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2.2.3 Void scheme functions

Sometimes a procedure is executed in order to perform an action rather than return a value. Some programming languages (like C and Scheme) use functions for either concept and just discard the returned value (usually by allowing any expression to act as statement, ignoring the result). This is clever but error-prone: most C compilers nowadays offer warnings for various non-“void” expressions being discarded. For many functions executing an action, the Scheme standards declare the return value to be unspecified. LilyPond’s Scheme interpreter Guile has a unique value *unspecified* that it usually (such when using set! directly on a variable) but unfortunately not consistently returns in such cases.

Defining a LilyPond function with define-void-function makes sure that this special value (the only value satisfying the predicate void? ) will be returned.

noPointAndClick =
#(define-void-function
   (ly:set-option 'point-and-click #f))
\noPointAndClick   % disable point and click
If you want to evaluate an expression only for its side-effect and
don’t want any value it may return interpreted, you can do so by
prefixing it with \void:
\void #(hashq-set! some-table some-key some-value)
That way, you can be sure that LilyPond will not assign meaning to the
returned value regardless of where it encounters it.  This will also
work for music functions such as \displayMusic.
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About automatic language selection.
      
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 Internal music representation
    
1.3 Building complicated functions
      
1.3.1 Displaying music expressions
      
1.3.2 Music properties
      
1.3.3 Doubling a note with slurs (example)
      
1.3.4 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
  
A. GNU Free Documentation License
  
B. LilyPond index