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.