Note that both the value `2` and the guile prompt `guile` showed up on the same line. This can be avoided by calling the newline procedure or displaying a newline character.

    guile> (display a)(newline)
    2
    guile> (display a)(display "\n")
    2
    guile>

Once a variable has been created, its value can be changed with `set!`:

    guile> (set! a 12345)
    guile> a
    12345
    guile>

#### Extending LilyPond v2.26.0 (stable-branch).

- 1 Scheme tutorial
  - 1.1 Introduction to Scheme
    - 1.1.1 Scheme sandbox
    - 1.1.2 Scheme variables
    - 1.1.3 Scheme simple data types
    - 1.1.4 Scheme compound data types
      - Pairs
      - Lists
      - Association lists (alists)
      - Hash tables
    - 1.1.5 Calculations in Scheme
    - 1.1.6 Scheme procedures
      - Defining procedures
      - Predicates
      - Return values
    - 1.1.7 Scheme conditionals
      - if
      - cond
  - 1.2 Scheme in LilyPond
    - 1.2.1 LilyPond Scheme syntax
    - 1.2.2 LilyPond variables
    - 1.2.3 Debugging Scheme code
    - 1.2.4 Input variables and Scheme
    - 1.2.5 Importing Scheme in LilyPond
    - 1.2.6 Object properties
    - 1.2.7 LilyPond compound variables
      - Offsets
      - Fractions
      - Extents
      - Property alists
      - Alist chains
    - 1.2.8 LilyPond’s box model
    - 1.2.9 Internal music representation
  - 1.3 Building complicated functions
    - 1.3.1 A first approach (example)
    - 1.3.2 Displaying music expressions
    - 1.3.3 Music properties
    - 1.3.4 Doubling a note with slurs (example)
    - 1.3.5 Adding articulation to notes (example)
- 2 Interfaces for programmers
  - 2.1 LilyPond code blocks
  - 2.2 Scheme functions
    - 2.2.1 Scheme function definitions
    - 2.2.2 Scheme function usage
    - 2.2.3 Void Scheme functions
  - 2.3 Music functions
    - 2.3.1 Music function definitions
    - 2.3.2 Music function usage
    - 2.3.3 Simple substitution functions
    - 2.3.4 Intermediate substitution functions
    - 2.3.5 Mathematics in functions
    - 2.3.6 Functions without arguments
    - 2.3.7 Void music functions
  - 2.4 Event functions
  - 2.5 Markup functions
    - 2.5.1 Markup construction in Scheme
    - 2.5.2 How markups work internally
    - 2.5.3 New markup command definition
      - Markup command definition syntax
      - On properties
      - A complete example
      - Adapting builtin commands
      - Converting markups to strings
    - 2.5.4 New markup list command definition
  - 2.6 Contexts for programmers
    - 2.6.1 Context evaluation
    - 2.6.2 Running a function on all layout objects
  - 2.7 Callback functions
  - 2.8 Unpure-pure containers
  - 2.9 Difficult tweaks
- 3 LilyPond Scheme interfaces
- Appendix A GNU Free Documentation License
- Appendix B Index